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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. (2012). Underlying event characteristics and their dependence on jet size of charged-particle jet events in pp collisions at root(s)=7 TeV with the ATLAS detector. Phys. Rev. D, 86(7), 072004–34pp.
Abstract: Distributions sensitive to the underlying event are studied in events containing one or more chargedparticle jets produced in pp collisions at root s = 7 TeV with the ATLAS detector at the Large Hadron Collider (LHC). These measurements reflect 800 μb(-1) of data taken during 2010. Jets are reconstructed using the anti-k(t) algorithm with radius parameter R varying between 0.2 and 1.0. Distributions of the charged-particle multiplicity, the scalar sum of the transverse momentum of charged particles, and the average charged-particle p(T) are measured as functions of p(T)(jet) in regions transverse to and opposite the leading jet for 4 GeV < p(T)(jet) < 100 GeV. In addition, the R dependence of the mean values of these observables is studied. In the transverse region, both the multiplicity and the scalar sum of the transverse momentum at fixed p(T)(jet) vary significantly with R, while the average charged- particle transverse momentum has a minimal dependence on R. Predictions from several Monte Carlo tunes have been compared to the data; the predictions from Pythia 6, based on tunes that have been determined using LHC data, show reasonable agreement with the data, including the dependence on R. Comparisons with other generators indicate that additional tuning of soft-QCD parameters is necessary for these generators. The measurements presented here provide a testing ground for further development of the Monte Carlo models.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Measurement of the inclusive W-+/- and Z/gamma* cross sections in the e and μdecay channels in pp collisions at root s=7 TeV with the ATLAS detector. Phys. Rev. D, 85(7), 072004–39pp.
Abstract: The production cross sections of the inclusive Drell-Yan processes W-+/- -> l nu and Z/gamma* -> ll (l = e, mu) are measured in proton-proton collisions at root s = 7 TeV with the ATLAS detector. The cross sections are reported integrated over a fiducial kinematic range, extrapolated to the full range, and also evaluated differentially as a function of the W decay lepton pseudorapidity and the Z boson rapidity, respectively. Based on an integrated luminosity of about 35 pb(-1) collected in 2010, the precision of these measurements reaches a few percent. The integrated and the differential W-+/- and Z/gamma* cross sections in the e and μchannels are combined, and compared with perturbative QCD calculations, based on a number of different parton distribution sets available at next-to-next-to-leading order.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for excited leptons in proton-proton collisions at root s=7 TeV with the ATLAS detector. Phys. Rev. D, 85(7), 072003–23pp.
Abstract: The ATLAS detector is used to search for excited leptons in the electromagnetic radiative decay channel l* -> l gamma. Results are presented based on the analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 2.05 fb(-1). No evidence for excited leptons is found, and limits are set on the compositeness scale Lambda as a function of the excited lepton mass m(l*). In the special case where Lambda = m(l*), excited electron and muon masses below 1.87 TeV and 1.75 TeV are excluded at 95% C.L., respectively.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for Pair Production of a New b ' Quark that Decays into a Z Boson and a Bottom Quark with the ATLAS Detector. Phys. Rev. Lett., 109(7), 071801–19pp.
Abstract: A search is reported for the pair production of a new quark b' with at least one b' decaying to a Z boson and a bottom quark. The data, corresponding to 2.0 fb(-1) of integrated luminosity, were collected from pp collisions at root s = 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. Using events with a b-tagged jet and a Z boson reconstructed from opposite-charge electrons, the mass distribution of large transverse momentum b' candidates is tested for an enhancement. No evidence for a b' signal is detected in the observed mass distribution, resulting in the exclusion at a 95% confidence level of b' quarks with masses m(b)' < 400 GeV that decay entirely via b' -> Z + b. In the case of a vectorlike singlet b' mixing solely with the third standard model generation, masses m(b)' < 358 GeV are excluded.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Escobar, C., et al. (2011). Properties of jets measured from tracks in proton-proton collisions at center-of-mass energy sqrt(s)=7 TeV with the ATLAS detector. Phys. Rev. D, 84(5), 054001–27pp.
Abstract: Jets are identified and their properties studied in center-of-mass energy sqrt(s) = 7 TeV proton-proton collisions at the Large Hadron Collider using charged particles measured by the ATLAS inner detector. Events are selected using a minimum bias trigger, allowing jets at very low transverse momentum to be observed and their characteristics in the transition to high-momentum fully perturbative jets to be studied. Jets are reconstructed using the anti-k(t) algorithm applied to charged particles with two radius parameter choices, 0.4 and 0.6. An inclusive charged jet transverse momentum cross section measurement from 4 GeV to 100 GeV is shown for four ranges in rapidity extending to 1.9 and corrected to charged particle-level truth jets. The transverse momenta and longitudinal momentum fractions of charged particles within jets are measured, along with the charged particle multiplicity and the particle density as a function of radial distance from the jet axis. Comparison of the data with the theoretical models implemented in existing tunings of Monte Carlo event generators indicates reasonable overall agreement between data and Monte Carlo. These comparisons are sensitive to Monte Carlo parton showering, hadronization, and soft physics models.
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