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  • 1.
    Bergström, Johannes
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Ohlsson, Tommy
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Constraining new physics with a positive or negative signal of neutrino-less double beta decay2011Inngår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, Vol. 2011, nr 05, s. 122-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigate numerically how accurately one could constrain the strengths of different short-range contributions to neutrino-less double beta decay in effective field theory. Depending on the outcome of near-future experiments yielding information on the neutrino masses, the corresponding bounds or estimates can be stronger or weaker. A particularly interesting case, resulting in strong bounds, would be a positive signal of neutrino-less double beta decay that is consistent with complementary information from neutrino oscillation experiments, kinematical determinations of the neutrino mass, and measurements of the sum of light neutrino masses from cosmological observations. The keys to more robust bounds are improvements of the knowledge of the nuclear physics involved and a better experimental accuracy.

  • 2.
    Bonnevier, Johan
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Melbéus, Henrik
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Ohlsson, Tommy
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Monoenergetic gamma rays from nonminimal Kaluza-Klein dark matter annihilations2012Inngår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 85, nr 4, s. 043524-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigate monoenergetic gamma-ray signatures from annihilations of dark matter comprised of Z(1), the first Kaluza-Klein (KK) excitation of the Z boson in a nonminimal universal extra dimensions (UED) model. The self interactions of the non-Abelian Z(1) gauge boson give rise to a large number of contributing Feynman diagrams that do not exist for annihilations of the Abelian gauge boson B-1, which is the standard Kaluza-Klein dark matter (KKDM) candidate. We find that the annihilation rate is indeed considerably larger for the Z(1) than for the B-1. Even though relic density calculations indicate that the mass of the Z(1) should be larger than the mass of the B-1, the predicted monoenergetic gamma fluxes are of the same order of magnitude. We compare our results to existing experimental limits, as well as to future sensitivities, for image air Cherenkov telescopes, and we find that the limits are reached already with a moderately large boost factor. The realistic prospects for detection depend on the experimental energy resolution.

  • 3.
    Bouchand, Romain
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik.
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik.
    Running of radiative neutrino masses: the scotogenic model2012Inngår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 7, s. 084-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the renormalization group equations of Ma's scotogenic model, which generates an active neutrino mass at 1-loop level. In addition to other benefits, the main advantage of the mechanism exploited in this model is to lead to a natural loop-suppression of the neutrino mass, and therefore to an explanation for its smallness. However, since the structure of the neutrino mass matrix is altered compared to the ordinary type I seesaw case, the corresponding running is altered as well. We have derived the full set of renormalization group equations for the scotogenic model which, to our knowledge, had not been presented previously in the literature. This set of equations reflects some interesting structural properties of the model, and it is an illustrative example for how the running of neutrino parameters in radiative models is modified compared to models with tree-level mass generation. We also study a simplified numerical example to illustrate some general tendencies of the running. Interestingly, the structure of the RGEs can be exploited such that a bimaximal leptonic mixing pattern at the high-energy scale is translated into a valid mixing pattern at low energies, featuring a large value of theta(13). This suggests very interesting connections to flavour symmetries.

  • 4.
    de Woul, Jonas
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik.
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik.
    Ohlsson, Tommy
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Establishing analogies between the physics of extra dimensions and carbon nanotubes2012Inngår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 714, nr 1, s. 44-47Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We point out a conceptual analogy between the physics of extra spatial dimensions and the physics of carbon nanotubes which arises for principle reasons, although the corresponding energy scales are at least ten orders of magnitude apart. For low energies, one can apply the Kaluza-Klein description to both types of systems, leading to two completely different but consistent interpretations of the underlying physics. In particular, we discuss in detail the Kaluza-Klein description of armchair and zig-zag carbon nanotubes. Furthermore, we describe how certain experimental results for carbon nanotubes could be re-interpreted in terms of the Kaluza-Klein description. Finally, we present ideas for new measurements that could allow to probe concepts of models with extra spatial dimensions in table-top experiments, providing further links between condensed matter and particle physics.

  • 5.
    Di Bari, Pasquale
    et al.
    University of Southampton.
    King, Stephen
    University of Southampton.
    Luhn, Christoph
    University of Southampton.
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Schmidt-May, Angnis
    Max-Planck-Institut fuer Kernphysik.
    Radiative Inflation and Dark Energy2011Inngår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 84, nr 8Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We propose a model based on radiative symmetry breaking that combines inflation with dark energy and is consistent with the Wilkinson Microwave Anisotropy Probe 7-year regions. The radiative inflationary potential leads to the prediction of a spectral index 0.955 less than or similar to n(S) less than or similar to 0.967 and a tensor to scalar ratio 0.142 less than or similar to r less than or similar to 0.186, both consistent with current data but testable by the Planck experiment. The radiative symmetry breaking close to the Planck scale gives rise to a pseudo Nambu-Goldstone boson with a gravitationally suppressed mass which can naturally play the role of a quintessence field responsible for dark energy. Finally, we present a possible extra dimensional scenario in which our model could be realized.

  • 6. Duerr, Michael
    et al.
    Lindner, Manfred
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    On the quantitative impact of the Schechter-Valle theorem2011Inngår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 6, s. 091-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We evaluate the Schechter-Valle (Black Box) theorem quantitatively by considering the most general Lorentz invariant Lagrangian consisting of point-like operators for neutrinoless double beta decay. It is well known that the Black Box operators induce Majorana neutrino masses at four-loop level. This warrants the statement that an observation of neutrinoless double beta decay guarantees the Majorana nature of neutrinos. We calculate these radiatively generated masses and find that they are many orders of magnitude smaller than the observed neutrino masses and splittings. Thus, some lepton number violating New Physics (which may at tree-level not be related to neutrino masses) may induce Black Box operators which can explain an observed rate of neutrinoless double beta decay. Although these operators guarantee finite Majorana neutrino masses, the smallness of the Black Box contributions implies that other neutrino mass terms (Dirac or Majorana) must exist. If neutrino masses have a significant Majorana contribution then this will become the dominant part of the Black Box operator. However, neutrinos might also be predominantly Dirac particles, while other lepton number violating New Physics dominates neutrinoless double beta decay. Translating an observed rate of neutrinoless double beta decay into neutrino masses would then be completely misleading. Although the principal statement of the Schechter-Valle theorem remains valid, we conclude that the Black Box diagram itself generates radiatively only mass terms which are many orders of magnitude too small to explain neutrino masses. Therefore, other operators must give the leading contributions to neutrino masses, which could be of Dirac or Majorana nature.

  • 7. King, Stephen F.
    et al.
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik.
    Warm Dark Matter from keVins2012Inngår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, nr 8, s. 016-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We propose a simple model for Warm Dark Matte (WDM) in which two femions are added to the Standard,Model: (quasi-) stable "keVins" (keV inert fermions) which account for WDM and their unstable brothers, the "GeVins" (GeV inert fermions), both of which carry zero electric charge and zero lepton number, and are (approximately) "inert", in the sense that their only interactions are via suppressed couplings to the Z. We consider scenarios in which stable keVins are thermally produced and their abundance is subsequently diluted by entropy production from the decays of the heavier unstable GeVins. This mechanism could be implemented in a wide variety of models, including E-6 inspired supersymmetric models or models involving sterile neutrinos.

  • 8. Lindner, M.
    et al.
    Merle, Alexander
    Max-Planck-Institut für Kernphysik, Heidelberg, Germany.
    Niro, V.
    Enhancing dark matter annihilation into neutrinos2010Inngår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 82, nr 12, s. 123529-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We perform a detailed and quasi model-independent analysis of direct annihilation of dark matter into neutrinos. Considering different cases for scalar and fermionic dark matter, we identify several settings in which this annihilation is enhanced, contrary to some statements in the literature. The key point is that several restrictions of, e.g., a supersymmetric framework do not hold in general. The mass generation mechanism of the neutrinos plays an important role, too. We illustrate our considerations by two examples that are not (as usually) suppressed by the smallness of the neutrino mass, for which we also present a numerical analysis. Our results can be easily used as guidelines for model building.

  • 9. Lindner, Manfred
    et al.
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Niro, Viviana
    Soft L-e - L-mu - L-tau flavour symmetry breaking and sterile neutrino keV Dark Matter2011Inngår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, Vol. 2011, nr 01, s. 034-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We discuss how a L-e - L-mu - L-tau flavour symmetry that is softly broken leads to keV sterile neutrinos, which are a prime candidate for Warm Dark Matter. This is to our knowledge the first model where flavour symmetries are applied simultaneously to active and sterile neutrinos explaining at the same time active neutrino properties and this peculiar Dark Matter scenario. The essential point is that different scales of the symmetry breaking and the symmetry preserving entries in the mass matrix lead to one right- handed neutrino which is nearly massless compared to the other two. Furthermore, we naturally predict vanishing theta(13) and maximal theta(23), while the correct value of theta(12) must come from the mixing of the charged leptons. We can furthermore predict an exact mass spectrum for the light neutrinos, which will be testable in the very near future.

  • 10.
    Melbéus, Henrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Ohlsson, Tommy
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Continuum photon spectrum from Z1Z1 annihilations in universal extra dimensions2012Inngår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 706, nr 4-5, s. 329-332Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We calculate the continuum photon spectrum from the pair annihilation of a Z1 LKP in non-minimal universal extra dimensions. We find that, due to the preferred annihilation into W+ W- pairs, the continuum flux of collinear photons is relatively small compared to the standard case of the B1 as the LKP. This conclusion applies in particular to the spectral endpoint, where also the additional fermionic contributions are not large enough to increase the flux significantly. When searching for the line signal originating from Z1 Z1 annihilations, this is actually a perfect situation, since the continuum signal can be regarded as background to the smoking gun signature of a peak in the photon flux at an energy that is nearly equal to the mass of the dark matter particle. This signal, in combination with (probably) a non-observation of the continuum signal at lower photon energies, constitutes a perfect handle to probe the hypothesis of the Z1 LKP being the dominant component of the dark matter observed in the Universe.

  • 11.
    Melbéus, Henrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik. Royal Inst Technol KTH, Dept Theoret Phys, Sch Engn Sci, S-10691 Stockholm, Sweden..
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik. Royal Inst Technol KTH, Dept Theoret Phys, Sch Engn Sci, S-10691 Stockholm, Sweden..
    Ohlsson, Tommy
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik. Royal Inst Technol KTH, Dept Theoret Phys, Sch Engn Sci, S-10691 Stockholm, Sweden..
    Erratum to: Continuum photon spectrum from Z(1)Z(1) annihilations in universal extra dimensions2012Inngår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 713, nr 3, s. 350-350Artikkel i tidsskrift (Fagfellevurdert)
  • 12.
    Melbéus, Henrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Ohlsson, Tommy
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Higgs dark matter in UEDs: A good WIMP with bad detection prospects2012Inngår i: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 715, nr 1-3, s. 164-169Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We study the first Kaluza-Klein excitation of the Higgs boson in universal extra dimensions as a dark matter candidate. The first-level Higgs boson could be the lightest Kaluza-Klein particle, which is stable due to the conservation of Kaluza-Klein parity, in non-minimal models where boundary localized terms modify the mass spectrum. We calculate the relic abundance and find that it agrees with the observed dark matter density if the mass of the first-level Higgs boson is slightly above 2 TeV, not considering coannihilations and assuming no relative mass splitting among the first-level Kaluza-Klein modes. In the case of coannihilations and a non-zero mass splitting, the mass of the first-level Higgs boson can range from 1 TeV to 4 TeV. We study also the prospects for detection of this dark matter candidate in direct as well as indirect detection experiments. Although the first-level Higgs boson is a typical weakly interacting massive particle, an observation in any of the conventional experiments is very challenging.

  • 13.
    Melbéus, Henrik
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Ohlsson, Tommy
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Higgs Kaluza–Klein dark matter in universal extra dimensions2008Manuskript (preprint) (Annet vitenskapelig)
  • 14.
    Merle, Alexander
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    keV sterile Neutrino Dark Matter and Neutrino Model Building2012Inngår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 375, nr Part 2, s. 012047-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A sterile neutrino with a mass around the keV scale could be an interesting candidate for warm dark matter. Although there are several scenarios and production mechanisms known in which such a particle could yield the correct abundance, there are astonishingly few models around that can actually yield an explanation for the appearance of a keV-like scale. We here review three main classes of such mass models for keV sterile neutrino dark matter, based on split seesaw, on L-e - L-mu - L-tau symmetry, and on the Froggatt-Nielsen mechanism, respectively.

  • 15.
    Merle, Alexander
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik, Teoretisk partikelfysik.
    Niro, Viviana
    University of Turino.
    Deriving models for keV sterile neutrino Dark Matter with the Froggatt-Nielsen mechanism2011Inngår i: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, Vol. 2011, nr 7, s. 023-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Sterile neutrinos with a mass around the keV scale are an attractive particle physics candidate for Warm Dark Matter. Although many frameworks have been presented in which these neutrinos can fulfill all phenomenological constraints, there are hardly any models known that can explain such a peculiar mass pattern, one sterile neutrino at the keV scale and the other two considerably heavier, while at the same time being compatible with low-energy neutrino data. In this paper, we present models based on the Froggatt-Nielsen mechanism, which can give such an explanation. We explain how to assign Froggatt-Nielsen charges in a successful way, and we give a detailed discussion of all conditions to be fulfilled. It turns out that the typical arbitrariness of the charge assignments is greatly reduced when trying to carefully account for all constraints. We furthermore present analytical calculations of a few simplified models, while quasi-perfect models are found numerically.

  • 16.
    Merle, Alexander
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Teoretisk fysik.
    Zwicky, Roman
    Explicit and spontaneous breaking of SU(3) into its finite subgroups2012Inngår i: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, nr 2, s. 128-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We investigate the breaking of SU(3) into its subgroups from the viewpoints of explicit and spontaneous breaking. A one-to-one link between these two approaches is given by the complex spherical harmonics, which form a complete set of SU(3)-representation functions. An invariant of degrees p and q in complex conjugate variables corresponds to a singlet, or vacuum expectation value, in a (p; q)-representation of SU(3). We review the formalism of the Molien function, which contains information on primary and secondary invariants. Generalizations of the Molien function to the tensor generating functions are discussed. The latter allows all branching rules to be deduced. We have computed all primary and secondary invariants for all proper finite subgroups of order smaller than 512, for the entire series of groups Delta(3n(2)), Delta(6n(2)), and for all crystallographic groups. Examples of sufficient conditions for breaking into a subgroup are worked out for the entire Tn[a]-, Delta(3n(2))-, Delta(6n(2))-series and for all crystallographic groups Sigma(X). The corresponding invariants provide an alternative definition of these groups. A Mathematica package, SUtree, is provided which allows the extraction of the invariants, Molien and generating functions, syzygies, VEVs, branching rules, character tables, matrix (p; q)(SU(3))-representations, Kronecker products, etc. for the groups discussed above.

1 - 16 of 16
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