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  • 51. Baggioli, M.
    et al.
    Goutéraux, Blaise
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Kiritsis, E.
    Li, W. -J
    Higher derivative corrections to incoherent metallic transport in holography2017Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, Vol. 2017, nr 3, artikel-id 170Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Transport in strongly-disordered, metallic systems is governed by diffusive processes. Based on quantum mechanics, it has been conjectured that these diffusivities obey a lower bound D/v2 ≳ ℏ/kBT , the saturation of which provides a mechanism for the T-linear resistivity of bad metals. This bound features a characteristic velocity v, which was later argued to be the butterfly velocity vB, based on holographic models of transport. This establishes a link between incoherent metallic transport, quantum chaos and Planckian timescales. Here we study higher derivative corrections to an effective holographic action of homogeneous disorder. The higher derivative terms involve only the charge and translation symmetry breaking sector. We show that they have a strong impact on the bound on charge diffusion Dc/νB 2 ≳ ℏ/kBT, by potentially making the coefficient of its right-hand side arbitrarily small. On the other hand, the bound on energy diffusion is not affected.

  • 52.
    Baggioli, Matteo
    et al.
    UAM, CSIC, Inst Fis Teor, C Nicolas Cabrera 13-15, Madrid 28049, Spain..
    Gran, Ulf
    Chalmers Univ Technol, Dept Phys, Div Theoret Phys, SE-41296 Gothenburg, Sweden..
    Alba, Amadeo Jimenez
    UAM, CSIC, Inst Fis Teor, C Nicolas Cabrera 13-15, Madrid 28049, Spain..
    Tornso, Marcus
    Chalmers Univ Technol, Dept Phys, Div Theoret Phys, SE-41296 Gothenburg, Sweden..
    Zingg, Tobias
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, NORDITA, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Holographic plasmon relaxation with and without broken translations2019Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 9, artikel-id 013Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the dynamics and the relaxation of bulk plasmons in strongly coupled and quantum critical systems using the holographic framework. We analyze the dispersion relation of the plasmonic modes in detail for an illustrative class of holographic bottom-up models. Comparing to a simple hydrodynamic formula, we entangle the complicated interplay between the three least damped modes and shed light on the underlying physical processes. Such as the dependence of the plasma frequency and the effective relaxation time in terms of the electromagnetic coupling, the charge and the temperature of the system. Introducing momentum dissipation, we then identify its additional contribution to the damping. Finally, we consider the spontaneous symmetry breaking (SSB) of translational invariance. Upon dialing the strength of the SSB, we observe an increase of the longitudinal sound speed controlled by the elastic moduli and a decrease in the plasma frequency of the gapped plasmon. We comment on the condensed matter interpretation of this mechanism.

  • 53.
    Bai, Nan
    et al.
    Guangxi Normal Univ, Dept Phys, Guilin 541004, Peoples R China..
    Chen, Hui-Huang
    Jiangxi Normal Univ, Coll Phys & Commun Elect, Nanchang 330022, Jiangxi, Peoples R China..
    Ouyang, Hao
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Wu, Jun-Bao
    Tianjin Univ, Ctr Joint Quantum Studies, 135 Yaguan Rd, Tianjin 300350, Peoples R China.;Tianjin Univ, Sch Sci, Dept Phys, 135 Yaguan Rd, Tianjin 300350, Peoples R China.;Peking Univ, Ctr High Energy Phys, 5 Yiheyuan Rd, Beijing 100871, Peoples R China..
    Two-loop integrability of ABJM open spin chain from giant graviton2019Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 3, artikel-id 193Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We prove the integrability of the two-loop open spin chain Hamiltonian from ABJM determinant like operators given in [1]. By explicitly constructing R-matrices and K-matrices, we successfully obtain the two-loop Hamiltonian from the double row transfer matrices. This proves the integrability of our two-loop Hamiltonian. Based on the vacuum eigenvalues of the transfer matrices, we make a conjecture on the eigenvalues of the transfer matrices for general excited states. Bethe ansatz equations are simply obtained from the analytic conditions at the superficial poles of the eigenvalues.

  • 54. Balasubramanian, V.
    et al.
    Bernamonti, A.
    Craps, B.
    Keränen, Ville
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Keski-Vakkuri, E.
    Mueller, B.
    Thorlacius, Larus
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Vanhoof, J.
    Thermalization of the spectral function in strongly coupled two dimensional conformal field theories2013Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 4, s. 069-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Using Wigner transforms of Green functions, we discuss non-equilibrium generalizations of spectral functions and occupation numbers. We develop methods for computing time-dependent spectral functions in conformal field theories holographically dual to thin-shell AdS-Vaidya spacetimes.

  • 55.
    Balatsky, Alexander
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Gudnason, Sven Bjarke
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Kedem, Yaron
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Krikun, Alexander
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Thorlacius, Larus
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Zarembo, Konstantin
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Classical and quantum temperature fluctuations via holography2015Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 1, artikel-id 011Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study local temperature fluctuations in a 2+1 dimensional CFT on the sphere, dual to a black hole in asymptotically AdS spacetime. The fluctuation spectrum is governed by the lowest-lying hydrodynamic modes of the system whose frequency and damping rate determine whether temperature fluctuations are thermal or quantum. We calculate numerically the corresponding quasinormal frequencies and match the result with the hydrodynamics of the dual CFT at high temperature. As a by-product of our analysis we determine the appropriate boundary conditions for calculating low-lying quasinormal modes for a four-dimensional Reissner-Nordstrom black hole in global AdS.

  • 56.
    Balatsky, Alexander V.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Institute for Materials Science, Los Alamos National Laboratory, Los Alamos, NM, United States .
    Balatsky, Galina I.
    Borysov, Stanislav S.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik, Nanostrukturfysik.
    Resource Demand Growth and Sustainability Due to Increased World Consumption2015Ingår i: Sustainability, ISSN 2071-1050, E-ISSN 2071-1050, Vol. 7, nr 3, s. 3430-3440Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The paper aims at continuing the discussion on sustainability and attempts to forecast the impossibility of the expanding consumption worldwide due to the planet's limited resources. As the population of China, India and other developing countries continue to increase, they would also require more natural and financial resources to sustain their growth. We coarsely estimate the volumes of these resources (energy, food, freshwater) and the gross domestic product (GDP) that would need to be achieved to bring the population of India and China to the current levels of consumption in the United States. We also provide estimations for potentially needed immediate growth of the world resource consumption to meet this equality requirement. Given the tight historical correlation between GDP and energy consumption, the needed increase of GDP per capita in the developing world to the levels of the U.S. would deplete explored fossil fuel reserves in less than two decades. These estimates predict that the world economy would need to find a development model where growth would be achieved without heavy dependence on fossil fuels.

  • 57. Balazs, Csaba
    et al.
    Li, Tong
    Savage, Chris
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. University of Utah, United States.
    White, Martin
    Interpreting the Fermi-LAT gamma ray excess in the simplified framework2015Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 92, nr 12, artikel-id 123520Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We test the plausibility of the hypothesis that the annihilation of a Majorana fermion dark matter particle via a scalar mediator explains the gamma ray excess from the Galactic center. Assuming that the mediator couples to all third generation fermions we calculate observables for dark matter abundance and scattering on nuclei, gamma, positron, and antiproton cosmic ray fluxes, radio emission from dark matter annihilation, and the effect of dark matter annihilations on the CMB. After discarding the controversial radio observation, we show that the dark matter model simultaneously fits the observed excesses in the cosmic gamma ray, the positron, and antiproton fluxes, while evading constraints from the CMB and direct detection. The experimental data are consistent with a dark matter (mediator) mass in the 10-100 (3-1000) GeV region and with weakly correlated couplings to bottom quarks and tau leptons with values of 10(-3) - 1 at the 68% credibility level.

  • 58.
    Banerjee, R.
    et al.
    Nanyang Technol Univ, Sch Phys & Math Sci, Div Phys & Appl Phys, Singapore 637371, Singapore..
    Liew, T. C. H.
    Nanyang Technol Univ, Sch Phys & Math Sci, Div Phys & Appl Phys, Singapore 637371, Singapore..
    Kyriienko, O.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Realization of Hofstadter's butterfly and a one-way edge mode in a polaritonic system2018Ingår i: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, nr 7, artikel-id 075412Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a scheme to generate an artificial gauge field for the system of neutral bosons, represented by polaritons in micropillars arranged into a square lattice. The splitting between the two polarizations of the micropillars breaks the time-reversal symmetry (TRS) and results in the effective phase-dependent hopping between cavities. This can allow for engineering a nonzero flux on the plaquette, corresponding to an artificial magnetic field. Changing the phase, we observe a characteristic Hofstadter's butterfly pattern and the appearance of chiral edge states for a finite-size structure. For long-lived polaritons, we show that the propagation of wave packets at the edge is robust against disorder. Moreover, given the inherent driven-dissipative nature of polariton lattices, we find that the system can exhibit topological lasing, recently discovered for active ring cavity arrays. The results point to a static way to realize artificial magnetic field in neutral spinful systems, avoiding the periodic modulation of the parameters or strong spin-orbit interaction. Ultimately, the described system can allow for high-power topological single-mode lasing which is robust to imperfections.

  • 59.
    Banerjee, saikat
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Universal trends in interacting two-dimensional Dirac materialsManuskript (preprint) (Övrigt vetenskapligt)
  • 60.
    Banerjee, Saikat
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi. KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Institute for Materials Science, Los Alamos National Laboratory, USA.
    Fransson, J.
    Black-Schaffer, A. M.
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Balatsky, A. V.
    Granular superconductor in a honeycomb lattice as a realization of bosonic Dirac material2016Ingår i: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, nr 13, artikel-id 134502Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We examine the low-energy effective theory of phase oscillations in a two-dimensional granular superconducting sheet where the grains are arranged in a honeycomb lattice structure. Using the example of graphene, we present evidence for the engineered Dirac nodes in the bosonic excitations: the spectra of the collective bosonic modes cross at the K and K' points in the Brillouin zone and form Dirac nodes. We show how two different types of collective phase oscillations are obtained and that they are analogous to the Leggett and the Bogoliubov-Anderson-Gorkov modes in a two-band superconductor. We show that the Dirac node is preserved in the presence of an intergrain interaction, despite induced changes of the qualitative features of the two collective modes. Finally, breaking the sublattice symmetry by choosing different on-site potentials for the two sublattices leads to a gap opening near the Dirac node, in analogy with fermionic Dirac materials. The Dirac node dispersion of bosonic excitations is thus expanding the discussion of the conventional Dirac cone excitations to the case of bosons. We call this case as a representative of bosonic Dirac materials (BDM), similar to the case of Fermionic Dirac materials extensively discussed in the literature.

  • 61.
    Banerjee, Saikat
    et al.
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi. KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Los Alamos Natl Lab, USA.
    Ågren, Hans
    KTH, Skolan för bioteknologi (BIO), Teoretisk kemi och biologi.
    Balatsky, Alexander V.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Landau-like states in neutral particles2016Ingår i: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, nr 23, artikel-id 235134Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We show the emergence of a new type of dispersion relation for neutral atoms with an interesting similarity to the spectrum of two-dimensional electrons in an applied perpendicular constant magnetic field. These neutral atoms can be confined in toroidal optical traps and give quasi-Landau spectra. In strong contrast to the equidistant infinitely degenerate Landau levels for charged particles, the spectral gap for such two-dimensional neutral particles increases in particular electric-field configurations. The idea in the paper is motivated by the development in cold atom experiments and builds on the seminal paper of Aharonov and Casher.

  • 62.
    Barekat, Atefeh
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Department of Astronomy, AlbaNova University Center, Stockholm University, Stockholm, Sweden .
    Brandenburg, A.
    Near-polytropic stellar simulations with a radiative surface2014Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 571, s. A68-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Studies of solar and stellar convection often employ simple polytropic setups using the diffusion approximation instead of solving the proper radiative transfer equation. This allows one to control separately the polytropic index of the hydrostatic reference solution, the temperature contrast between top and bottom, and the Rayleigh and Peclet numbers. Aims. Here we extend such studies by including radiative transfer in the gray approximation using a Kramers-like opacity with freely adjustable coefficients. We study the properties of such models and compare them with results from the diffusion approximation. Methods. We use the Pencil code, which is a high-order finite difference code where radiation is treated using the method of long characteristics. The source function is given by the Planck function. The opacity is written as kappa = kappa(0)rho T-a(b), where a = 1 in most cases, b is varied from -3.5 to +5, and kappa(0) is varied by four orders of magnitude. We adopt a perfect monatomic gas. We consider sets of one-dimensional models and perform a comparison with the diffusion approximation in one-and two-dimensional models. Results. Except for the case where b = 5, we find one-dimensional hydrostatic equilibria with a nearly polytropic stratification and a polytropic index close to n = (3 -b)/(1 + a), covering both convectively stable (n > 3/2) and unstable (n < 3/2) cases. For b = 3 and a = -1, the value of n is undefined a priori and the actual value of n depends then on the depth of the domain. For large values of kappa(0), the thermal adjustment time becomes long, the Peclet and Rayleigh numbers become large, and the temperature contrast increases and is thus no longer an independent input parameter, unless the Stefan-Boltzmann constant is considered adjustable. Conclusions. Proper radiative transfer with Kramers-like opacities provides a useful tool for studying stratified layers with a radiative surface in ways that are more physical than what is possible with polytropic models using the diffusion approximation.

  • 63. Bartl, A.
    et al.
    Pethick, Christopher J.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Univ Copenhagen, Denmark.
    Schwenk, A.
    Supernova Matter at Subnuclear Densities as a Resonant Fermi Gas: Enhancement of Neutrino Rates2014Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 113, nr 8, s. 081101-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    At low energies nucleon-nucleon interactions are resonant and therefore supernova matter at subnuclear densities has many similarities to atomic gases with interactions dominated by a Feshbach resonance. We calculate the rates of neutrino processes involving nucleon-nucleon collisions and show that these are enhanced in mixtures of neutrons and protons at subnuclear densities due to the large scattering lengths. As a result, the rate for neutrino pair bremsstrahlung and absorption is significantly larger below 10(13) g cm(-3) compared to rates used in supernova simulations.

  • 64. Battistin, C.
    et al.
    Hertz, John
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Niels Bohr Institute, Denmark .
    Tyrcha, J.
    Roudi, Yasser
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Centre for Neural Computation, Norway.
    Belief propagation and replicas for inference and learning in a kinetic Ising model with hidden spins2015Ingår i: Journal of Statistical Mechanics: Theory and Experiment, ISSN 1742-5468, E-ISSN 1742-5468, nr 5, artikel-id P05021Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We propose a new algorithm for inferring the state of hidden spins and reconstructing the connections in a synchronous kinetic Ising model, given the observed history. Focusing on the case in which the hidden spins are conditionally independent of each other given the state of observable spins, we show that calculating the likelihood of the data can be simplified by introducing a set of replicated auxiliary spins. Belief propagation (BP) and susceptibility propagation (SusP) can then be used to infer the states of hidden variables and to learn the couplings. We study the convergence and performance of this algorithm for networks with both Gaussian-distributed and binary bonds. We also study how the algorithm behaves as the fraction of hidden nodes and the amount of data are changed, showing that it outperforms the Thouless-Anderson-Palmer (TAP) equations for reconstructing the connections.

  • 65.
    Baum, S.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. The Oskar Klein Centre for Cosmoparticle Physics, Department of PhysicsStockholm UniversityStockholmSweden.
    Carena, M.
    Shah, N. R.
    Wagner, C. E. M.
    Higgs portals for thermal Dark Matter. EFT perspectives and the NMSSM2018Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, Vol. 2018, nr 4, artikel-id 69Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We analyze a low energy effective model of Dark Matter in which the thermal relic density is provided by a singlet Majorana fermion which interacts with the Higgs fields via higher dimensional operators. Direct detection signatures may be reduced if blind spot solutions exist, which naturally appear in models with extended Higgs sectors. Explicit mass terms for the Majorana fermion can be forbidden by a Z3 symmetry, which in addition leads to a reduction of the number of higher dimensional operators. Moreover, a weak scale mass for the Majorana fermion is naturally obtained from the vacuum expectation value of a scalar singlet field. The proper relic density may be obtained by the s-channel interchange of Higgs and gauge bosons, with the longitudinal mode of the Z boson (the neutral Goldstone mode) playing a relevant role in the annihilation process. This model shares many properties with the Next-to-Minimal Supersymmetric extension of the Standard Model (NMSSM) with light singlinos and heavy scalar and gauge superpartners. In order to test the validity of the low energy effective field theory, we compare its predictions with those of the ultraviolet complete NMSSM. Extending our framework to include Z3 neutral Majorana fermions, analogous to the bino in the NMSSM, we find the appearance of a new bino-singlino well tempered Dark Matter region. 

  • 66.
    Baum, Sebastian
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Oskar Klein Ctr, Dept Phys, SE-10691 Stockholm, SwedenStockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Catena, Riccardo
    Chalmers Univ Technol, Dept Phys, SE-41296 Gothenburg, Sweden..
    Krauss, Martin B.
    Chalmers Univ Technol, Dept Phys, SE-41296 Gothenburg, Sweden.;Univ Roma Tre, Dipartimento Matemat & Fis, Via Vasca Navale 84, I-00146 Rome, Italy..
    Impact of a XENONnT signal on LHC dijet searches2019Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 7, artikel-id 015Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    It is well-known that dark matter (DM) direct detection experiments and the LHC are complementary, since they probe physical processes occurring at different energy scales. And yet, there are aspects of this complementarity which are still not fully understood, or exploited. For example, what is the impact that the discovery of DM at XENONnT would have on present and future searches for DM in LHC final states involving a pair of hadronic jets? In this work we investigate the impact of a XENONnT signal on the interpretation of current dijet searches at the LHC, and on the prospects for dijet signal discovery at the High-Luminosity (HL) LHC in the framework of simplified models. Specifically, we focus on a general class of simplified models where DM can have spin 0, 1/2 or 1, and interacts with quarks through the exchange of a scalar, pseudo-scalar, vector, or pseudo-vector mediator. We find that exclusion limits on the mediator's mass and its coupling to quarks from dijet searches at the LHC are significantly affected by a signal at XENONnT, and that O(100) signal events at XENONnT would drastically narrow the region in the parameter space of simplified models where a dijet signal can be discovered at 5 sigma C.L. at the HL-LHC.

  • 67.
    Baum, Sebastian
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Dept Phys, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Freese, Katherine
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Dept Phys, Oskar Klein Ctr Cosmoparticle Phys, S-10691 Stockholm, Sweden.; Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Univ Michigan, Dept Phys, Leinweber Ctr Theoret Phys, Ann Arbor, MI 48109 USA..
    Kelso, Chris
    Univ North Florida, Dept Phys, Jacksonville, FL 32224 USA..
    Dark Matter implications of DAMA/LIBRA-phase2 results2019Ingår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 789, s. 262-269Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recently, the DAMA/LIBRA collaboration released updated results from their search for the annual modulation signal from Dark Matter (DM) scattering in the detector. Besides approximately doubling the exposure of the DAMA/LIBRA data set, the updated photomultiplier tubes of the experiment allow a lower recoil energy threshold of I keV electron equivalent compared to the previous threshold of 2 keV electron equivalent. We study the compatibility of the observed modulation signal with DM scattering. Due to a conspiracy of multiple effects, the new data at low recoil energies is very powerful for testing the DM hypothesis. We find that canonical (isospin conserving) spin-independent DM-nucleon interactions are no longer a good fit to the observed modulation signal in the standard halo model. The canonical spin independent case is disfavored by the new data, with best fit points of a DM mass of similar to 8 GeV, disfavored by 5.2 sigma, or a mass of similar to 54GeV, disfavored by 2.5 sigma. Allowing for isospin violating spin independent interactions, we find a region with a good fit to the data with suppressed effective couplings to iodine for DM masses of similar to 10 GeV. We also consider spin-dependent DM-nucleon interactions, which yield good fits for similar DM masses of similar to 10 GeV or similar to 45 GeV. Elsevier B.V.

  • 68.
    Baum, Sebastian
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Freese, Katherine
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. University of Michigan, United States; Stockholm University, Sweden.
    Shah, N. R.
    Shakya, B.
    NMSSM Higgs boson search strategies at the LHC and the mono-Higgs signature in particular2017Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 95, nr 11, artikel-id 115036Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study the collider phenomenology of the extended Higgs sector of the next-to-minimal supersymmetric Standard Model (NMSSM). The region of NMSSM parameter space favored by a 125 GeV SM-like Higgs and naturalness generically features a light Higgs and neutralino spectrum as well as a large O(1) coupling between the Higgs doublets and the NMSSM singlet fields. In such regimes, the heavier Higgs bosons can decay dominantly into lighter Higgs bosons and neutralinos. We study the prospects of observing such decays at the 13 TeV LHC, focusing on mono-Higgs signatures as probes of such regions of parameter space. We present results for the mono-Higgs reach in a framework easily applicable to other models featuring similar decay topologies. In the NMSSM, we find that the mono-Higgs channel can probe TeV scale Higgs bosons and has sensitivity even in the low tanβ, large mA regime that is difficult to probe in the MSSM. Unlike for many conventional Higgs searches, the reach of the mono-Higgs channel will improve significantly with the increased luminosity expected to be collected at the LHC in the ongoing and upcoming runs.

  • 69.
    Baum, Sebastian
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Dept Phys, Oskar Klein Ctr Cosmoparticle Phys, Alba Nova, S-10691 Stockholm, Sweden.;KTH Royal Inst Technol, NORDITA, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Shah, Nausheen R.
    Wayne State Univ, Dept Phys & Astron, Detroit, MI 48201 USA..
    Two Higgs doublets and a complex singlet: disentangling the decay topologies and associated phenomenology2018Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 12, artikel-id 044Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a systematic study of an extension of the Standard Model (SM) with two Higgs doublets and one complex singlet (2HDM+S). In order to gain analytical understanding of the parameter space, we re-parameterize the 27 parameters in the Lagrangian by quantities more closely related to physical observables: physical masses, mixing angles, trilinear and quadratic couplings, and vacuum expectation values. Embedding the 125 GeV SM-like Higgs boson observed at the LHC places stringent constraints on the parameter space. In particular, the mixing of the SM-like interaction state with the remaining states is severely constrained, requiring approximate alignment without decoupling in the region of parameter space where the additional Higgs bosons are light enough to be accessible at the LHC. In contrast to 2HDM models, large branching ratios of the heavy Higgs bosons into two lighter Higgs bosons or a light Higgs and a Z boson, so-called Higgs cascade decays, are ubiquitous in the 2HDM+S. Using currently available limits, future projections, and our own collider simulations, we show that combining different final states arising from Higgs cascades would allow to probe most of the interesting region of parameter space with Higgs boson masses up to 1 TeV at the LHC with L = 3000 fb(-1) of data.

  • 70.
    Baum, Sebastian
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Oskar Klein Ctr Cosmoparticle Phys, Dept Phys, Alba Nova, S-10691 Stockholm, Sweden; Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.
    Shah, Nausheen R.
    Wayne State Univ, Dept Phys & Astron, Detroit, MI 48201 USA..
    Freese, Katherine
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Oskar Klein Ctr Cosmoparticle Phys, Dept Phys, Alba Nova, S-10691 Stockholm, Sweden.; Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden; Univ Michigan, Leinweber Ctr Theoret Phys, Dept Phys, Ann Arbor, MI 48109 USA.
    The NMSSM is within reach of the LHC: mass correlations & decay signatures2019Ingår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 4, artikel-id 011Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Next-to-Minimal Supersymmetric Standard Model (NMSSM), the singlet extension of the MSSM which fixes many of the MSSM's shortcomings, is shown to be within reach of the upcoming runs of the Large Hadron Collider (LHC). A systematic treatment of the various Higgs decay channels and their interplay has been lacking due to the seemingly large number of free parameters in the NMSSM's Higgs sector. We demonstrate that due to the SM-like nature of the observed Higgs boson, the NMSSM's Higgs and neutralino sectors have highly correlated masses and couplings and can effectively be described by four physically intuitive parameters: the physical masses of the two CP-odd states and their mixing angle, and tan , which plays a minor role. The heavy Higgs bosons in the NMSSM have large branching ratios into pairs of lighter Higgs bosons or a light Higgs and a Z boson. Search channels arising via these Higgs cascades are unique to models like the NMSSM with a Higgs sector larger than that of the MSSM. In order to cover as much of the NMSSM parameter space as possible, one must combine conventional search strategies employing decays of the additional Higgs bosons into pairs of SM particles with Higgs cascade channels. We demonstrate that such a combination would allow a significant fraction of the viable NMSSM parameter space containing additional Higgs bosons with masses below 1 TeV to be probed at future runs of the LHC.

  • 71.
    Baum, Sebastian
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Visinelli, Luca
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Freese, Katherine
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Stengel, Patrick
    Dark matter capture, subdominant WIMPs, and neutrino observatories2017Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 95, nr 4, artikel-id 043007Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Weakly interacting massive particles (WIMPs), which are among the best motivated dark matter (DM) candidates, could make up all or only a fraction of the total DM budget. We consider a scenario in which WIMPs are a subdominant DM component; such a scenario would affect both current direct and indirect bounds on the WIMP-nucleon scattering cross section. In this paper we focus on indirect searches for the neutrino flux produced by annihilation of subdominant WIMPs captured by the Sun or the Earth via either spin-dependent or spin-independent scattering. We derive the annihilation rate and the expected neutrino flux at neutrino observatories. In our computation, we include an updated chemical composition of the Earth with respect to the previous literature, leading to an increase of the Earth's capture rate for spin-dependent scattering by a factor of 3. Results are compared with current bounds from Super-Kamiokande and IceCube. We discuss the scaling of bounds from both direct and indirect detection methods with the WIMP abundance.

  • 72. Baym, Gordon
    et al.
    Beck, D. H.
    Pethick, Christopher J.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Transport in ultradilute solutions of He-3 in superfluid He-42015Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, nr 2, artikel-id 024504Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We calculate the effect of a heat current on transporting He-3 dissolved in superfluid He-4 at ultralow concentration, as will be utilized in a proposed experimental search for the electric dipole moment of the neutron (nEDM). In this experiment, a phonon wind will be generated to drive (partly depolarized) 3He down a long pipe. In the regime of 3He concentrations less than or similar to 10(-9) and temperatures similar to 0.5 K, the phonons comprising the heat current are kept in a flowing local equilibrium by small angle phonon-phonon scattering, while they transfer momentum to the walls via the He-4 first viscosity. On the other hand, the phonon wind drives the 3He out of local equilibrium via phonon-He-3 scattering. For temperatures below 0.5 K, both the phonon and He-3 mean free paths can reach the centimeter scale, and we calculate the effects on the transport coefficients. We derive the relevant transport coefficients, the phonon thermal conductivity, and the He-3 diffusion constants from the Boltzmann equation. We calculate the effect of scattering from the walls of the pipe and show that it may be characterized by the average distance from points inside the pipe to the walls. The temporal evolution of the spatial distribution of the He-3 atoms is determined by the time dependent He-3 diffusion equation, which describes the competition between advection by the phonon wind and He-3 diffusion. As a consequence of the thermal diffusivity being small compared with the He-3 diffusivity, the scale height of the final He-3 distribution is much smaller than that of the temperature gradient. We present exact solutions of the time dependent temperature and He-3 distributions in terms of a complete set of normal modes.

  • 73. Baym, Gordon
    et al.
    Beck, D. H.
    Pethick, Christopher J.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Transport in very dilute solutions of He-3 in superfluid He-42013Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, nr 1, s. 014512-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Motivated by a proposed experimental search for the electric dipole moment of the neutron (nEDM) utilizing neutron-He-3 capture in a dilute solution of He-3 in superfluid He-4, we derive the transport properties of dilute solutions in the regime where the He-3 are classically distributed and rapid He-3-He-3 scatterings keep the He-3 in equilibrium. Our microscopic framework takes into account phonon-phonon, phonon-He-3, and He-3-He-3 scatterings. We then apply these calculations to measurements by Rosenbaum et al. [J. Low Temp. Phys. 16, 131 (1974)] and by Lamoreaux et al. [Europhys. Lett. 58, 718 (2002)] of dilute solutions in the presence of a heat flow. We find satisfactory agreement of theory with the data, serving to confirm our understanding of the microscopics of the helium in the future nEDM experiment.

  • 74. Baym, Gordon
    et al.
    Patil, Subodh P.
    Pethick, Christopher
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. University of Copenhagen, Denmark.
    Damping of gravitational waves by matter2017Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 96, nr 8, artikel-id 084033Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We develop a unified description, via the Boltzmann equation, of damping of gravitational waves by matter, incorporating collisions. We identify two physically distinct damping mechanisms-collisional and Landau damping. We first consider damping in flat spacetime, and then generalize the results to allow for cosmological expansion. In the first regime, maximal collisional damping of a gravitational wave, independent of the details of the collisions in the matter is, as we show, significant only when its wavelength is comparable to the size of the horizon. Thus damping by intergalactic or interstellar matter for all but primordial gravitational radiation can be neglected. Although collisions in matter lead to a shear viscosity, they also act to erase anisotropic stresses, thus suppressing the damping of gravitational waves. Damping of primordial gravitational waves remains possible. We generalize Weinberg's calculation of gravitational wave damping, now including collisions and particles of finite mass, and interpret the collisionless limit in terms of Landau damping. While Landau damping of gravitational waves cannot occur in flat spacetime, the expansion of the universe allows such damping by spreading the frequency of a gravitational wave of given wave vector.

  • 75. Baym, Gordon
    et al.
    Pethick, Christopher J.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Landau critical velocity in weakly interacting Bose gases2012Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 86, nr 2, s. 023602-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The flow of a uniform Bose gas at speeds greater than the Landau critical velocity nu(c) does not necessarily destroy superfluidity, but rather needs only to lead to a decrease of the superfluid mass density rho(s). Analyzing a weakly interacting Bose gas with a finite-range interparticle interaction that leads to a Landau critical velocity at nonzero quasiparticle momentum, we explicitly construct the (nonuniform) condensate for fluid flow faster than nu(c) and calculate the accompanying decrease in rho s. We briefly comment on the relation of the physics to other problems in superfluids, e.g., solitons and vortices in Bose-Einstein condensates, and critical currents in superconductors.

  • 76. Baym, Gordon
    et al.
    Pethick, Christopher J.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Normal mass density of a superfluid Fermi gas at unitarity2013Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 88, nr 4, s. 043631-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We calculate the normal mass density of a paired Fermi gas at unitarity. The dominant contribution near the superfluid transition is from fermionic quasiparticle excitations and is, thus, sensitive to the pairing gap. A comparison with the recent experiment of Sidorenkov et al. [Nature (London) 498, 78 (2013)] suggests that the superfluid gap near the transition temperature is larger than the BCS value, but the data do not permit a quantitative inference of the gap. Calculations of the quenched moment of inertia of a BCS superfluid in a harmonic trap are in reasonable agreement with the earlier experiment of Riedl et al. [New J. Phys. 13, 035003 (2011)].

  • 77.
    Behrens, C.
    et al.
    Scuola Normale Super Pisa, Piazza Cavalieri 7, I-56126 Pisa, Italy..
    Pallottini, A.
    Scuola Normale Super Pisa, Piazza Cavalieri 7, I-56126 Pisa, Italy.;Museo Stor Fis, Ctr Fermi, Piazza Viminale 1, I-00184 Rome, Italy.;Ctr Studi & Ric Enrico Fermi, Piazza Viminale 1, I-00184 Rome, Italy.;Univ Cambridge, Cavendish Lab, 19 JJ Thomson Ave, Cambridge CB3 0HE, England.;Univ Cambridge, Kavli Inst Cosmol, Madingley Rd, Cambridge CB3 0HA, England..
    Ferrara, A.
    Scuola Normale Super Pisa, Piazza Cavalieri 7, I-56126 Pisa, Italy.;Univ Tokyo, Kavli IPMU, 5-1-5 Kashiwanoha, Kashiwa, Chiba 2778583, Japan..
    Gallerani, S.
    Scuola Normale Super Pisa, Piazza Cavalieri 7, I-56126 Pisa, Italy..
    Vallini, L.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Ly alpha emission from galaxies in the Epoch of eionization2019Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 486, nr 2, s. 2197-2209Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The intrinsic strength of the Ly alpha line in young, star-forming systems makes it a special tool for studying high-redshift galaxies. However, interpreting observations remains challenging due to the complex radiative transfer involved. Here, we combine state-of-the-art hydrodynamical simulations of 'Althaea', a prototypical Lyman Break Galaxy (LBG; stellar mass M-star similar or equal to 10(10) M-circle dot) at z = 7.2, with detailed radiative transfer computations of dust/continuum, [C II] 158 mu m, and Ly alpha to clarify the relation between the galaxy properties and its Ly alpha emission. Althaea exhibits low (f(alpha) < 1 per cent) Ly alpha escape fractions and equivalent widths, EW less than or similar to 6 angstrom for the simulated lines of sight, with a large scatter. The correlation between escape fraction and inclination is weak, as a result of the rather chaotic structure of high-redshift galaxies. Low f(alpha) values persist even if we artificially remove neutral gas around star-forming regions to mimic the presence of H II regions. The high attenuation is primarily caused by dust clumps co-located with young stellar clusters. We can turn Althaea into a Lyman Alpha Emitter (LAE) only if we artificially remove dust from the clumps, yielding EWs up to 22 angstrom. Our study suggests that the LBG-LAE duty-cycle required by recent clustering measurements poses the challenging problem of a dynamically changing dust attenuation. Finally, we find an anticorrelation between the magnitude of Ly alpha-[C II] line velocity shift and Ly alpha luminosity.

  • 78.
    Behrens, C.
    et al.
    Scuola Normale Super Pisa, Piazza Cavalieri 7, I-56126 Pisa, Italy..
    Pallottini, A.
    Scuola Normale Super Pisa, Piazza Cavalieri 7, I-56126 Pisa, Italy.;Museo Stor Fis, Ctr Fermi, Piazza Viminale 1, I-00184 Rome, Italy.;Ctr Studi & Ric Enrico Fermi, Piazza Viminale 1, I-00184 Rome, Italy.;Univ Cambridge, Cavendish Lab, 19 JJ Thomson Ave, Cambridge CB3 OHE, England.;Univ Cambridge, Kavli Inst Cosmol, Madingley Rd, Cambridge CB3 0HA, England..
    Ferrara, A.
    Scuola Normale Super Pisa, Piazza Cavalieri 7, I-56126 Pisa, Italy.;Univ Tokyo, Kavli IPMU, 5-1-5 Kashiwanoha, Kashiwa, Chiba 2778583, Japan..
    Gallerani, S.
    Scuola Normale Super Pisa, Piazza Cavalieri 7, I-56126 Pisa, Italy..
    Vallini, Livia
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Dusty galaxies in the Epoch of Reionization: simulations2018Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 477, nr 1, s. 552-565Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The recent discovery of dusty galaxies well into the Epoch of Reionization (redshift z > 6) poses challenging questions about the properties of the interstellar medium in these pristine systems. By combining state-of-the-art hydrodynamic and dust radiative transfer simulations, we address these questions focusing on the recently discovered dusty galaxy A2744_YD4 (z = 8.38, Laporte et al.). We show that we can reproduce the observed spectral energy distribution (SED) only using different physical values with respect to the inferred ones by Laporte et al., i.e. a star formation rate of SFR = 78 M(circle dot)yr(-1), a factor approximate to 4 higher than deduced from simple SED fitting. In this case, we find: (i) dust attenuation (corresponding to tau(v) = 1.4) is consistent with a Milky Way (MW) extinction curve; (ii) the dust-to-metal ratio is low, f(d) similar to 0.08, implying that early dust formation is rather inefficient;(iii) the luminosity-weighted dust temperature is high, T-d = 91 23 K, as a result of the intense (approximate to 100 x MW) interstellar radiation field; and (iv) due to the high T-d, the Atacama Large Millimeter/submillimeter Array Band 7 detection can be explained by a limited dust mass, M-d = 1.6 x 10(6) M-circle dot. Finally, the high dust temperatures might solve the puzzling low infrared excess (IRX) recently deduced for high-z galaxies from the IRX-beta relation.

  • 79. Behringer, H.
    et al.
    Eichhorn, Ralf
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Brownian dynamics simulations with hard-body interactions: Spherical particles2012Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 137, nr 16, s. 164108-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A novel approach to account for hard-body interactions in (overdamped) Brownian dynamics simulations is proposed for systems with non-vanishing force fields. The scheme exploits the analytically known transition probability for a Brownian particle on a one-dimensional half-line. The motion of a Brownian particle is decomposed into a component that is affected by hard-body interactions and into components that are unaffected. The hard-body interactions are incorporated by replacing the affected component of motion by the evolution on a half-line. It is discussed under which circumstances this approach is justified. In particular, the algorithm is developed and formulated for systems with space-fixed obstacles and for systems comprising spherical particles. The validity and justification of the algorithm is investigated numerically by looking at exemplary model systems of soft matter, namely at colloids in flow fields and at protein interactions. Furthermore, a thorough discussion of properties of other heuristic algorithms is carried out.

  • 80. Behringer, Hans
    et al.
    Eichhorn, Ralf
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Wallin, Stefan
    Dynamics of biomolecular processes PREFACE2013Ingår i: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. 87, nr 5, s. 058501-Artikel i tidskrift (Övrigt vetenskapligt)
  • 81. Bellini, E.
    et al.
    Barreira, A.
    Frusciante, N.
    Hu, B.
    Peirone, S.
    Raveri, M.
    Zumalacarregui, Miguel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Avilez-Lopez, A.
    Ballardini, M.
    Battye, R. A.
    Bolliet, B.
    Calabrese, E.
    Dirian, Y.
    Ferreira, P. G.
    Finelli, F.
    Huang, Z.
    Ivanov, M. M.
    Lesgourgues, J.
    Li, B.
    Lima, N. A.
    Pace, F.
    Paoletti, D.
    Sawicki, I.
    Silvestri, A.
    Skordis, C.
    Umilta, C.
    Vernizzi, F.
    Comparison of Einstein-Boltzmann solvers for testing general relativity2018Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 97, nr 2, artikel-id 023520Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We compare Einstein-Boltzmann solvers that include modifications to general relativity and find that, for a wide range of models and parameters, they agree to a high level of precision. We look at three general purpose codes that primarily model general scalar-tensor theories, three codes that model Jordan-Brans-Dicke (JBD) gravity, a code that models f(R) gravity, a code that models covariant Galileons, a code that models Horava-Lifschitz gravity, and two codes that model nonlocal models of gravity. Comparing predictions of the angular power spectrum of the cosmic microwave background and the power spectrum of dark matter for a suite of different models, we find agreement at the subpercent level. This means that this suite of Einstein-Boltzmann solvers is now sufficiently accurate for precision constraints on cosmological and gravitational parameters.

  • 82.
    Belonoshko, Anatoly B.
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Kondenserade materiens teori.
    Lukinov, Timofiy
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Kondenserade materiens teori.
    Rosengren, Anders
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Kondenserade materiens teori. KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. AlbaNova University Center, Sweden.
    Bryk, Taras
    Litasov, Konstantin D.
    Synthesis of heavy hydrocarbons at the core-mantle boundary2015Ingår i: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 5, artikel-id 18382Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The synthesis of complex organic molecules with C-C bonds is possible under conditions of reduced activity of oxygen. We have found performing ab initio molecular dynamics simulations of the C-O-H- Fe system that such conditions exist at the core-mantle boundary (CMB). H2O and CO2 delivered to the CMB by subducting slabs provide a source for hydrogen and carbon. The mixture of H2O and CO2 subjected to high pressure (130 GPa) and temperature (4000 to 4500 K) does not lead to synthesis of complex hydrocarbons. However, when Fe is added to the system, C-C bonds emerge. It means that oil might be a more abundant mineral than previously thought.

  • 83.
    Beltran Jimenez, Jose
    et al.
    Univ Autonoma Madrid, Inst Fis Teor, UAM CSIC, E-28049 Madrid, Spain.;Univ Salamanca, Dept Fis Fundamental, E-37008 Salamanca, Spain..
    Heisenberg, Lavinia
    Swiss Fed Inst Technol, Inst Theoret Studies, Clausiusstr 47, CH-8092 Zurich, Switzerland..
    Koivisto, Tomi S.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Coincident general relativity2018Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 98, nr 4, artikel-id 044048Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The metric-affine variational principle is applied to generate teleparallel and symmetric teleparallel theories of gravity. From the latter we discover an exceptional class which is consistent with a vanishing affine connection. Based on this remarkable property, this work proposes a simpler geometrical formulation of general relativity that is oblivious to the affine spacetime structure, thus fundamentally depriving gravity of any inertial character. The resulting theory is described by the Hilbert action purged from the boundary term and is more robustly underpinned by the spin-2 field theory, where an extra symmetry is now manifest, possibly related to the double-copy structure of the gravity amplitudes. This construction also provides a novel starting point for modified gravity theories, and the paper presents new and simple generalizations where analytical self-accelerating cosmological solutions arise naturally in the early-and late-time Universe.

  • 84.
    Beltran Jimenez, Jose
    et al.
    Univ Autonoma Madrid, CSIC, Inst Fis Teor, E-28049 Madrid, Spain.;Univ Salamanca, Dept Fis Fundamental, Plaza Merced, E-37008 Salamanca, Spain..
    Heisenberg, Lavinia
    Swiss Fed Inst Technol, Inst Theoret Studies, Clausiusstr 47, CH-8092 Zurich, Switzerland..
    Koivisto, Tomi S.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Teleparallel Palatini theories2018Ingår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, nr 8, artikel-id 039Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Palatini formalism, which assumes the metric and the affine connection as independent variables, is developed for gravitational theories in flat geometries. We focus on two particularly interesting scenarios. First, we fix the connection to be metric compatible, as done in the usual teleparallel theories, but we follow a completely covariant approach by imposing the constraints with suitable Lagrange multipliers. For a general quadratic theory we show how torsion naturally propagates and we reproduce the Teleparallel Equivalent of General Relativity as a particular quadratic action that features an additional Lorentz symmetry. We then study the much less explored theories formulated in a geometry with neither curvature nor torsion, so that all the geometrical information is encoded in the non-metricity. We discuss how this geometrical framework leads to a purely inertial connection that can thus be completely removed by a coordinate gauge choice, the coincident gauge. From the quadratic theory we recover a simpler formulation of General Relativity in the form of the Einstein action, which enjoys an enhanced symmetry that reduces to a second linearised diffeomorphism at linear order. More general theories in both geometries can be formulated consistently by taking into account the inertial connection and the associated additional degrees of freedom. As immediate applications, the new cosmological equations and their Newtonian limit are considered, where the role of the lapse in the consistency of the equations is clarified, and the Schwarzschild black hole entropy is computed by evaluating the corresponding Euclidean action. We discuss how the boundary terms in the usual formulation of General Relativity are related to different choices of coordinates in its coincident version and show that in isotropic coordinates the Euclidean action is finite without the need to introduce boundary or normalisation terms. Finally, we discuss the double-copy structure of the gravity amplitudes and the bootstrapping of gravity within the framework of coincident General Relativity.

  • 85.
    Beltran Jimenez, Jose
    et al.
    Univ Salamanca, Dept Fis Fundamental, E-37008 Salamanca, Spain..
    Heisenberg, Lavinia
    Swiss Fed Inst Technol, Inst Theoret Studies, Clausiusstr 47, CH-8092 Zurich, Switzerland..
    Koivisto, Tomi S.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    The canonical frame of purified gravity2019Ingår i: International Journal of Modern Physics D, ISSN 0218-2718, Vol. 28, nr 14, artikel-id 1944012Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the recently introduced gauge theory of translations, dubbed Coincident General Relativity (CGR), gravity is described with neither torsion nor curvature in the spacetime affine geometry. The action of the theory enjoys an enhanced symmetry and avoids the second derivatives that appear in the conventional Einstein-Hilbert action. While it implies the equivalent classical dynamics, the improved action principle can make a difference in considerations of energetics, thermodynamics and quantum theory. This paper reports on possible progress in those three aspects of gravity theory. In the so-called purified gravity, (1) energy-momentum is described locally by a conserved, symmetric tensor, (2) the Euclidean path integral is convergent without the addition of boundary or regulating terms and (3) it is possible to identify a canonical frame for quantization.

  • 86.
    Beltran Jimenez, Jose
    et al.
    Univ Salamanca, Dept Fis Fundamental, E-37008 Salamanca, Spain.;Univ Salamanca, IUFFyM, E-37008 Salamanca, Spain..
    Heisenberg, Lavinia
    Swiss Fed Inst Technol, Inst Theoret Phys, Wolfgang Pauli Str 27, CH-8093 Zurich, Switzerland..
    Koivisto, Tomi S.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. KTH Royal Inst Technol, NORDITA, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Univ Tartu, Inst Phys, Lab Theoret Phys, W Ostwaldi 1, EE-50411 Tartu, Estonia..
    The Geometrical Trinity of Gravity2019Ingår i: UNIVERSE, ISSN 2218-1997, Vol. 5, nr 7, artikel-id 173Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The geometrical nature of gravity emerges from the universality dictated by the equivalence principle. In the usual formulation of General Relativity, the geometrisation of the gravitational interaction is performed in terms of the spacetime curvature, which is now the standard interpretation of gravity. However, this is not the only possibility. In these notes, we discuss two alternative, though equivalent, formulations of General Relativity in flat spacetimes, in which gravity is fully ascribed either to torsion or to non-metricity, thus putting forward the existence of three seemingly unrelated representations of the same underlying theory. Based on these three alternative formulations of General Relativity, we then discuss some extensions.

  • 87. Beniwal, A.
    et al.
    Rajec, F.
    Savage, Christopher
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Scott, P.
    Weniger, C.
    White, M.
    Williams, A. G.
    Combined analysis of effective Higgs portal dark matter models2016Ingår i: Proceedings of Science, Proceedings of Science (PoS) , 2016, Vol. Part F128556Konferensbidrag (Refereegranskat)
    Abstract [en]

    We perform a combined and extended analysis of effective scalar, vector, Majorana and Dirac fermion Higgs portal models of dark matter (DM). In these models, DM couples to the Standard Model (SM) Higgs boson via an operator of the form ODMH†H. For the fermion models, we allow an admixture of the scalar ΨΨ and pseudoscalar Ψiγ5Ψ interaction terms. On each model, we impose constraints from the Planck measured DM relic density, LHC limits on the Higgs invisible branching ratio, indirect search limits from the WMAP7/Planck observations of the cosmic microwave background, a combined analysis of 15 dwarf spheroidal galaxies by Fermi-LAT and the upcoming Cherenkov Telescope Array (CTA) as well as the direct search limits from the LUX and projected XENON1T experiments. We find in line with previous studies that the direct search experiments will continue to exclude large portions of the model parameter space. Due to the momentum suppression of the direct search cross section in the case of a pure pseudoscalar interaction of a fermionic DM candidate, future indirect searches are the only class of experiments capable of probing the high mass range of the theory.

  • 88. Beniwal, Ankit
    et al.
    Rajec, Filip
    Savage, Christopher
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Scott, Pat
    Weniger, Christoph
    White, Martin
    Williams, Anthony G.
    Combined analysis of effective Higgs portal dark matter models2016Ingår i: Physical Review D, ISSN 2470-0010, Vol. 93, nr 11, artikel-id 115016Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We combine and extend the analyses of effective scalar, vector, Majorana and Dirac fermion Higgs portal models of dark matter (DM), in which DM couples to the Standard Model (SM) Higgs boson via an operator of the form (ODMHH)-H-dagger. For the fermion models, we take an admixture of scalar.. and pseudoscalar (psi) over bari gamma(5)psi interaction terms. For each model, we apply constraints on the parameter space based on the Planck measured DM relic density and the LHC limits on the Higgs invisible branching ratio. For the first time, we perform a consistent study of the indirect detection prospects for these models based on the WMAP7/Planck observations of the cosmic microwave background, a combined analysis of 15 dwarf spheroidal galaxies by Fermi-LAT and the upcoming Cherenkov Telescope Array (CTA). We also perform a correct treatment of the momentum-dependent direct search cross section that arises from the pseudoscalar interaction term in the fermionic DM theories. We find, in line with previous studies, that current and future direct search experiments such as LUX and XENON1T can exclude much of the parameter space, and we demonstrate that a joint observation in both indirect and direct searches is possible for high mass weakly interacting massive particles. In the case of a pure pseudoscalar interaction of a fermionic DM candidate, future gamma-ray searches are the only class of experiment capable of probing the high mass range of the theory.

  • 89.
    Beresnyak, Andrey
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    On the Parallel Spectrum and Anisotropy in MHD Turbulence2015Ingår i: NUMERICAL MODELING OF SPACE PLASMA FLOWS: ASTRONUM-2014 / [ed] Pogorelov, NV Audit, E Zank, GP, ASTRONOMICAL SOC PACIFIC , 2015, Vol. 498, s. 3-9Konferensbidrag (Refereegranskat)
    Abstract [en]

    MHD turbulence is anisotropic and shows different spectral slopes measured parallel and perpendicular to the local magnetic field. Solar wind observations typically show a k(-2) power law scaling, which is much steeper than the perpendicular scaling, typically k(-5/3). In this paper I show that this scaling could be interpreted as the Lagrangian frequency spectrum omega(-2) of the evolving Afven perturbations. As Alfven waves propagate exactly along magnetic field lines, the measurement of the spectrum along the field line is equivalent to the measurement of the frequency spectrum. This was tested using high-resolution simulations of MHD turbulence, e.g., I verified that the cutoff of the parallel spectrum scales as Kolmogorov timescale, not lengthscale.

  • 90.
    Beresnyak, Andrey
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Los Alamos National Laboratory, United States .
    On the parallel spectrum in magnetohydrodynamic turbulence2015Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 801, nr 1, artikel-id L9Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Anisotropy of MHD turbulence has been studied extensively for many years, most prominently by measurements in the solar wind and high-resolution simulations. The spectrum parallel to the local magnetic field was observed to be steeper than the perpendicular spectrum, typically k(-2), consistent with the widely accepted Goldreich & Sridhar model. In this Letter, I looked deeper into the nature of the relation between parallel and perpendicular spectra and argue that this k(-2) scaling has the same origin as the omega(-2) scaling of the Lagrangian frequency spectrum in strong hydrodynamic turbulence. This follows from the fact that Alfven waves propagate along magnetic field lines. It has now became clear that the observed anisotropy can be argued without invocation of the "critical balance" argument and is more robust that was previously thought. The relation between parallel (Lagrangian) and perpendicular (Eulerian) spectra is an inevitable consequence of strong turbulence of Alfven waves, rather than a conjecture based on the uncertainty relation. I tested this using high-resolution simulations of MHD turbulence, in particular, I verified that the cutoff of the parallel spectrum scales as a Kolmogorov timescale, not lengthscale.

  • 91.
    Beresnyak, Andrey
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Los Alamos National Laboratory, United States.
    THE FLOW AROUND A COSMIC STRING. I. HYDRODYNAMIC SOLUTION2015Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 804, nr 2, artikel-id 121Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Cosmic strings are linear topological defects which are hypothesized to be produced during inflation. Most searches for strings have relied on the string's lensing of background galaxies or the cosmic microwave background. In this paper, I obtained a solution for the supersonic flow of collisional gas past the cosmic string which has two planar shocks with a shock compression ratio that depends on the angle defect of the string and its speed. The shocks result in the compression and heating of the gas and, given favorable conditions, particle acceleration. Gas heating and over-density in an unusual wedge shape can be detected by observing the HI line at high redshifts. Particle acceleration can occur in the present-day universe when the string crosses the hot gas contained in galaxy clusters and, since the consequences of such a collision persist for cosmological timescales, could be located by looking at unusual large-scale radio sources situated on a single spatial plane.

  • 92.
    Beresnyak, Andrey
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Miniati, Francesco
    TURBULENT AMPLIFICATION AND STRUCTURE OF THE INTRACLUSTER MAGNETIC FIELD2016Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 817, nr 2, artikel-id 127Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We compare DNS calculations of homogeneous isotropic turbulence with the statistical properties of intracluster turbulence from the Matryoshka Run and find remarkable similarities between their inertial ranges. This allowed us to use the time-dependent statistical properties of intracluster turbulence to evaluate dynamo action in the intracluster medium, based on earlier results from a numerically resolved nonlinear magneto- hydrodynamic turbulent dynamo. We argue that this approach is necessary (a) to properly normalize dynamo action to the available intracluster turbulent energy and (b) to overcome the limitations of low Re affecting current numerical models of the intracluster medium. We find that while the properties of intracluster magnetic field are largely insensitive to the value and origin of the seed field, the resulting values for the Alfven speed and the outer scale of the magnetic field are consistent with current observational estimates, basically confirming the idea that the magnetic field in today's galaxy clusters is a record of its past turbulent activity.

  • 93. Bern, Z.
    et al.
    Carrasco, J. J.
    Chen, W. -M
    Johansson, Henrik
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Uppsala University, Sweden.
    Roiban, R.
    Gravity Amplitudes as Generalized Double Copies of Gauge-Theory Amplitudes2017Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 118, nr 18, artikel-id 181602Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Whenever the integrand of a gauge-theory loop amplitude can be arranged into a form where the Bern-Carrasco-Johansson duality between color and kinematics is manifest, a corresponding gravity integrand can be obtained simply via the double-copy procedure. However, finding such gauge-theory representations can be challenging, especially at high loop orders. Here, we show that we can, instead, start from generic gauge-theory integrands, where the duality is not manifest, and apply a modified double-copy procedure to obtain gravity integrands that include contact terms generated by violations of dual Jacobi identities. We illustrate this with three-, four- and five-loop examples in N=8 supergravity.

  • 94.
    Bern, Zvi
    et al.
    Univ Calif Los Angeles, Mani L Bhaumik Inst Theoret Phys, Dept Phys & Astron, Los Angeles, CA 90095 USA..
    Carrasco, John Joseph
    Univ Paris Saclay, CEA Saclay, Inst Theoret Phys, F-91191 Gif Sur Yvette, France.;CNRS, F-91191 Gif Sur Yvette, France..
    Chen, Wei-Ming
    Univ Calif Los Angeles, Mani L Bhaumik Inst Theoret Phys, Dept Phys & Astron, Los Angeles, CA 90095 USA..
    Edison, Alex
    Univ Calif Los Angeles, Mani L Bhaumik Inst Theoret Phys, Dept Phys & Astron, Los Angeles, CA 90095 USA..
    Johansson, Karl H.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Uppsala Univ, Dept Phys & Astron, S-75108 Uppsala, Sweden..
    Parra-Martinez, Julio
    Univ Calif Los Angeles, Mani L Bhaumik Inst Theoret Phys, Dept Phys & Astron, Los Angeles, CA 90095 USA..
    Roiban, Radu
    Penn State Univ, Inst Gravitat & Cosmos, University Pk, PA 16802 USA..
    Zeng, Mao
    Univ Calif Los Angeles, Mani L Bhaumik Inst Theoret Phys, Dept Phys & Astron, Los Angeles, CA 90095 USA..
    Ultraviolet properties of N=8 supergravity at five loops2018Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 98, nr 8, artikel-id 086021Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We use the recently developed generalized double-copy construction to obtain an improved representation of the five-loop four-point integrand of N = 8 supergravity whose leading ultraviolet behavior we analyze using state-of-the-art loop-integral expansion and reduction methods. We find that the five-loop critical dimension where ultraviolet divergences first occur is D-c = 24/5, corresponding to a (DR4)-R-8 counterterm. This ultraviolet behavior stands in contrast to the cases of four-dimensional N = 4 supergravity at three loops and N = 5 supergravity at four loops whose improved ultraviolet behavior demonstrates enhanced cancellations beyond implications from standard symmetry considerations. We express this D-c = 24/5 divergence in terms of two relatively simple positive-definite integrals with vanishing external momenta, excluding any additional ultraviolet cancellations at this loop order. We note nontrivial relations between the integrals describing this leading ultraviolet behavior and integrals describing lower-loop behavior. This observation suggests not only a path towards greatly simplifying future calculations at higher loops, but may even allow us to directly investigate ultraviolet behavior in terms of simplified integrals, avoiding the construction of complete integrands.

  • 95. Bern, Zvi
    et al.
    Carrasco, John Joseph
    Chen, Wei-Ming
    Johansson, Henrik
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Roiban, Radu
    Zeng, Mao
    Five-loop four-point integrand of N=8 supergravity as a generalized double copy2017Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 96, nr 12, artikel-id 126012Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We use the recently developed generalized double-copy procedure to construct an integrand for the fiveloop four-point amplitude of N = 8 supergravity. This construction starts from a naive double copy of the previously computed corresponding amplitude of N = 4 super-Yang-Mills theory. This is then systematically modified by adding contact terms generated in the context of the method of maximal unitarity cuts. For the simpler generalized cuts, whose corresponding contact terms tend to be the most complicated, we derive a set of formulas relating the contact contributions to the violations of the dual Jacobi identities in the relevant gauge-theory amplitudes. For more complex generalized unitarity cuts, which tend to have simpler contact terms associated with them, we use the method of maximal cuts more directly. The five-loop fourpoint integrand is a crucial ingredient towards future studies of ultraviolet properties of N = 8 supergravity at five loops and beyond. We also present a nontrivial check of the consistency of the integrand, based on modern approaches for integrating over the loop momenta in the ultraviolet region.

  • 96. Bern, Zvi
    et al.
    Davies, Scott
    Di Vecchia, Paolo
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. University of Copenhagen, Denmark .
    Nohle, Josh
    Low-energy behavior of gluons and gravitons from gauge invariance2014Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 90, nr 8, s. 084035-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We show that at tree level, on-shell gauge invariance can be used to fully determine the first subleading soft-gluon behavior and the first two subleading soft-graviton behaviors. Our proofs of the behaviors for n-gluon and n-graviton tree amplitudes are valid in D dimensions and are similar to Low's proof of universality of the first subleading behavior of photons. In contrast to photons coupling to massive particles, in four dimensions the soft behaviors of gluons and gravitons are corrected by loop effects. We comment on how such corrections arise from this perspective. We also show that loop corrections in graviton amplitudes arising from scalar loops appear only at the second soft subleading order. This case is particularly transparent because it is not entangled with graviton infrared singularities. Our result suggests that if we set aside the issue of infrared singularities, soft-graviton Ward identities of extended Bondi, van der Burg, Metzner and Sachs symmetry are not anomalous through the first subleading order.

  • 97. Bern, Zvi
    et al.
    Davies, Scott
    Nohle, Josh
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Double-copy constructions and unitarity cuts2016Ingår i: PHYSICAL REVIEW D, ISSN 2470-0010, Vol. 93, nr 10, artikel-id 105015Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The duality between color and kinematics enables the construction of multiloop gravity integrands directly from corresponding gauge-theory integrands. This has led to new nontrivial insights into the structure of gravity theories, including the discovery of enhanced ultraviolet cancellations. To continue to gain deeper understandings and probe these new properties, it is crucial to further improve techniques for constructing multiloop gravity integrands. In this paper, we show by example how one can alleviate difficulties encountered at the multiloop level by relaxing the color-kinematics duality conditions to hold manifestly only on unitarity cuts instead of globally on loop integrands. As an example, we use a minimal Ansatz to construct an integrand for the two-loop four-point nonsupersymmetric pure Yang-Mills amplitude in D dimensions that is compatible with these relaxed color-kinematics duality constraints. We then immediately obtain a corresponding gravity integrand through the double-copy procedure. Comments on ultraviolet divergences are also included.

  • 98. Bernard, Laura
    et al.
    Deffayet, Cedric
    Hinterbichler, Kurt
    von Strauss, Mikael
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. UPMC, France.
    Partially massless graviton on beyond Einstein spacetimes2017Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 95, nr 12, artikel-id 124036Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We show that a partially massless graviton can propagate on a large set of spacetimes which are not Einstein spacetimes. Starting from a recently constructed theory for a massive graviton that propagates the correct number of degrees of freedom on an arbitrary spacetime, we first give the full explicit form of the scalar constraint responsible for the absence of a sixth degree of freedom. We then spell out generic conditions for the constraint to be identically satisfied, so that there is a scalar gauge symmetry which makes the graviton partially massless. These simplify if one assumes that spacetime is Ricci symmetric. Under this assumption, we find explicit non-Einstein spacetimes (some, but not all, with vanishing Bach tensors) allowing for the propagation of a partially massless graviton. These include in particular the Einstein static Universe.

  • 99.
    Bettoni, Dario
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Sweden.
    Ezquiaga, Jose Maria
    Hinterbichler, Kurt
    Zumalacarregui, Miguel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Sweden; Univ Calif Berkeley, USA.
    Speed of gravitational waves and the fate of scalar-tensor gravity2017Ingår i: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 95, nr 8, artikel-id 084029Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The direct detection of gravitational waves (GWs) is an invaluable new tool to probe gravity and the nature of cosmic acceleration. A large class of scalar-tensor theories predicts that GWs propagate with velocity different than the speed of light, a difference that can be O(1) for many models of dark energy. We determine the conditions behind the anomalous GW speed, namely, that the scalar field spontaneously breaks Lorentz invariance and couples to the metric perturbations via the Weyl tensor. If these conditions are realized in nature, the delay between GW and electromagnetic signals from distant events will run beyond human time scales, making it impossible to measure the speed of GWs using neutron star mergers or other violent events. We present a robust strategy to exclude or confirm an anomalous speed of GWs using eclipsing binary systems, the electromagnetic phase of which can be exquisitely determined. The white dwarf binary J0651 + 2844 is a known example of such a system that can be used to probe deviations in the GW speed as small as cg/ c - 1 greater than or similar to 2 x 10(-12) when LISA comes online. This test will either eliminate many contender models for cosmic acceleration or wreck a fundamental pillar of general relativity.

  • 100.
    Bhat, P.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Hydraulic effects in a radiative atmosphere with ionization2016Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 587, artikel-id A90Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. In his 1978 paper, Eugene Parker postulated the need for hydraulic downward motion to explain magnetic flux concentrations at the solar surface. A similar process has also recently been seen in simplified (e.g., isothermal) models of flux concentrations from the negative effective magnetic pressure instability (NEMPI). Aims. We study the effects of partial ionization near the radiative surface on the formation of these magnetic flux concentrations. Methods. We first obtain one-dimensional (1D) equilibrium solutions using either a Kramers-like opacity or the H-opacity. The resulting atmospheres are then used as initial conditions in two-dimensional (2D) models where flows are driven by an imposed gradient force that resembles a localized negative pressure in the form of a blob. To isolate the effects of partial ionization and radiation, we ignore turbulence and convection. Results. Because of partial ionization, an unstable stratification always forms near the surface. We show that the extrema in the specific entropy profiles correspond to the extrema in the degree of ionization. In the 2D models without partial ionization, strong flux concentrations form just above the height where the blob is placed. Interestingly, in models with partial ionization, such flux concentrations always form at the surface well above the blob. This is due to the corresponding negative gradient in specific entropy. Owing to the absence of turbulence, the downflows reach transonic speeds. Conclusions. We demonstrate that, together with density stratification, the imposed source of negative pressure drives the formation of flux concentrations. We find that the inclusion of partial ionization affects the entropy profile dramatically, causing strong flux concentrations to form closer to the surface. We speculate that turbulence effects are needed to limit the strength of flux concentrations and homogenize the specific entropy to a stratification that is close to marginal.

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