ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Search for top quark decays t -> qH,with H -> gamma gamma, in root s=13 TeV pp collisions using the ATLAS detector. J. High Energy Phys., 10(10), 129–43pp.
Abstract: This article presents a search for flavour-changing neutral currents in the decay of a top quark into an up-type (q = c; u) quark and a Higgs boson, where the Higgs boson decays into two photons. The proton-proton collision data set analysed amounts to 36.1 fb(-1) at root s = 13TeV collected by the ATLAS experiment at the LHC. Top quark pair events are searched for, where one top quark decays into qH and the other decays into bW. Both the hadronic and leptonic decay modes of the W boson are used. No significant excess is observed and an upper limit is set on the t -> cH branching ratio of 2 : 2 x 10(-3) at the 95% confidence level, while the expected limit in the absence of signal is 1 : 6 x 10(-3). The corresponding limit on the tcH coupling is 0.090 at the 95% confidence level. The observed upper limit on the t -> uH branching ratio is 2 : 4 x 10(-3).
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Muon reconstruction performance of the ATLAS detector in proton-proton collision data at root s=13 TeV. Eur. Phys. J. C, 76(5), 292–30pp.
Abstract: This article documents the performance of the ATLAS muon identification and reconstruction using the LHC dataset recorded at root s = 13 TeVin 2015. Using a large sample of J/.psi -> μμand Z -> μμdecays from 3.2 fb(-1) of pp collision data, measurements of the reconstruction efficiency, as well as of the momentum scale and resolution, are presented and compared to Monte Carlo simulations. The reconstruction efficiency is measured to be close to 99 % over most of the covered phase space (vertical bar eta vertical bar < 2.5 and 5 < p(T) < 100 GeV). The isolation efficiency varies between 93 and 100 % depending on the selection applied and on the momentum of the muon. Both efficiencies are well reproduced in simulation. In the central region of the detector, the momentum resolution is measured to be 1.7 % (2.3 %) for muons from J/psi -> μmu(Z -> μmu) decays, and the momentum scale is known with an uncertainty of 0.05 %. In the region vertical bar eta vertical bar > 2.2, the p(T) resolution for muons from Z -> μdecays is 2.9 % while the precision of the momentum scale for low-p(T) muons from J/psi -> μμdecays is about 0.2 %.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Measurement of the ZZ Production Cross Section in pp Collisions at root s=13 TeV with the ATLAS Detector. Phys. Rev. Lett., 116(10), 101801–19pp.
Abstract: The ZZ production cross section in proton-proton collisions at 13 TeV center-of-mass energy is measured using 3.2 fb(-1) of data recorded with the ATLAS detector at the Large Hadron Collider. The considered Z boson candidates decay to an electron or muon pair of mass 66-116 GeV. The cross section is measured in a fiducial phase space reflecting the detector acceptance. It is also extrapolated to a total phase space for Z bosons in the same mass range and of all decay modes, giving 16.7(-2.0)(+2.2) (stat)(-0.7)(+0.9) (syst)(-0.7)(+1.0) (lumi) pb. The results agree with standard model predictions.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Measurement of W boson angular distributions in events with high transverse momentum jets at root s=8 TeV using the ATLAS detector. Phys. Lett. B, 765, 132–153.
Abstract: The W boson angular distribution in events with high transverse momentum jets is measured using data collected by the ATLAS experiment from proton-proton collisions at a centre-of-mass energy root s = 8 TeV at the Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb(-1). The focus is on the contributions to W + jets processes from real W emission, which is achieved by studying events where a muon is observed close to a high transverse momentum jet. At small angular separations, these contributions are expected to be large. Various theoretical models of this process are compared to the data in terms of the absolute cross-section and the angular distributions of the muon from the leptonic W decay.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Performance of the ATLAS Transition Radiation Tracker in Run 1 of the LHC: tracker properties. J. Instrum., 12, P05002–42pp.
Abstract: The tracking performance parameters of the ATLAS Transition Radiation Tracker (TRT) as part of the ATLAS inner detector are described in this paper for different data-taking conditions in proton-proton, proton-lead and lead-lead collisions at the Large Hadron Collider (LHC). The performance is studied using data collected during the first period of LHC operation (Run 1) and is compared with Monte Carlo simulations. The performance of the TRT, operating with two different gas mixtures (xenon-based and argon-based) and its dependence on the TRT occupancy is presented. These studies show that the tracking performance of the TRT is similar for the two gas mixtures and that a significant contribution to the particle momentum resolution is made by the TRT up to high particle densities.
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