A PDF file should load here. If you do not see its contents
the file may be temporarily unavailable at the journal website
or you do not have a PDF plug-in installed and enabled in your browser.
Alternatively, you can download the file locally and open with any standalone PDF reader:
https://link.springer.com/content/pdf/10.1007%2FJHEP09%282014%29145.pdf
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)