Measurement of the hadronic activity in events with a Z and two jets and extraction of the cross section for the electroweak production of a Z with two jets in pp collisions at sqrt(s) = 7 TeV

TL;DR

The paper reports the first measurement of the electroweak production cross section for Z bosons with two jets in 7 TeV pp collisions, using CMS data and two independent jet reconstructions. By combining a dijet-mass fit and a multivariate analysis, the authors extract an EW Zjj cross section of 154 fb (per ll flavor) with detailed systematic breakdowns and find good agreement with NLO SM predictions. In addition to the cross section, the study characterizes hadronic activity in the rapidity interval between tagging jets and validates modeling of radiation patterns in Z+jets events, reinforcing the groundwork for vector boson fusion and Higgs-related analyses. The results demonstrate a significant, albeit modest (2.6σ), signal presence and establish methods to probe EW processes in VBF-like topology at the LHC.

Abstract

The first measurement of the electroweak production cross section of a Z boson with two jets (Zjj) in pp collisions at sqrt(s) = 7 TeV is presented, based on a data sample recorded by the CMS experiment at the LHC with an integrated luminosity of 5 inverse femtobarns. The cross section is measured for the lljj (l = e, mu) final state in the kinematic region m[ll] >50 GeV, m[jj] > 120 GeV, transverse momenta pt[j] > 25 GeV and pseudorapidity abs(eta[j]) < 4.0. The measurement, combining the muon and electron channels, yields sigma = 154 +/- 24 (stat.) +/- 46 (exp. syst.) +/- 27 (th. syst.) +/- 3 (lum.) fb, in agreement with the theoretical cross section. The hadronic activity, in the rapidity interval between the jets, is also measured. These results establish an important foundation for the more general study of vector boson fusion processes, of relevance for Higgs boson searches and for measurements of electroweak gauge couplings and vector boson scattering.

Figures (12)

• Figure 1: Representative diagrams for EW $\ell\ell\Pq\Pq'$ production (for $\ell$$=$$\Pgm$): VBF (left), bremsstrahlung (middle), and multiperipheral (right).
• Figure 2: Representative diagram for Drell--Yan production in association with two jets.
• Figure 3: Distribution of the absolute difference in the pseudorapidity of the tagging jets, $\Delta\eta_{{\mathrm{j}\xspace_{1}\mathrm{j}\xspace_{2}}} = \abs{\eta_{{\mathrm{j}\xspace_{1}}} - \eta_{{\mathrm{j}\xspace_{2}}}}$ (left) and the tagging jet $\pt$ for both jets, $\mathrm{j}\xspace_{1}$ and $\mathrm{j}\xspace_{2}$ (right) for the DY $\Pgm\Pgm \mathrm{j}\xspace\mathrm{j}\xspace$, EW $\Pgm\Pgm \mathrm{j}\xspace\mathrm{j}\xspace$, and VBF Higgs boson production processes.
• Figure 4: The $\pt^{\mathrm{j}\xspace_{1}}$ (left) and $\pt^{\mathrm{j}\xspace_{2}}$ (right) distributions after applying the $\cPZ_{\Pgm\Pgm}$ selection and the jet tagging requirement TJ1. The expected contributions from the signal and background processes are evaluated from simulation. The bottom panels show the ratio of data over the expected contribution of the signal plus background. The region between the two lines with the labels JES Up and JES Down shows the 1 $\sigma$ uncertainty of the simulation prediction due to the jet energy scale uncertainty. The data points are shown with the statistical uncertainties.
• Figure 5: The absolute difference in the pseudorapidity of the two tagging jets (left), and the dimuon $\pt$ (right) after the $\cPZ_{\Pgm\Pgm}$ selection and the tagging jet requirement TJ1. The expected contributions from the signal and background processes are evaluated from simulation. The bottom panels show the ratio of data over the expected contribution of the signal plus background. The region between the two lines with the labels JES Up and JES Down shows the 1$\,\sigma$ uncertainty of the simulation prediction due to the jet energy scale uncertainty. The data points are shown with the statistical uncertainties.