ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Measurements of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC. Phys. Lett. B, 726(1-3), 88–119.
Abstract: Measurements are presented of production properties and couplings of the recently discovered Higgs boson using the decays into boson pairs, H -> gamma gamma, H -> ZZ* -> 4l and H -> WW* -> l nu l nu. The results are based on the complete pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at centre-of-mass energies of root s = 7 TeV and root s = 8 TeV, corresponding to an integrated luminosity of about 25 fb(-1). Evidence for Higgs boson production through vector-boson fusion is reported. Results of combined fits probing Higgs boson couplings to fermions and bosons, as well as anomalous contributions to loop-induced production and decay modes, are presented. All measurements are consistent with expectations for the Standard Model Higgs boson.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2014). Search for new phenomena in photon plus jet events collected in proton-proton collisions at root s=8 TeV with the ATLAS detector. Phys. Lett. B, 728, 562–578.
Abstract: This Letter describes a model-independent search for the production of new resonances in photon + jet (gamma + jet) events using 20 fb(-1) of proton-proton LHC data recorded with the ATLAS detector at a centre-of-mass energy of root s = 8 TeV. The gamma + jet mass distribution is compared to a background model fit from data; no significant deviation from the background-only hypothesis is found. Limits are set at 95% credibility level on generic Gaussian-shaped signals and two benchmark phenomena beyond the Standard Model: non-thermal quantum black holes and excited quarks. Non-thermal quantum black holes are excluded below masses of 4.6 TeV and excited quarks are excluded below masses of 3.5 TeV.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Search for new phenomena in the WW -> vertical bar v vertical bar ' v ' final state in pp collisions at root s=7 TeV with the ATLAS detector. Phys. Lett. B, 718(3), 860–878.
Abstract: This Letter reports a search for a heavy particle that decays to WW using events produced in pp collisions at root s = 7 TeV. The data were recorded in 2011 by the ATLAS detector and correspond to an integrated luminosity of 4.7 fb(-1). WW -> lvl'v' (l,l' = e or mu) final states are considered and the distribution of the transverse mass of the W W candidates is found to be consistent with Standard Model expectations. Upper limits on the production cross section times branching ratio into W boson pairs are set for Randall-Sundrum and bulk Randall-Sundrum gravitons, which result in observed 95% CL lower limits on the masses of the two particles of 1.23 TeV and 0.84 TeV, respectively.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2014). Search for Higgs boson decays to a photon and a Z boson in pp collisions at root s=7 and 8 TeV with the ATLAS detector. Phys. Lett. B, 732, 8–27.
Abstract: A search is reported for a neutral Higgs boson in the decay channel H -> Z gamma, Z -> l(+)l(-) (l = e, mu), using 4.5 fb(-1) of pp collisions at root s = 7 TeV and 20.3 fb(-1) of pp collisions at root s = 8 TeV, recorded by the ATLAS detector at the CERN Large Hadron Collider. The observed distribution of the invariant mass of the three final-state particles, m(ll gamma), is consistent with the Standard Model hypothesis in the investigated mass range of 120-150 GeV. For a Higgs boson with a mass of 125.5 GeV, the observed upper limit at the 95% confidence level is 11 times the Standard Model expectation. Upper limits are set on the cross section times branching ratio of a neutral Higgs boson with mass in the range 120-150 GeV between 0.13 and 0.5 pb for root s = 8 TeV at 95% confidence level.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2014). Search for WZ resonances in the fully leptonic channel using pp collisions at root s=8 TeV with the ATLAS detector. Phys. Lett. B, 737, 223–243.
Abstract: A search for resonant WZ production in the l nu l'l' (l, l' = e, mu) decay channel using 20.3 fb(-1) of root s = 8 TeV pp collision data collected by the ATLAS experiment at LHC is presented. No significant deviation from the Standard Model prediction is observed and upper limits on the production cross sections of WZ resonances from an extended gauge model W' and from a simplified model of heavy vector triplets are derived. A corresponding observed (expected) lower mass limit of 1.52 (1.49) TeV is derived for the W' at the 95% confidence level. (C) 2014 CERN for the benefit of the ATLAS Collaboration.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2014). Measurement of the cross section of high transverse momentum Z -> b(b)over-bar production in proton-proton collisions at root s=8 TeV with the ATLAS detector. Phys. Lett. B, 738, 25–43.
Abstract: This Letter reports the observation of a high transverse momentum Z -> b (b) over bar signal in proton-proton collisions at root s = 8TeVand the measurement of its production cross section. The data analysed were collected in 2012 with the ATLAS detector at the LHC and correspond to an integrated luminosity of 19.5 fb(-1). The Z -> b (b) over bar decay is reconstructed from a pair of b-tagged jets, clustered with the anti- k(t) jet algorithm with R = 0.4, that have low angular separation and form a dijet with p(T) > 200 GeV. The signal yield is extracted from a fit to the dijet invariant mass distribution, with the dominant, multi-jet background mass shape estimated by employing a fully data-driven technique that reduces the dependence of the analysis on simulation. The fiducial cross section is determined to be sigma(fid)(Z -> b (b) over bar) = 2.02 +/- 0.20 (stat.) +/- 0.25 (syst.) +/- 0.06 (lumi.) pb = 2.02 +/- 0.33 pb, in good agreement with next-to-leading-order theoretical predictions.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2014). Measurement of inclusive jet charged-particle fragmentation functions in Pb plus Pb collisions at root S-NN=2.76 TeV with the ATLAS detector. Phys. Lett. B, 739, 320–342.
Abstract: Measurements of charged-particle fragmentation functions of jets produced in ultra-relativistic nuclear collisions can provide insight into the modification of parton showers in the hot, dense medium created in the collisions. ATLAS has measured jets in root S-NN = 2.76 TeV Pb+Pb collisions at the LHC using a data set recorded in 2011 with an integrated luminosity of 0.14 nb(-1). Jets were reconstructed using the anti-k(t) algorithm with distance parameter values R = 0.2, 0.3, and 0.4. Distributions of charged-particle transverse momentum and longitudinal momentum fraction are reported for seven bins in collision centrality for R = 0.4 jets with p(T)(jet) > 100 GeV. Commensurate minimum p(T) values are used for the other radii. Ratios of fragment distributions in each centrality bin to those measured in the most peripheral bin are presented. These ratios show a reduction of fragment yield in central collisions relative to peripheral collisions at intermediate z values, 0.04 less than or similar to z less than or similar to 0.2, and an enhancement in fragment yield for z less than or similar to 0.04. A smaller, less significant enhancement is observed at large z and large p(T) in central collisions.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2015). Centrality and rapidity dependence of inclusive jet production in root(NN)-N-S=5.02 TeV proton-lead collisions with the ATLAS detector. Phys. Lett. B, 748, 392–413.
Abstract: Measurements of the centrality and rapidity dependence of inclusive jet production in root(NN)-N-S = 5.02 TeV proton-lead (p + Pb) collisions and the jet cross-section in root s = 2.76 TeV proton-proton collisions are presented. These quantities are measured in datasets corresponding to an integrated luminosity of 27.8 nb(-1) and 4.0 pb(-1), respectively, recorded with the ATLAS detector at the Large Hadron Collider in 2013. The p + Pb collision centrality was characterised using the total transverse energy measured in the pseudorapidity interval -4.9 < eta < -3.2 in the direction of the lead beam. Results are presented for the double-differential per-collision yields as a function of jet rapidity and transverse momentum (p(T)) for minimum-bias and centrality-selected p + Pb collisions, and are compared to the jet rate from the geometric expectation. The total jet yield in minimum-bias events is slightly enhanced above the expectation in a p(T)-dependent manner but is consistent with the expectation within uncertainties. The ratios of jet spectra from different centrality selections show a strong modification of jet production at all p(T) at forward rapidities and for large pT at mid-rapidity, which manifests as a suppression of the jet yield in central events and an enhancement in peripheral events. These effects imply that the factorisation between hard and soft processes is violated at an unexpected level in proton-nucleus collisions. Furthermore, the modifications at forward rapidities are found to be a function of the total jet energy only, implying that the violations may have a simple dependence on the hard parton-parton kinematics.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2015). Measurement of the production of neighbouring jets in lead-lead collisions at root s(NN)=2.76TeV with the ATLAS detector. Phys. Lett. B, 751, 376–395.
Abstract: This Letter presents measurements of correlated production of nearby jets in Pb+Pb collisions at root s(NN) = 2.76 TeV using the ATLAS detector at the Large Hadron Collider. The measurement was performed using 0.14 nb(-1)of data recorded in 2011. The production of correlated jet pairs was quantified using the rate, R-Delta R, of “neighbouring” jets that accompany “test” jets within a given range of angular distance, Delta R, in the pseudorapidity-azimuthal angle plane. The jets were measured in the ATLAS calorimeter and were reconstructed using the anti-ktalgorithm with radius parameters d = 0.2, 0.3, and 0.4. R-Delta R was measured in different Pb+Pbcollision centrality bins, characterized by the total transverse energy measured in the forward calorimeters. Acentrality dependence of R-Delta R is observed for all three jet radii with R-Delta R found to be lower in central collisions than in peripheral collisions. The ratios formed by the R-Delta R values in different centrality bins and the values in the 40-80% centrality bin are presented.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2014). Fiducial and differential cross sections of Higgs boson production measured in the four-lepton decay channel in pp collisions at root s=8 TeV with the ATLAS detector. Phys. Lett. B, 738, 234–253.
Abstract: Measurements of fiducial and differential cross sections of Higgs boson production in the H -> ZZ* -> 4l decay channel are presented. The cross sections are determined within a fiducial phase space and corrected for detection efficiency and resolution effects. They are based on 20.3 fb(-1) of pp collision data, produced at root s= 8 TeV centre-of-mass energy at the LHC and recorded by the ATLAS detector. The differential measurements are performed in bins of transverse momentum and rapidity of the four-lepton system, the invariant mass of the subleading lepton pair and the decay angle of the leading lepton pair with respect to the beam line in the four-lepton rest frame, as well as the number of jets and the transverse momentum of the leading jet. The measured cross sections are compared to selected theoretical calculations of the Standard Model expectations. No significant deviation from any of the tested predictions is found.
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