Measurements of jet multiplicity and differential production cross sections of Z + jets events in proton-proton collisions at sqrt(s) = 7 TeV

TL;DR

This study measures fiducial ${Z}$+jets production at $ oot s = 7$ TeV with ${4.9~fb^{-1}}$ in CMS, obtaining jet-multiplicity and differential cross sections for the four leading jets and for $H_T$. The analysis uses robust lepton+jet selections, data/MC efficiency corrections, and a detector-level to particle-level unfolding with RooUnfold. The results are confronted with LO MadGraph and NLO Sherpa/Powheg predictions, revealing that LO predictions describe many features but can over- or under-predict high-$p_T$ regions, while NLO approaches improve overall agreement but show residual discrepancies in high-energy tails, guiding future refinements in merging schemes and scale settings. The work emphasizes Z+jets as both a precise test of pQCD and a key background for beyond-Standard-Model searches, underlining the need for accurate modeling of multi-jet final states at the LHC.

Abstract

Measurements of differential cross sections are presented for the production of a Z boson and at least one hadronic jet in proton-proton collisions at sqrt(s) = 7 TeV, recorded by the CMS detector, using a data sample corresponding to an integrated luminosity of 4.9 inverse femtobarns. The jet multiplicity distribution is measured for up to six jets. The differential cross sections are measured as a function of jet transverse momentum and pseudorapidity for the four highest transverse momentum jets. The distribution of the scalar sum of jet transverse momenta is also measured as a function of the jet multiplicity. The measurements are compared with theoretical predictions at leading and next-to-leading order in perturbative QCD.

Figures (9)

• Figure 1: Distributions of the exclusive jet multiplicity for the electron channel (top) and muon channel (bottom). Data are compared to the simulation, which is the sum of signal and background events. Scale factors have been used to correct simulation distributions for residual efficiency differences with respect to data. No unfolding procedure is applied. Only statistical uncertainties are shown.
• Figure 2: Exclusive (left) and inclusive (right) jet multiplicity distributions, after the unfolding procedure, compared with sherpa, powheg, and MadGraph predictions. Error bars around the experimental points represent the statistical uncertainty, while cross-hatched bands represent statistical plus systematic uncertainty. The bands around theory predictions correspond to the statistical uncertainty of the generated sample and, for NLO calculations, to its combination with the systematic uncertainty related to scale variations.
• Figure 3: Unfolded differential cross section as a function of $p_{\mathrm{T}}$ for the first (top left), second (top right), third (bottom left), and fourth (bottom right) highest $p_{\mathrm{T}}$ jets, compared with sherpa, powheg, and MadGraph predictions. Error bars around the experimental points represent the statistical uncertainty, while cross-hatched bands represent statistical plus systematic uncertainty. The bands around theory predictions correspond to the statistical uncertainty of the generated sample and, for NLO calculations, to its combination with systematic uncertainty related to scale variations.
• Figure 4: Unfolded differential cross section as a function of the jet absolute pseudorapidity $\lvert \eta \rvert$ for the first (top left), second (top right), third (bottom left), and fourth (bottom right) highest $p_{\mathrm{T}}$ jets, compared with sherpa, powheg, and MadGraph predictions. Error bars around the experimental points represent the statistical uncertainty, while cross-hatched bands represent statistical plus systematic uncertainty. The bands around theory predictions correspond to the statistical uncertainty of the generated sample and, for NLO calculations, to its combination with systematic uncertainty related to scale variations.
• Figure 5: Unfolded differential cross section as a function of $H_{\mathrm{T}}$ for events with at least one (top left), two (top right), three (bottom left), and four (bottom right) jets compared with sherpa, powheg, and MadGraph predictions. Error bars around the experimental points represent the statistical uncertainty, while cross-hatched bands represent statistical plus systematic uncertainty. The bands around theory predictions correspond to the statistical uncertainty of the generated sample and, for NLO calculations, to its combination with systematic uncertainty related to scale variations.