Measurement of the production cross section for Z/gamma* in association with jets in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

TL;DR

This ATLAS study measures inclusive jet production in Z/γ*→ℓℓ events in pp collisions at √s=7 TeV using 36 pb^-1 of data, with jets reconstructed by the anti-kT algorithm (R=0.4) and pT>30 GeV in |y|<4.4. The analysis corrects detector effects to the particle level and compares results to NLO pQCD (BlackHat) and LO matrix-element+parton-shower MCs (ALPGEN, Sherpa, PYTHIA), including non-perturbative and QED corrections. Across observables such as jet multiplicity, pT, rapidity, and dijet kinematics, the data are described well by all major predictions, with PYTHIA generally underestimating at high jet counts or large jet separations. The results validate pQCD calculations and MC modeling for Z/γ*+jets, supporting their use as backgrounds in new-physics searches and as benchmark tests of perturbative QCD at the LHC.

Abstract

Results are presented on the production of jets of particles in association with a Z/gamma* boson, in proton-proton collisions at sqrt(s) = 7 TeV with the ATLAS detector. The analysis includes the full 2010 data set, collected with a low rate of multiple proton-proton collisions in the accelerator, corresponding to an integrated luminosity of 36 pb^-1. Inclusive jet cross sections in Z/gamma* events, with Z/gamma* decaying into electron or muon pairs, are measured for jets with transverse momentum pT > 30 GeV and jet rapidity |y| < 4.4. The measurements are compared to next-to-leading-order perturbative QCD calculations, and to predictions from different Monte Carlo generators implementing leading-order matrix elements supplemented by parton showers.

Figures (26)

• Figure 1: Uncorrected dilepton invariant mass in (top) $Z/\gamma^*\rightarrow e^+ e^-$ and (bottom) $Z/\gamma^*\rightarrow \mu^+ \mu^-$ events with at least one jet in the final state, shown in a wider dilepton mass region than the one selected (left), and uncorrected inclusive jet multiplicity (right), for jets with $p_{ T} > 30$ GeV and $|y|<4.4$ (black dots), and in the mass range $66$ GeV$< m_{\ell^+ \ell^-} < 116$ GeV ($\ell = e, \mu$). Only statistical uncertainties are shown. The data are compared to predictions for signal (ALPGEN and Sherpa, both normalized to the FEWZ value for the total cross section) and background processes (filled histograms).
• Figure 2: Relative systematic uncertainties from different sources in the (top) $Z/\gamma^*(\rightarrow e^+ e^-)$+jets and (bottom) $Z/\gamma^*(\rightarrow \mu^+ \mu^-)$+jets analyses for the measured cross section as a function of inclusive jet multiplicity, and the inclusive differential cross sections as a function of $p_{ T}$, for events with at least one jet with $p_{ T} >30$ GeV and $|y|<4.4$ in the final state (see Section \ref{['sec:sys']}). The total systematic uncertainty is obtained by summing all contributions in quadrature.
• Figure 3: Measured cross section $\sigma_{N_{\rm jet}}$ (black dots) for (left) $Z/\gamma^*(\rightarrow e^+ e^-)$+jets and (right) $Z/\gamma^*(\rightarrow \mu^+ \mu^-)$+jets production as a function of the inclusive jet multiplicity, for events with at least one jet with $p_{ T} >30$ GeV and $|y|<4.4$ in the final state. In this and subsequent figures \ref{['fig:ratio']} - \ref{['fig:dr']} the error bars indicate the statistical uncertainty and the dashed areas the statistical and systematic uncertainties added in quadrature. The measurements are compared to NLO pQCD predictions from BlackHat, as well as the predictions from ALPGEN and Sherpa (both normalized to the FEWZ value for the total cross section), and PYTHIA (normalized to the data as discussed in Section \ref{['sec:results']}).
• Figure 4: Measured ratio of cross sections ($\sigma_{N_{\rm jet}} / \sigma_{N_{\rm jet} - 1}$) (black dots) for (left) $Z/\gamma^*(\rightarrow e^+ e^-)$+jets and (right) $Z/\gamma^*(\rightarrow \mu^+ \mu^-)$+jets production as a function of the inclusive jet multiplicity, for events with at least one jet with $p_{ T} >30$ GeV and $|y|<4.4$ in the final state.
• Figure 5: Measured normalized inclusive jet cross section $(1/\sigma_{Z/\gamma^*\rightarrow \ell^+ \ell^-}) \mathrm{d}\sigma / \mathrm{d}p_{ T}$ (black dots) in (left) $Z/\gamma^*(\rightarrow e^+ e^-)$+jets and (right) $Z/\gamma^*(\rightarrow \mu^+ \mu^-)$+jets production as a function of $p_{ T}$, in events with at least one jet with $p_{ T} >30$ GeV and $|y|<4.4$ in the final state, and normalized by $\sigma_{Z/\gamma^*\rightarrow e^+ e^-}$ and $\sigma_{Z/\gamma^*\rightarrow \mu^+ \mu^-}$ Drell-Yan cross sections, respectively.