Measurement of the Z/γ * boson transverse momentum distribution in pp collisions at \( \sqrt{s}=7 \) TeV with the ATLAS detector

Journal of High Energy Physics, Sep 2014

This paper describes a measurement of the Z/γ * boson transverse momentum spectrum using ATLAS proton-proton collision data at a centre-of-mass energy of \( \sqrt{s}=7 \) TeV at the LHC. The measurement is performed in the Z/γ * → e + e − and Z/γ * → μ + μ − channels, using data corresponding to an integrated luminosity of 4.7 fb−1. Normalized differential cross sections as a function of the Z/γ * boson transverse momentum are measured for transverse momenta up to 800 GeV. The measurement is performed inclusively for Z/γ * rapidities up to 2.4, as well as in three rapidity bins. The channel results are combined, compared to perturbative and resummed QCD calculations and used to constrain the parton shower parameters of Monte Carlo generators.

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Measurement of the Z/γ * boson transverse momentum distribution in pp collisions at \( \sqrt{s}=7 \) TeV with the ATLAS detector

The ATLAS collaboration This paper describes a measurement of the Z/ boson transverse momentum spectrum using ATLAS proton-proton collision data at a centre-of-mass energy of s = 7 TeV at the LHC. The measurement is performed in the Z/ e+e and Z/ + channels, using data corresponding to an integrated luminosity of 4.7 fb1. Normalized differential cross sections as a function of the Z/ boson transverse momentum are measured for transverse momenta up to 800 GeV. The measurement is performed inclusively for Z/ rapidities up to 2.4, as well as in three rapidity bins. The channel results are combined, compared to perturbative and resummed QCD calculations and used to constrain the parton shower parameters of Monte Carlo generators. 1 Introduction QCD predictions The ATLAS detector Event simulation Event reconstruction and selection Background estimation Unfolding and systematic uncertainties Comparison to QCD predictions 10 Tuning of PYTHIA8 and POWHEG + PYTHIA8 The ATLAS collaboration Introduction The transverse momentum distribution of W and Z bosons produced in hadronic collisions is a traditional probe of strong interaction dynamics. The low transverse momentum (pT) range is governed by initial-state parton radiation (ISR) and the intrinsic transverse momentum of the initial-state partons inside the proton, and modeled using soft-gluon resummation [1] or parton shower models [2, 3]. Quark-gluon scattering dominates at high pT and is described by perturbative QCD [46]. The correct modelling of the vector boson pT distribution is important in many physics analyses at the LHC for which the production of W or Z bosons constitutes a significant background. Moreover, it is crucial for a precise measurement of the W boson mass. The transverse momentum distribution also probes the gluon density of the proton [7]. Vector boson pT distribution measurements were published by ATLAS [8, 9] and CMS [10] based on 3540 pb1 of proton-proton collisions at a centre-of-mass energy of s = 7 TeV. The typical precision of these measurements is 4% to 10%. This paper presents a measurement of the normalized Z boson transverse momentum distribution (pZT) with the ATLAS detector, in the Z/ e+e and Z/ + channels, using LHC proton-proton collision data taken in 2011 at a centre-of-mass energy of s = 7 TeV and corresponding to an integrated luminosity of 4.7 fb1 [11]. The large integrated luminosity allows the measurement to be performed in three different Z boson rapidity (yZ ) bins, probing the transverse momentum dynamics over a wide range of the initial-state parton momentum fraction. With respect to previous results, the present analysis aims at reduced uncertainties, finer binning and extended measurement range. Reconstructed from the final-state lepton kinematics, pZT is affected by lepton energy and momentum measurement uncertainties. To minimize the impact of these uncertainties, the ? observable1 was introduced as an alternative probe of pZT [12], pioneered at the Tevatron [1315], and studied by ATLAS using the present data set [16] and LHCb [17]. The correlation between ? and pZT is, however, only partial and the good experimental resolution on ? is counterbalanced by a reduced sensitivity to the underlying transverse momentum distribution; in addition, interpreting ? as a probe of pZT assumes that the final-state lepton angular correlations are correctly modeled. The measurement presented in this paper allows the effects of the Z boson transverse momentum and the lepton angular correlations to be disentangled unambiguously. QCD predictions for the pZT distribution are described in the next section. After a brief description of the experiment in section 3, the measurement is presented in sections 48. The results are compared to available QCD predictions in section 9 and used to constrain phenomenological models describing the low-pZT region in section 10; the compatibility of the ? measurement with the pZT-constrained models is also tested. Section 11 concludes the paper. QCD predictions The measurements are compared to a representative set of theoretical predictions. They rely on perturbative QCD (pQCD) only, or include resummation of soft-gluon emissions. Resummation is treated either analytically, or using Monte Carlo methods. Fully differential inclusive boson-production cross sections can be obtained to second order in the strong coupling constant S (NNLO) using the Fewz3.1 [46] and Dynnlo1.3 [18, 19] programs. The O(S2) cross-section predictions are valid at large pZT, where the cross section is dominated by the radiation of high-pT gluons. At low pZT, multiple soft-gluon emissions predominate and fixed-order pQCD predictions are not appropriate. The ResBos calculation relies on soft-gluon resummation at low pZT and matches the O(S2) cross section at high pZT. It simulates the vector boson decays but does not include a description of the hadronic activity in the event. Two versions are used here, which di (...truncated)


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G. Aad, B. Abbott, J. Abdallah, S. Abdel Khalek. Measurement of the Z/γ * boson transverse momentum distribution in pp collisions at \( \sqrt{s}=7 \) TeV with the ATLAS detector, Journal of High Energy Physics, 2014, pp. 145, Volume 2014, Issue 9, DOI: 10.1007/JHEP09(2014)145