Measurement of the production cross section of jets in association with a Z boson in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

TL;DR

This ATLAS study provides a comprehensive Z+jets cross-section measurement at sqrt(s)=7 TeV, extending to up to seven jets and a wide set of differential observables. After unfolding to particle level, results are confronted with fixed-order NLO and multiple MC generators, revealing good overall agreement but notable shortcomings for MC@NLO at high jet multiplicities and for leading-jet p_T. A key finding is that inclusive HT and p_T observables are better described when higher jet multiplicities are included via exclusive-sum approaches, underscoring the importance of multi-jet contributions in precision QCD tests. The results validate pQCD in a broad phase space while guiding improvements in MC modeling for Z+jets as a background to Higgs and beyond-Standard-Model searches.

Abstract

Measurements of the production of jets of particles in association with a Z boson in pp collisions at sqrt(s) = 7 TeV are presented, using data corresponding to an integrated luminosity of 4.6/fb collected by the ATLAS experiment at the Large Hadron Collider. Inclusive and differential jet cross sections in Z events, with Z decaying into electron or muon pairs, are measured for jets with transverse momentum pT > 30 GeV and rapidity |y| < 4.4. The results are compared to next-to-leading-order perturbative QCD calculations, and to predictions from different Monte Carlo generators based on leading-order and next-to-leading-order matrix elements supplemented by parton showers.

Figures (17)

• Figure 1: Numbers of events observed in data and predicted in simulation that pass the $Z\,(\to ee)\,\texttt{+}\,\mathrm{jets}$ and $Z\,(\to\mu\mu)\,\texttt{+}\,\mathrm{jets}$ selection as a function of the invariant mass of the $Z$ candidate, (a) $m^{ee}$ and (b) $m^{\mu\mu}$ , for events with at least one jet with $p^{\rm jet}_{\rm T} > 30 \mathrm{\ Ge V} \textrm{Ge V}$ and $|y^{\rm jet}| <4.4$, and as a function of the inclusive jet multiplicity, $N_{\rm jet}$ , in (c) di-electron and (d) di-muon events. The individual contributions of the various backgrounds are also shown, as detailed in the legend. The hatched band corresponds to the combined statistical and systematic uncertainty on the prediction, obtained using ALPGEN to model the $Z\,\texttt{+}\,\mathrm{jets}$ process. The error bars on each data point show the statistical uncertainty. The bottom panel shows the corresponding MC/data ratio. The shaded band corresponds to the total systematic uncertainty and the error bars to the statistical uncertainty on the MC/data ratio.
• Figure 2: (a) Measured cross section for $Z\,(\to\ell \ell)\,\texttt{+}\,\mathrm{jets}$ as a function of the inclusive jet multiplicity, $N_{\rm jet}$ , and (b) ratio of cross sections for successive inclusive jet multiplicities. The data are compared to NLO pQCD predictions from BlackHat+SHERPA corrected to the particle level, and the ALPGEN , SHERPA and MC@NLO event generators (see legend for details). The error bars indicate the statistical uncertainty on the data, and the hatched (shaded) bands the statistical and systematic uncertainties on data (prediction) added in quadrature.
• Figure 3: (a) Ratio of cross sections for successive exclusive jet multiplicities, $N_{\rm jet}$ , in events selected with the standard selection and (b) in events with at least one jet with $p^{\rm jet}_{\rm T} > 150 \mathrm{\ Ge V} \textrm{Ge V}$ and $|y^{\rm jet}|<4.4$. The data are compared to NLO pQCD predictions from BlackHat+SHERPA corrected to the particle level, and the ALPGEN , SHERPA and MC@NLO event generators (see legend for details). The error bars indicate the statistical uncertainty on the data, and the hatched (shaded) bands the statistical and systematic uncertainties on data (prediction) added in quadrature. The shaded bands on the theory calculations show the systematic uncertainty excluding the scale uncertainty (dark shaded) and the total systematic uncertainties using the naive approach (medium shaded) and the nominal approach (light shaded) to derive the scale uncertainty (see section \ref{['sec:Theory']}). The figures include (a) a linear fit $R_{(n+1)/n} = R_o+\frac{{\rm d}R}{{\rm d}n}\cdot n$ in the range $R_{2/1} <R_{(n+1)/n}<R_{5/4}$ and (b) a Poisson fit $R_{(n+1)/n} = \frac{\bar{n}}{n}$ to the data points, with the free parameters $R_o$, $\frac{{\rm d}R}{{\rm d}n}$ and $\bar{n}$.
• Figure 4: (a) Measured cross section for $Z\,(\to\ell \ell)\,\texttt{+}\,\mathrm{jets}$ as a function of the exclusive jet multiplicity, $N_{\rm jet}$ , and (b) ratio of the cross sections for two successive multiplicities, in events passing the VBF preselection (at least two jets with $p^{\rm jet}_{\rm T} > 30 \mathrm{\ Ge V} \textrm{Ge V}$ and $|y^{\rm jet}|<4.4$ and $m^{jj} >350\mathrm{\ Ge V} \textrm{Ge V}$ and $|\Delta y^{jj}| >3.0$ for the two leading jets). The other details are as in Figure \ref{['fig:ElMuComb1']}.
• Figure 5: (a) Measured cross section for $Z\,(\to\ell \ell)\,\texttt{+}\,\mathrm{jets}$ as a function of the transverse momentum, $p^{\rm jet}_{\rm T}$ , of the leading jet for events with at least one jet with $p^{\rm jet}_{\rm T} > 30 \mathrm{\ Ge V} \textrm{Ge V}$ and $|y^{\rm jet}|<4.4$ in the final state and (b) as a function of $p^{\rm jet}_{\rm T}$ of the second leading jet for events with at least two jets. The cross sections are normalized to the inclusive $Z\,(\to\ell \ell)$ cross section. The other details are as in Figure \ref{['fig:ElMuComb0']}.