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KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of event shapes at large momentum transfer with the ATLAS detector in pp collisions at root s=7 TeV2012In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, no 11, p. 2211-Article in journal (Refereed)

A measurement of event shape variables is presented for large momentum transfer proton-proton collisions using the ATLAS detector at the Large Hadron Collider. Six event shape variables calculated using hadronic jets are studied in inclusive multi-jet events in 35 pb(-1) of integrated luminosity at a center-of-mass energy of root s = 7 TeV. These measurements are compared to predictions by three Monte Carlo event generators containing leading-logarithmic parton showers matched to leading order matrix elements for 2 -> 2 and 2 -> n (n = 2, ... , 6) scattering. Measurements of the third-jet resolution parameter, aplanarity, thrust, sphericity, and transverse sphericity are generally well described. The mean value of each event shape variable is evaluated as a function of the average momentum of the two leading jets p(T,1) and p(T,2), with a mean p(T) approaching 1 TeV.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of hard double-parton interactions in W(-> lv) plus 2-jet events at root s=7 TeV with the ATLAS detector2013In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 15, p. 033038-Article in journal (Refereed)

The production of W bosons in association with two jets in proton-proton collisions at a centre-of-mass energy of root s = 7 TeV has been analysed for the presence of double-parton interactions using data corresponding to an integrated luminosity of 36 pb(-1), collected with the ATLAS detector at the Large Hadron Collider. The fraction of events arising from double-parton interactions, f(DP)((D)), has been measured through the p(T) balance between the two jets and amounts to f(DP)((D)) = 0.08 +/- 0.01 (stat.) +/- 0.02 (sys.) for jets with transverse momentum p(T) > 20 GeV and rapidity vertical bar y vertical bar < 2.8. This corresponds to a measurement of the effective area parameter for hard double-parton interactions of sigma(eff) = 15 +/- 3 (stat.)(-3)(+5) (sys.) mb.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of isolated-photon pair production in pp collisions at root s=7 TeV with the ATLAS detector2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 1, p. 086-Article in journal (Refereed)

The ATLAS experiment at the LHC has measured the production cross section of events with two isolated photons in the final state, in proton-proton collisions at root s = 7 TeV. The full data set collected in 2011, corresponding to an integrated luminosity of 4.9 fb(-1), is used. The amount of background, from hadronic jets and isolated electrons, is estimated with data-driven techniques and subtracted. The total cross section, for two isolated photons with transverse energies above 25 GeV and 22 GeV respectively, in the acceptance of the electromagnetic calorimeter (vertical bar eta vertical bar < 1.37 and 1.52 < vertical bar eta vertical bar 2.37) and with an angular separation Delta R > 0.4, is 44.0(-4.2)(+3.2) pb. The differential cross sections as a function of the di-photon invariant mass, transverse momentum, azimuthal separation, and cosine of the polar angle of the largest transverse energy photon in the Collins-Soper di-photon rest frame are also measured. The results are compared to the prediction of leading-order parton-shower and next-to-leading-order and next-to-next-to-leading-order parton-level generators.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of jet shapes in top-quark pair events at root s=7 TeV using the ATLAS detector2013In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 73, no 12, p. 1-31Article in journal (Refereed)

A measurement of jet shapes in top-quark pair events using 1.8 fb(-1) of pp collision data recorded by the ATLAS detector at the LHC is presented. Samples of top-quark pair events are selected in both the single-lepton and dilepton final states. The differential and integrated shapes of the jets initiated by bottom-quarks from the top-quark decays are compared with those of the jets originated by light-quarks from the hadronic W-boson decays in the single-lepton channel. The light-quark jets are found to have a narrower distribution of the momentum flow inside the jet area than b-quark jets.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of k(T) splitting scales in W -> l nu events at root s=7 TeV with the ATLAS detector2013In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 73, no 5, p. 2432-Article in journal (Refereed)

A measurement of splitting scales, as defined by the kT clustering algorithm, is presented for final states containing a W boson produced in proton-proton collisions at a centre-of-mass energy of 7 TeV. The measurement is based on the full 2010 data sample corresponding to an integrated luminosity of 36 pb(-1) which was collected using the ATLAS detector at the CERN Large Hadron Collider. Cluster splitting scales are measured in events containing W bosons decaying to electrons or muons. The measurement comprises the four hardest splitting scales in a k(T) cluster sequence of the hadronic activity accompanying the W boson, and ratios of these splitting scales. Backgrounds such as multi-jet and top-quark-pair production are subtracted and the results are corrected for detector effects. Predictions from various Monte Carlo event generators at particle level are compared to the data. Overall, reasonable agreement is found with all generators, but larger deviations between the predictions and the data are evident in the soft regions of the splitting scales.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of tau polarization in W -> tau nu decays with the ATLAS detector in pp collisions at root s=7 TeV2012In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, no 7, p. 2062-Article in journal (Refereed)

In this paper, a measurement of tau polarization in W -> tau nu decays is presented. It is measured from the energies of the decay products in hadronic tau decays with a single final state charged particle. The data, corresponding to an integrated luminosity of 24 pb(-1), were collected by the ATLAS experiment at the Large Hadron Collider in 2010. The measured value of the tau polarization is P-tau = -1.06 +/- 0.04 (stat)(-0.07)(+0.05) (syst), in agreement with the Standard Model prediction, and is consistent with a physically allowed 95 % CL interval [-1,-0.91]. Measurements of tau polarization have not previously been made at hadron colliders.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the azimuthal anisotropy for charged particle production in root s(NN)=2.76 TeV lead-lead collisions with the ATLAS detector2012In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 86, no 1, p. 014907-Article in journal (Refereed)

Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at root sNN = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 mu b(-1). This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle relative to the reaction plane, with the coefficients v(n) denoting the magnitude of the anisotropy. Significant v(2)-v(6) values are obtained as a function of transverse momentum (0.5 < p(T) < 20 GeV), pseudorapidity (|eta| < 2.5), and centrality using an event plane method. The v(n) values for n >= 3 are found to vary weakly with both eta and centrality, and their p(T) dependencies are found to follow an approximate scaling relation, v(n)(1/n)(p(T)) proportional to v(2)(1/2)(p(T)), except in the top 5% most central collisions. A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Delta phi = phi(a)-phi(b)) is performed to extract the coefficients v(n,n) = < cos n Delta phi >. For pairs of charged particles with a large pseudorapidity gap (|Delta eta = eta(a) - eta(b)| > 2) and one particle with p(T) < 3 GeV, the v(2,2)-v(6,6) values are found to factorize as v(n,n)(p(T)(a), p(T)(b)) approximate to v(n) (p(T)(a))v(n)(p(T)(b)) in central and midcentral events. Such factorization suggests that these values of v(2,2)-v(6,6) are primarily attributable to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the v(1,1)(p(T)(a), p(T)(b)) data are consistent with the combined contributions from a rapidity-even v(1) and global momentum conservation. A two-component fit is used to extract the v(1) contribution. The extracted v(1) isobserved to cross zero at pT approximate to 1.0 GeV, reaches a maximum at 4-5 GeV with a value comparable to that for v(3), and decreases at higher p(T).

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the b-hadron production cross section using decays to D*(+)mu X- final states in pp collisions at root s=7 TeV with the ATLAS detector2012In: Nuclear Physics B, ISSN 0550-3213, E-ISSN 1873-1562, Vol. 864, no 3, p. 341-381Article in journal (Refereed)

The b-hadron production cross section is measured with the ATLAS detector in pp collisions at root s = 7 TeV, using 3.3 pb(-1) of integrated luminosity, collected during the 2010 LHC run. The b-hadrons are selected by partially reconstructing D*(+)mu X- final states. Differential cross sections are measured as functions of the transverse momentum and pseudorapidity. The measured production cross section for a b-hadron with p(T) > 9 GeV and vertical bar eta vertical bar < 2.5 is 32.7 +/- 0.8(stat.)(-6.8)(+4.5)(syst.) mu b, higher than the next-to-leading-order QCD predictions but consistent within the experimental and theoretical uncertainties.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the cross-section for W boson production in association with b-jets in pp collisions at root s=7 TeV with the ATLAS detector2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 6, p. 084-Article in journal (Refereed)

This paper reports a measurement of the W+b-jets (W+b+X and W+b (b) over bar +X) production cross-section in proton-proton collisions at a centre-of-mass energy of 7 TeV at the LHC. These results are based on data corresponding to an integrated luminosity of 4.6 fb(-1), collected with the ATLAS detector. Cross-sections are presented as a function of jet multiplicity and of the transverse momentum of the leading b-jet for both the muon and electron decay modes of the W boson. The W+b-jets cross-section, corrected for all known detector effects, is quoted in a limited kinematic range. Combining the muon and electron channels, the fiducial cross-section for W+b-jets is measured to be 7.1 +/- 0.5 (stat) +/- 1.4 (syst) pb, consistent with the next-to-leading order QCD prediction, corrected for non-perturbative and double-parton interactions (DPI) contributions, of 4.70 +/- 0.09 (stat) (+0.60)(-0.49) (scale) +/- 0.06 (PDF) +/- 0.16 (non-pert) (+0.52)(-0.38) (DPI) pb.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the distributions of event-by-event flow harmonics in lead-lead collisions at root s(NN)=2.76 TeV with the ATLAS detector at the LHC2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 11, p. 83-Article in journal (Refereed)

The distributions of event-by-event harmonic flow coefficients v (n) for n = 2- 4 are measured in = 2.76 TeV Pb + Pb collisions using the ATLAS detector at the LHC. The measurements are performed using charged particles with transverse momentum p (T) > 0.5 GeV and in the pseudorapidity range |eta| < 2.5 in a dataset of approximately 7 mu b(-1) recorded in 2010. The shapes of the v (n) distributions suggest that the associated flow vectors are described by a two-dimensional Gaussian function in central collisions for v (2) and over most of the measured centrality range for v (3) and v (4). Significant deviations from this function are observed for v (2) in mid-central and peripheral collisions, and a small deviation is observed for v (3) in mid-central collisions. In order to be sensitive to these deviations, it is shown that the commonly used multi-particle cumulants, involving four particles or more, need to be measured with a precision better than a few percent. The v (n) distributions are also measured independently for charged particles with 0.5 < p (T) < 1 GeV and p (T) > 1 GeV. When these distributions are rescaled to the same mean values, the adjusted shapes are found to be nearly the same for these two p (T) ranges. The v (n) distributions are compared with the eccentricity distributions from two models for the initial collision geometry: a Glauber model and a model that includes corrections to the initial geometry due to gluon saturation effects. Both models fail to describe the experimental data consistently over most of the measured centrality range.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the flavour composition of dijet events in pp collisions at root s=7 TeV with the ATLAS detector2013In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 73, no 2, p. 2301-Article in journal (Refereed)

This paper describes a measurement of the flavour composition of dijet events produced in pp collisions at root s = 7 TeV using the ATLAS detector. The measurement uses the full 2010 data sample, corresponding to an integrated luminosity of 39 pb(-1). Six possible combinations of light, charm and bottom jets are identified in the dijet events, where the jet flavour is defined by the presence of bottom, charm or solely light flavour hadrons in the jet. Kinematic variables, based on the properties of displaced decay vertices and optimised for jet flavour identification, are used in a multidimensional template fit to measure the fractions of these dijet flavour states as functions of the leading jet transverse momentum in the range 40 GeV to 500 GeV and jet rapidity vertical bar y vertical bar < 2.1. The fit results agree with the predictions of leading-and next-to-leading-order calculations, with the exception of the dijet fraction composed of bottom and light flavour jets, which is underestimated by all models at large transverse jet momenta. The ability to identify jets containing two b-hadrons, originating from e. g. gluon splitting, is demonstrated. The difference between bottom jet production rates in leading and subleading jets is consistent with the next-to-leading-order predictions.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the high-mass Drell-Yan differential cross-section in pp collisions at root s=7 TeV with the ATLAS detector2013In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 725, no 4-5, p. 223-242Article in journal (Refereed)

This Letter reports a measurement of the high-mass Drell-Yan differential cross-section in proton-proton collisions at a centre-of-mass energy of 7 TeV at the LHC. Based on an integrated luminosity of 4.9 fb(-1), the differential cross-section in the Z/gamma* -> e(+)e(-) channel is measured with the ATLAS detector as a function of the invariant mass, m(ee), in the range 116 < m(ee) < 1500 GeV, for a fiducial region in which both the electron and the positron have transverse momentum p(T) > 25 GeV and pseudorapidity vertical bar n vertical bar < 2.5. A comparison is made to various event generators and to the predictions of perturbative QCD calculations at next-to-next-to-leading order.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the inclusive isolated prompt photons cross section in pp collisions at root s=7 TeV with the ATLAS detector using 4.6 fb(-1)2014In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 89, no 5, p. 052004-Article in journal (Refereed)

A measurement of the cross section for the production of isolated prompt photons in pp collisions at a center-of-mass energy root s = 7 TeV is presented. The results are based on an integrated luminosity of 4.6 fb(-1) collected with the ATLAS detector at the LHC. The cross section is measured as a function of photon pseudorapidity eta(gamma) and transverse energy E-T(gamma) in the kinematic range 100 <= E-T(gamma) < 1000 GeV and in the regions vertical bar eta(gamma)vertical bar < 1.37 and 1.52 <= vertical bar eta(gamma)vertical bar < 2.37. The results are compared to leading-order parton-shower Monte Carlo models and next-to-leading-order perturbative QCD calculations. Next-to-leading-order perturbative QCD calculations agree well with the measured cross sections as a function of E-T(gamma) and eta(gamma) .

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the inclusive jet cross-section in pp collisions at root s=2.76 TeV and comparison to the inclusive jet cross-section at root s=7 TeV using the ATLAS detector2013In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 73, no 8, p. 2509-Article in journal (Refereed)

The inclusive jet cross-section has been measured in proton-proton collisions at root s = 2.76 TeV in a dataset corresponding to an integrated luminosity of 0.20 pb(-1) collected with the ATLAS detector at the Large Hadron Collider in 2011. Jets are identified using the anti-k(t) algorithm with two radius parameters of 0.4 and 0.6. The inclusive jet double-differential cross-section is presented as a function of the jet transverse momentum p(T) and jet rapidity y, covering a range of 20 <= p(T) < 430 GeV and vertical bar y vertical bar < 4.4. The ratio of the cross-section to the inclusive jet cross-section measurement at root s = 7 TeV, published by the ATLAS Collaboration, is calculated as a function of both transverse momentum and the dimensionless quantity x(T) = 2p(T)/root s, in bins of jet rapidity. The systematic uncertainties on the ratios are significantly reduced due to the cancellation of correlated uncertainties in the two measurements. Results are compared to the prediction from next-to-leading order perturbative QCD calculations corrected for non-perturbative effects, and next-to-leading order Monte Carlo simulation. Furthermore, the ATLAS jet cross-section measurements at root s = 2.76 TeV and root s = 7 TeV are analysed within a framework of next-to-leading order perturbative QCD calculations to determine parton distribution functions of the proton, taking into account the correlations between the measurements.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the jet radius and transverse momentum dependence of inclusive jet suppression in lead-lead collisions at root S-NN=2.76 TeV with the ATLAS detector2013In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 719, no 4-5, p. 220-241Article in journal (Refereed)

Measurements of inclusive jet suppression in heavy ion collisions at the LHC provide direct sensitivity to the physics of jet quenching. In a sample of lead-lead collisions at root S-NN = 2.76 TeV corresponding to an integrated luminosity of approximately 7 mu b(-1), ATLAS has measured jets with a calorimeter system over the pseudorapidity interval vertical bar eta vertical bar < 2.1 and over the transverse momentum range 38 < pT <210 GeV. Jets were reconstructed using the anti-k(t) algorithm with values for the distance parameter that determines the nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of the jet yield is characterized by the jet "central-to-peripheral ratio," R-CP. Jet production is found to be suppressed by approximately a factor of two in the 10% most central collisions relative to peripheral collisions. R-CP varies smoothly with centrality as characterized by the number of participating nucleons. The observed suppression is only weakly dependent on jet radius and transverse momentum. These results provide the first direct measurement of inclusive jet suppression in heavy ion collisions and complement previous measurements of dijet transverse energy imbalance at the LHC.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the Lambda(0)(b) lifetime and mass in the ATLAS experiment2013In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 87, no 3, p. 032002-Article in journal (Refereed)

A measurement of the Lambda(0)(b) lifetime and mass in the decay channel Lambda(0)(b) -> J/psi (mu(+) mu(-))Lambda(0)(p pi(-)) is presented. The analysis uses a signal sample of about 2200 Lambda(0)(b) and (Lambda) over bar (0)(b) decays that are reconstructed in 4.9 fb(-1) of ATLAS pp collision data collected in 2011 at the LHC center-of-mass energy of 7 TeV. A simultaneous mass and decay time maximum likelihood fit is used to extract the Lambda(0)(b) lifetime and mass. They are measured to be tau(Lambda b) = 1.449 +/- 0.036(stat) +/- 0.017(syst) ps and m(Lambda b) = 5619.7 +/- 0.7(stat) +/- 1.1(syst) MeV.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at root s=7 TeV with the ATLAS experiment2012In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, no 5, p. 2001-Article in journal (Refereed)

This paper describes an analysis of the angular distribution of W -> e nu and W -> mu nu decays, using data from pp collisions at root s = 7 TeV recorded with the ATLAS detector at the LHC in 2010, corresponding to an integrated luminosity of about 35 pb(-1). Using the decay lepton transverse momentum and the missing transverse momentum, the W decay angular distribution projected onto the transverse plane is obtained and analysed in terms of helicity fractions f(0), f(L) and f(R) over two ranges of W transverse momentum (p(T)(W)) : 35 < p(T)(W) < 50 GeV and p(T)(W) > 50 GeV. Good agreement is found with theoretical predictions. For p(T)(W) > 50 GeV, the values of f(0) and f(L) - f(R), averaged over charge and lepton flavour, are measured to be: f(0) = 0.127 +/- 0.030 +/- 0.108 and f(L) - f(R) = 0.252 +/- 0.017 +/- 0.030, where the first uncertainties are statistical, and the second include all systematic effects.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the production cross section of an isolated photon associated with jets in proton-proton collisions at root s=7 TeV with the ATLAS detector2012In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 85, no 9, p. 092014-Article in journal (Refereed)

A measurement of the cross section for the production of an isolated photon in association with jets in proton-proton collisions at a center-of-mass energy root s = 7 TeV is presented. Photons are reconstructed in the pseudorapidity range vertical bar eta(gamma)vertical bar < 1.37 and with a transverse energy E-T(gamma) > 25 GeV. Jets are reconstructed in the rapidity range vertical bar y(jet)vertical bar < 4.4 and with a transverse momentum p(T)(jet) > 20 GeV. The differential cross section d sigma/dE(T)(gamma) is measured, as a function of the photon transverse energy, for three different rapidity ranges of the leading-p(T) jet: vertical bar y(jet)vertical bar < 1.2, 1.2 <= vertical bar y(jet)vertical bar < 2.8 and 2.8 <= vertical bar y(jet)vertical bar < 4.4. For each rapidity configuration the same-sign (eta(gamma)y(jet) >= 0) and opposite-sign (eta(gamma) y(jet) < 0) cases are studied separately. The results are based on an integrated luminosity of 37 pb(-1), collected with the ATLAS detector at the LHC. Next-to-leading order perturbative QCD calculations are found to be in fair agreement with the data, except for E-T(gamma) <= 45 GeV, where the theoretical predictions overestimate the measured cross sections.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the production cross section of jets in association with a Z boson in pp collisions at √s=7 TeV with the ATLAS detector2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 7, p. 032-Article in journal (Refereed)

Measurements of the production of jets of particles in association with a Z boson in pp collisions at √s=7 TeV are presented, using data corresponding to an integrated luminosity of 4.6 fb-1 collected by the ATLAS experiment at the Large Hadron Collider. Inclusive and differential jet cross sections in Z events, with Z decaying into electron or muon pairs, are measured for jets with transverse momentum p T &gt; 30 GeV and rapidity |y| &lt; 4.4. The results are compared to next-to-leading-order perturbative QCD calculations, and to predictions from different Monte Carlo generators based on leading-order and next-to-leading-order matrix elements supplemented by parton showers.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the t-channel single top-quark production cross section in pp collisions at root s=7 TeV with the ATLAS detector2012In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 717, no 4-5, p. 330-350Article in journal (Refereed)

We report a measurement of the cross section of single top-quark production in the t-channel using 1.04 fb(-1) of pp collision data at root s = 7 TeV recorded with the ATLAS detector at the LHC. Selected events feature one electron or muon, missing transverse momentum, and two or three jets, exactly one of them identified as originating from a b quark. The cross section is measured by fitting the distribution of a multivariate discriminant constructed with a neural network, yielding sigma(t) = 83 +/- 4 (stat.)(-19)(+20) (syst.) pb, which is in good agreement with the prediction of the Standard Model. Using the ratio of the measured to the theoretically predicted cross section and assuming that the top-quark-related CKM matrix elements obey the relation vertical bar V-tb vertical bar >> vertical bar V-ts vertical bar, vertical bar V-td vertical bar, the coupling strength at the W-t-b vertex is determined to be vertical bar V-tb vertical bar = 1.13(-0.13)(+0.14). If it is assumed that vertical bar V-tb vertical bar <= 1 a lower limit of vertical bar V-tb vertical bar > 0.75 is obtained at the 95% confidence level.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the top quark charge in pp collisions at root s=7 TeV with the ATLAS detector2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 11, p. 031-Article in journal (Refereed)

A measurement of the top quark electric charge is carried out in the ATLAS experiment at the Large Hadron Collider using 2.05 fb(-1) of data at a centre-of-mass energy of 7TeV. In units of the elementary electric charge, the top quark charge is determined to be 0.64 +/- 0.02 (stat.) +/- 0.08 (syst.) from the charges of the top quark decay products in single lepton t (t) over bar candidate events. This excludes models that propose a heavy quark of electric charge -4/3, instead of the Standard Model top quark, with a significance of more than 8 sigma.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the top quark pair cross section with ATLAS in pp collisions at root s=7 TeV using final states with an electron or a muon and a hadronically decaying tau lepton2012In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 717, no 1-3, p. 89-108Article in journal (Refereed)

A measurement of the cross section of top quark pair production in proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb(-1). Events with an isolated electron or muon and a tau lePton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, sigma(t (t) over bar) = 186 +/- 13 (stat.) +/- 20 (syst.) +/- 7 (lumi.) pb, is in good agreement with the Standard Model prediction.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the t(t)over-bar production cross section in the tau plus jets channel using the ATLAS detector2013In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 73, no 3, p. 2328-Article in journal (Refereed)

A measurement of the top quark pair production cross section in the final state with a hadronically decaying tau lepton and jets is presented. The analysis is based on proton-proton collision data recorded by the ATLAS experiment at the LHC, with a centre-of-mass energy of 7 TeV. The data sample corresponds to an integrated luminosity of 1.67 fb(-1). The cross section is measured to be sigma(t (t) over bar) = 194 +/- 18 (stat.) +/- 46 (syst.) pb and is in agreement with other measurements and with the Standard Model prediction.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of the W boson polarization in top quark decays with the ATLAS detector2012In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 6, p. 088-Article in journal (Refereed)

This paper presents measurements of the polarization of W bosons in top quark decays, derived from t (t) over bar events with missing transverse momentum, one charged lepton and at least four jets, or two charged leptons and at least two jets. Data from pp collisions at a centre-of-mass energy of 7 TeV were collected with the ATLAS experiment at the LHC and correspond to an integrated luminosity of 1.04 fb(-1). The measured fractions of longitudinally, left-and right-handed polarization are F-0 = 0.67 +/- 0.07, F-L = 0.32 +/- 0.04 and F-R = 0.01 +/- 0.05, in agreement with the Standard Model predictions. As the polarization of the W bosons in top quark decays is sensitive to the Wtb vertex Lorentz structure and couplings, the measurements were used to set limits on anomalous contributions to the Wtb couplings.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of t(t)over-bar production with a veto on additional central jet activity in pp collisions at root s=7 TeV using the ATLAS detector2012In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, no 6, p. 2043-Article in journal (Refereed)

A measurement of the jet activity in t (t) over bar events produced in proton-proton collisions at a centre-of-mass energy of 7 TeV is presented, using 2.05 fb(-1) of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. The t (t) over bar events are selected in the dilepton decay channel with two identified b-jets from the top quark decays. Events are vetoed if they contain an additional jet with transverse momentum above a threshold in a central rapidity interval. The fraction of events surviving the jet veto is presented as a function of this threshold for four different central rapidity interval definitions. An alternate measurement is also performed, in which events are vetoed if the scalar transverse momentum sum of the additional jets in each rapidity interval is above a threshold. In both measurements, the data are corrected for detector effects and compared to the theoretical models implemented in MC@NLO, POWHEG, ALPGEN and SHERPA. The experimental uncertainties are often smaller than the spread of theoretical predictions, allowing deviations between data and theory to be observed in some regions of phase space.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of upsilon production in 7 TeV pp collisions at ATLAS2013In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 87, no 5, p. 052004-Article in journal (Refereed)

Using 1.8 fb(-1) of pp collisions at a center- of- mass energy of 7 TeV recorded by the ATLAS detector at the Large Hadron Collider, we present measurements of the production cross sections of Upsilon(1S,2S,3S) mesons. Upsilon mesons are reconstructed using the dimuon decay mode. Total production cross sections for p(T) < 70 GeV and in the rapidity interval vertical bar y(Upsilon)vertical bar < 2. 25 are measured to be, 8.01 +/- 0.02 +/- 0.36 +/- 0.31 nb, 2.05 +/- 0.01 +/- 0.12 +/- 0.08 nb, and 0.92 +/- 0.01 +/- 0.07 +/- 0.04 nb, respectively, with uncertainties separated into statistical, systematic, and luminosity measurement effects. In addition, differential cross section times dimuon branching fractions for Upsilon(1S), Upsilon(2S), and Upsilon(3S) as a function of Upsilon transverse momentum pT and rapidity are presented. These cross sections are obtained assuming unpolarized production. If the production polarization is fully transverse or longitudinal with no azimuthal dependence in the helicity frame, the cross section may vary by approximately +/- 20%. If a nontrivial azimuthal dependence is considered, integrated cross sections may be significantly enhanced by a factor of 2 or more. We compare our results to several theoretical models of Upsilon meson production, finding that none provide an accurate description of our data over the full range of Upsilon transverse momenta accessible with this data set.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of W gamma and Z gamma production cross sections in pp collisions at root s=7 TeV and limits on anomalous triple gauge couplings with the ATLAS detector2012In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 717, no 1-3, p. 49-69Article in journal (Refereed)

This Letter presents measurements of l(+/-) nu gamma and l(+)l(-)gamma (l = e.mu) production in 1.02 fb(-1) of pp collision data recorded at root s = 7 TeV with the ATLAS detector at the LHC in the first half of 2011. Events dominated by W gamma and Z gamma production with leptonic decays of the W and Z bosons are selected, and their production cross sections and kinematic properties are measured in several ranges of the photon transverse energy. The results are compared to Standard Model predictions and are used to determine limits on anomalous WW gamma and ZZ gamma/Z gamma gamma couplings.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of W(+/-)Z production in proton-proton collisions at root s=7 TeV with the ATLAS detector2012In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, no 10, p. 2173-Article in journal (Refereed)

A study of W(+/-)Z production in proton-proton collisions at root s = 7 TeV is presented using data corresponding to an integrated luminosity of 4.6 fb(-1) collected with the ATLAS detector at the Large Hadron Collider in 2011. In total, 317 candidates, with a background expectation of 68 +/- 10 events, are observed in double-leptonic decay final states with electrons, muons and missing transverse momentum. The total cross-section is determined to be sigma(tot)(WZ) = 19.0(-1.3)(+1.4)(stat.) +/- 0.9(syst.) +/- 0.4(lumi.) pb, consistent with the Standard Model expectation of 17.6(-1.0)(+1.1) pb. Limits on anomalous triple gauge boson couplings are derived using the transverse momentum spectrum of Z bosons in the selected events. The cross-section is also presented as a function of Z boson transverse momentum and diboson invariant mass.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of Z Boson Production in Pb-Pb Collisions at root s(NN)=2.76 TeV with the ATLAS Detector2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, no 2, p. 022301-Article in journal (Refereed)

The ATLAS experiment has observed 1995 Z boson candidates in data corresponding to 0.15 nb(-1) of integrated luminosity obtained in the 2011 LHC Pb + Pb run at root s(NN) = 2.76 TeV. The Z bosons are reconstructed via dielectron and dimuon decay channels, with a background contamination of less than 3%. Results from the two channels are consistent and are combined. Within the statistical and systematic uncertainties, the per-event Z boson yield is proportional to the number of binary collisions estimated by the Glauber model. The elliptic anisotropy of the azimuthal distribution of the Z boson with respect to the event plane is found to be consistent with zero.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement of ZZ production in pp collisions at root s=7 TeV and limits on anomalous ZZZ and ZZ gamma couplings with the ATLAS detector2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 3, p. 128-Article in journal (Refereed)

A measurement of the ZZ production cross section in proton-proton collisions at root s = 7 TeV using data recorded by the ATLAS experiment at the Large Hadron Collider is presented. In a data sample corresponding to an integrated luminosity of 4.6 fb(-1) collected in 2011, events are selected that are consistent either with two Z bosons decaying to electrons or muons or with one Z boson decaying to electrons or muons and a second Z boson decaying to neutrinos. The ZZ((*)) -> l(+)l(-)l'(+)l'(-) and ZZ -> l(+)l(-) nu(nu) over bar cross sections are measured in restricted phase-space regions. These results are then used to derive the total cross section for ZZ events produced with both Z bosons in the mass range 66 to 116 GeV, sigma(tot)(ZZ) = 6.7 +/- 0.7 (stat.) (+0.4)(-0.3) (syst.) +/- 0.3 (lumi.) pb, which is consistent with the Standard Model prediction of 5.89(-0.18)(+0.22) pb calculated at next-to-leading order in QCD. The normalized differential cross sections in bins of various kinematic variables are presented. Finally, the differential event yield as a function of the transverse momentum of the leading Z boson is used to set limits on anomalous neutral triple gauge boson couplings in ZZ production.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurement with the ATLAS detector of multi-particle azimuthal correlations in p plus Pb collisions at root s(NN)=5.02 TeV2013In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 725, no 1-3, p. 60-78Article in journal (Refereed)

In order to study further the long-range correlations ("ridge") observed recently in p + Pb collisions at root s(NN) = 5.02 TeV, the second-order azimuthal anisotropy parameter of charged particles, v(2), has been measured with the cumulant method using the ATLAS detector at the LHC. In a data sample corresponding to an integrated luminosity of approximately 1 mu b(-1), the parameter v(2) has been obtained using two- and four-particle cumulants over the pseudorapidity range vertical bar eta vertical bar < 2.5. The results are presented as a function of transverse momentum and the event activity, defined in terms of the transverse energy summed over 3.1 < eta < 4.9 in the direction of the Pb beam. They show features characteristic of collective anisotropic flow, similar to that observed in Pb + Pb collisions. A comparison is made to results obtained using two-particle correlation methods, and to predictions from hydrodynamic models of p + Pb collisions. Despite the small transverse spatial extent of the p + Pb collision system, the large magnitude of v(2) and its similarity to hydrodynamic predictions provide additional evidence for the importance of final-state effects in p + Pb reactions.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurements of the pseudorapidity dependence of the total transverse energy in proton-proton collisions at root s=7 TeV with ATLAS2012In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 11, p. 033-Article in journal (Refereed)

This paper describes measurements of the sum of the transverse energy of particles as a function of particle pseudorapidity, eta, in proton-proton collisions at a centre-of-mass energy, root s = 7 TeV using the ATLAS detector at the Large Hadron Collider. The measurements are performed in the region \eta\ < 4.8 for two event classes: those requiring the presence of particles with a low transverse momentum and those requiring particles with a significant transverse momentum. In the second dataset measurements are made in the region transverse to the hard scatter. The distributions are compared to the predictions of various Monte Carlo event generators, which generally tend to underestimate the amount of transverse energy at high \eta\.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Measurements of top quark pair relative differential cross-sections with ATLAS in pp collisions at root s=7 TeV2013In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 73, no 1, p. 2261-Article in journal (Refereed)

Measurements are presented of differential cross-sections for top quark pair production in pp collisions at root s = 7 TeV relative to the total inclusive top quark pair production cross-section. A data sample of 2.05 fb(-1) recorded by the ATLAS detector at the Large Hadron Collider is used. Relative differential cross-sections are derived as a function of the invariant mass, the transverse momentum and the rapidity of the top quark pair system. Events are selected in the lepton (electron or muon) + jets channel. The background-subtracted differential distributions are corrected for detector effects, normalized to the total inclusive top quark pair production cross-section and compared to theoretical predictions. The measurement uncertainties range typically between 10 % and 20 % and are generally dominated by systematic effects. No significant deviations from the Standard Model expectations are observed.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC2012In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 716, no 1, p. 1-29Article in journal (Refereed)

A search for the Standard Model Higgs boson in proton-proton collisions with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities of approximately 4.8 fb(-1) collected at, root s = 7 TeV in 2011 and 5.8 fb(-1) at root s = 8 TeV in 2012. Individual searches in the channels H -> ZZ(()*()) -> 4l, H -> gamma gamma and H -> WW (()*()) -> ev mu v in the 8 TeV data are combined with previously published results of searches for H -> ZZ(()*()), WW(*()), b (b) over bar and tau(+)tau(-) in the 7 TeV data and results from improved analyses of the H -> ZZ(()*()) -> 4l and H -> gamma gamma channels in the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of 126.0 +/- 0.4 (stat) +/- 0.4 (sys) GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7 x 10(-9), is compatible with the production and decay of the Standard Model Higgs boson.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Performance of jet substructure techniques for large-R jets in proton-proton collisions at root s=7 TeV using the ATLAS detector2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 9, p. 076-Article in journal (Refereed)

This paper presents the application of a variety of techniques to study jet substructure. The performance of various modified jet algorithms, or jet grooming techniques, for several jet types and event topologies is investigated for jets with transverse momentum larger than 300 GeV. Properties of jets subjected to the mass-drop filtering, trimming, and pruning algorithms are found to have a reduced sensitivity to multiple proton-proton interactions, are more stable at high luminosity and improve the physics potential of searches for heavy boosted objects. Studies of the expected discrimination power of jet mass and jet substructure observables in searches for new physics are also presented. Event samples enriched in boosted W and Z bosons and top-quark pairs are used to study both the individual jet invariant mass scales and the efficacy of algorithms to tag boosted hadronic objects. The analyses presented use the full 2011 ATLAS dataset, corresponding to an integrated luminosity of 4.7 +/- 0.1 fb(-1) from proton-proton collisions produced by the Large Hadron Collider at a centre-of-mass energy of root s = 7 TeV.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for a fermiophobic Higgs boson in the diphoton decay channel with the ATLAS detector2012In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 72, no 9, p. 2157-Article in journal (Refereed)

A search for a fermiophobic Higgs boson using diphoton events produced in proton-proton collisions at a centre-of-mass energy of root s = 7 TeV is performed using data corresponding to an integrated luminosity of 4.9 fb(-1) collected by the ATLAS experiment at the Large Hadron Collider. A specific benchmark model is considered where all the fermion couplings to the Higgs boson are set to zero and the bosonic couplings are kept at the Standard Model values (fermiophobic Higgs model). The largest excess with respect to the background-only hypothesis is found at 125.5 GeV, with a local significance of 2.9 standard deviations, which reduces to 1.6 standard deviations when taking into account the look-elsewhere effect. The data exclude the fermiophobic Higgs model in the ranges 110.0-118.0 GeV and 119.5-121.0 GeV at 95 % confidence level.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for a heavy top-quark partner in final states with two leptons with the ATLAS detector at the LHC2012In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 11, p. 094-Article in journal (Refereed)

The results of a search for direct pair production of heavy top-quark partners in 4.7 fb(-1) of integrated luminosity from p p collisions at root s = 7 TeV collected by the ATLAS detector at the LHC are reported. Heavy top-quark partners decaying into a top quark and a neutral non-interacting particle are searched for in events with two leptons in the final state. No excess above the Standard Model expectation is observed. Limits are placed on the mass of a supersymmetric scalar top and of a spin-1/2 top-quark partner. A spin-1/2 top-quark partner with a mass between 300 GeV and 480 GeV, decaying to a top quark and a neutral non-interacting particle lighter than 100 GeV, is excluded at 95% confidence level.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for a light charged Higgs boson in the decay channel H+ -> c(s)over-bar in t(t)over-bar events using pp collisions at root s=7 TeV with the ATLAS detector2013In: European Physical Journal C, ISSN 1434-6044, E-ISSN 1434-6052, Vol. 73, no 6, p. 2465-Article in journal (Refereed)

A search for a charged Higgs boson (H+) in t (t) over bar decays is presented, where one of the top quarks decays via t -> H(+)b, followed by H+ -> two jets (c (s) over bar). The other top quark decays to Wb, where the W boson then decays into a lepton (e/mu) and a neutrino. The data were recorded in pp collisions at root s = 7 TeV by the ATLAS detector at the LHC in 2011, and correspond to an integrated luminosity of 4.7 fb(-1). With no observation of a signal, 95 % confidence level (CL) upper limits are set on the decay branching ratio of top quarks to charged Higgs bosons varying between 5 % and 1 % for H+ masses between 90 GeV and 150 GeV, assuming B(H+ -> c (s) over bar) = 100 %.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for a standard model Higgs boson in the H -> ZZ -> l(+)l(-) nu(nu)over-bar decay channel using 4.7 fb(-1) of root s=7 TeV data with the ATLAS detector2012In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 717, no 1-3, p. 29-48Article in journal (Refereed)

A search for a Standard Model Higgs boson decaying via H -> ZZ -> l(+)l(-) nu(nu) over bar, where l represents electrons or muons, is presented. It is based on proton-proton collision data at root s = 7 TeV, collected by the ATLAS experiment at the LHC during 2011 and corresponding to an integrated luminosity of 4.7 fb(-1). The data agree with the expected Standard Model backgrounds. Upper limits on the Higgs boson production cross section are derived for Higgs boson masses between 200 GeV and 600 GeV and the production of a Standard Model Higgs boson with a mass in the range 319-558 GeV is excluded at the 95% confidence level.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for a Standard Model Higgs boson in the mass range 200-600 GeV in the H -> ZZ -> l(+)l(-)q(q)over-bar decay channel with the ATLAS detector2012In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 717, no 1-3, p. 70-88Article in journal (Refereed)

A search for a heavy Standard Model Higgs boson decaying via H -> ZZ -> l(+)l(-)q (q) over bar, where l = e or mu, is presented. The search uses a data set of pp collisions at root s = 7 TeV, corresponding to an integrated luminosity of 4.7 fb(-1) collected in 2011 by the ATLAS detector at the CERN LHC. No significant excess of events above the estimated background is found. Upper limits at 95% confidence level on the production cross section of a Higgs boson with a mass in the range between 200 and 600 GeV are derived. A Standard Model Higgs boson with a mass in the range 300 GeV <= m(H) 322 <= GeV or 353 GeV <= m(H) <= 410 GeV is excluded at 95% CL The corresponding expected exclusion range is 351 GeV <= m(H) <= 404 GeV at 95% CL.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for a Supersymmetric Partner to the Top Quark in Final States with Jets and Missing Transverse Momentum at root s=7 TeV with the ATLAS Detector2012In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 109, no 21, p. 211802-Article in journal (Refereed)

A search for direct pair production of supersymmetric top squarks ((t) over tilde (1)) is presented, assuming the (t) over tilde (1) decays into a top quark and the lightest supersymmetric particle, (chi) over tilde (0)(1), and that both top quarks decay to purely hadronic final states. A total of 16 (4) events are observed compared to a predicted standard model background of 13.5(-3.6)(+3.7) (4.4(-1.3)(+1.7)) events in two signal regions based on integral Ldt = 4.7 fb(-1) of pp collision data taken at root s = 7 TeV with the ATLAS detector at the LHC. An exclusion region in the (t) over tilde (1) versus (chi) over tilde (0)(1) mass plane is evaluated: 370 < m(<(t)over tilde>1) < 465 GeV is excluded for m(<(chi)over tilde>10) similar to 0 GeV while m((t) over tilde1) = 445 GeV is excluded for m((chi) over tilde 10) <= 50 GeV.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for anomalous production of prompt like-sign lepton pairs at root s=7 TeV with the ATLAS detector2012In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 12, p. 007-Article in journal (Refereed)

An inclusive search for anomalous production of two prompt, isolated leptons with the same electric charge is presented. The search is performed in a data sample corresponding to 4.7 fb(-1) of integrated luminosity collected in 2011 at root s = 7TeV with the ATLAS detector at the LHC. Pairs of leptons (e(+/-)e(+/-), e(+/-)mu(+/-), and mu(+/-)mu(+/-)) with large transverse momentum are selected, and the dilepton invariant mass distribution is examined for any deviation from the Standard Model expectation. No excess is found, and upper limits on the production cross section of like-sign lepton pairs from physics processes beyond the Standard Model are placed as a function of the dilepton invariant mass within a fiducial region close to the experimental selection criteria. The 95% confidence level upper limits on the cross section of anomalous e(+/-)e(+/-), e(+/-)mu(+/-), or mu(+/-)mu(+/-) production range between 1.7 fb and 64 fb depending on the dilepton mass and flavour combination.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for charged Higgs bosons decaying via H-+/- -> tau nu in t(t)over-bar events using pp collision data at root s=7 TeV with the ATLAS detector2012In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 6, p. 039-Article in journal (Refereed)

The results of a search for charged Higgs bosons are presented. The analysis is based on 4.6 fb(-1) of proton-proton collision data at root s = 7TeV collected by the ATLAS experiment at the Large Hadron Collider, using top quark pair events with a tau lepton in the final state. The data are consistent with the expected background from Standard Model processes. Assuming that the branching ratio of the charged Higgs boson to a tau lepton and a neutrino is 100%, this leads to upper limits on the branching ratio of top quark decays to a b quark and a charged Higgs boson between 5% and 1% for charged Higgs boson masses ranging from 90GeV to 160GeV, respectively. In the context of the m(h)(max) scenario of the MSSM, tan beta above 12-26, as well as between 1 and 2-6, can be excluded for charged Higgs boson masses between 90GeV and 150GeV.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for charged Higgs bosons through the violation of lepton universality in t(t)over-bar events using pp collision data at root s=7 TeV with the ATLAS experiment2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 3, p. 076-Article in journal (Refereed)

In several extensions of the Standard Model, the top quark can decay into a bottom quark and a light charged Higgs boson H+, t -> bH(+), in addition to the Standard Model decay t -> bW. Since W bosons decay to the three lepton generations equally, while H+ may predominantly decay into tau nu, charged Higgs bosons can be searched for using the violation of lepton universality in top quark decays. The analysis in this paper is based on 4.6 fb(-1) of proton-proton collision data at root s = 7 TeV collected by the ATLAS experiment at the Large Hadron Collider. Signatures containing leptons (e or mu) and/or a hadronically decaying tau (tau(had)) are used. Event yield ratios between e+ tau(had) and e + mu, as well as between mu + tau(had) and mu + e, final states are measured in the data and compared to predictions from simulations. This ratio-based method reduces the impact of systematic uncertainties in the analysis. No significant deviation from the Standard Model predictions is observed. With the assumption that the branching fraction B(H+ -> tau nu) is 100%, upper limits in the range 3.2%-4.4% can be placed on the branching fraction B(t -> bH(+)) for charged Higgs boson masses m(H+) in the range 90-140GeV. After combination with results from a search for charged Higgs bosons in t (t) over bar decays using the tau(had) + jets final state, upper limits on B(t -> bH(+)) can be set in the range 0.8%-3.4%, for m(H+) in the range 90-160GeV.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for contact interactions and large extra dimensions in dilepton events from pp collisions at root s=7 TeV with the ATLAS detector2013In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 87, no 1, p. 015010-Article in journal (Refereed)

A search for nonresonant new phenomena, originating from either contact interactions or large extra spatial dimensions, has been carried out using events with two isolated electrons or muons. These events, produced at the LHC in proton-proton collisions at root s = 7 TeV, were recorded by the ATLAS detector. The data sample, collected throughout 2011, corresponds to an integrated luminosity of 4.9 and 5.0 fb(-1) in the e(+)e(-) and mu(+)mu(-) channels, respectively. No significant deviations from the Standard Model expectation are observed. Using a Bayesian approach, 95% confidence level lower limits ranging from 9.0 to 13.9 TeV are placed on the energy scale of llqq contact interactions in the left-left isoscalar model. Lower limits ranging from 2.4 to 3.9 TeV are also set on the string scale in large extra dimension models. After combining these limits with results from a similar search in the diphoton channel, slightly more stringent limits are obtained.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for dark matter candidates and large extra dimensions in events with a jet and missing transverse momentum with the ATLAS detector2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 4, p. UNSP 075-Article in journal (Refereed)

A search for new phenomena in events with a high-energy jet and large missing transverse momentum is performed using data from proton-proton collisions at root s = 7 TeV with the ATLAS experiment at the Large flatiron Collider. Four kinematic regions are explored using a dataset corresponding to an integrated luminosity of 4.7 fb(-1). No excess of events beyond expectations from Standard Model processes is observed, and limits are set on large extra dimensions and the pair production of dark matter particles.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for Dark Matter Candidates and Large Extra Dimensions in Events with a Photon and Missing Transverse Momentum in pp Collision Data at root s=7 TeV with the ATLAS Detector2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, no 1, p. 011802-Article in journal (Refereed)

Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum in proton-proton collisions at root s = 7 TeV are reported. Data collected by the ATLAS experiment at the LHC corresponding to an integrated luminosity of 4.6 fb(-1) are used. Good agreement is observed between the data and the standard model predictions. The results are translated into exclusion limits on models with large extra spatial dimensions and on pair production of weakly interacting dark matter candidates.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for diphoton events with large missing transverse momentum in 7 TeV proton-proton collision data with the ATLAS detector2012In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 718, no 2, p. 411-430Article in journal (Refereed)

A search for diphoton events with large missing transverse momentum has been performed using proton-proton collision data at is root s = 7 TeV recorded with the ATLAS detector, corresponding to an integrated luminosity of 4.8 fb(-1). No excess of events was observed above the Standard Model prediction and model-dependent 95% confidence level exclusion limits are set. In the context of a generalised model of gauge-mediated supersymmetry breaking with a bino-like lightest neutralino of mass above 50 GeV, gluinos (squarks) below 1.07 TeV (0.87 TeV) are excluded, while a breaking scale Lambda below 196 TeV is excluded for a minimal model of gauge-mediated supersymmetry breaking. For a specific model with one universal extra dimension, compactification scales 1/R < 1.40 TeV are excluded. These limits provide the most stringent tests of these models to date.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for direct chargino production in anomaly-mediated supersymmetry breaking models based on a disappearing-track signature in pp collisions at root s=7 TeV with the ATLAS detector2013In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 1, p. 131-Article in journal (Refereed)

A search for direct chargino production in anomaly-mediated supersymmetry breaking scenarios is performed in p p collisions at root s = 7 TeV using 4.7 fb(-1) of data collected with the ATLAS experiment at the LHC. In these models, the lightest chargino is predicted to have a lifetime long enough to be detected in the tracking detectors of collider experiments. This analysis explores such models by searching for chargino decays that result in tracks with few associated hits in the outer region of the tracking system. The transverse-momentum spectrum of candidate tracks is found to be consistent with the expectation from the Standard Model background processes and constraints on chargino properties are obtained.

KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in root s=7 TeV pp collisions with the ATLAS detector2013In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Vol. 718, no 3, p. 841-859Article in journal (Refereed)

A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb(-1) of root s = 7 TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results.

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