Measurement of electroweak production of two jets in association with a Z boson in proton-proton collisions at sqrt(s)= 8 TeV

TL;DR

This paper reports a measurement of the purely electroweak Z boson production in association with two jets in proton-proton collisions at sqrt(s)=8 TeV using CMS data. It employs three complementary analyses, including a data-driven background model based on γ+jets events, to isolate the EW Zjj signal from the dominant DY Zjj background and interference effects. The result, sigma_EW(lljj) = 174 ± 15 (stat) ± 40 (syst) fb, is consistent with the SM LO prediction and exceeds 5σ significance, with cross-checks confirming agreement in jet activity and radiation patterns. The analysis presents an enhanced precision over the 7 TeV measurements and provides robust validation of EW Zjj modeling and associated QCD radiation in hadronic final states.

Abstract

The purely electroweak (EW) cross section for the production of two jets in association with a Z boson, in proton-proton collisions at sqrt(s) = 8 TeV, is measured using data recorded by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 19.7 inverse femtobarns. The electroweak cross section for the lljj final state (with l = e or mu and j representing the quarks produced in the hard interaction) in the kinematic region defined by M[ll] > 50 GeV, M[jj] > 120 GeV, transverse momentum pt[j] > 25 GeV, and pseudorapidity abs(eta[j]) < 5, is found to be sigma[EW](lljj) = 174 +/- 15 (stat) +/- 40 (syst) fb, in agreement with the standard model prediction. The associated jet activity of the selected events is studied, in particular in a signal-enriched region of phase space, and the measurements are found to be in agreement with QCD predictions.

Figures (16)

• Figure 1: Representative Feynman diagrams for dilepton production in association with two jets from purely electroweak contributions: (left) vector boson fusion, (middle) bremsstrahlung-like, and (right) multiperipheral production.
• Figure 2: Representative diagrams for order $\alpha_\mathrm{S}^2$ corrections to DY production that comprise the main background (B) in this study.
• Figure 3: Distribution for (left) $Rp_{\mathrm{T}}\xspace^\text{hard}$ and $M_\mathrm{jj}$ for $\mu\mu$ events with (middle) $Rp_{\mathrm{T}}\xspace^\text{hard}\geq0.14$ (control region) and (right) $Rp_{\mathrm{T}}\xspace^\text{hard}<0.14$ (signal region). The contributions from the different background sources and the signal are shown stacked, with data points superimposed. The panels below the distributions show the ratio between the data and expectations as well as the uncertainty envelope for the impact of the uncertainty of the JES.
• Figure 4: Distribution for (left) the difference in the azimuthal angle and (middle) difference in the pseudorapidity of the tagging jets for ee events, with $Rp_{\mathrm{T}}\xspace^\text{hard}\geq0.14$. The $z^*$ distribution (right) is shown for the same category of events. The panels below the distributions show the ratio between the data and expectations as well as the uncertainty envelope for the impact of the uncertainty of the JES.
• Figure 5: Comparison of the $\mathrm{DY}\,{Z}\mathrm{jj}$ distributions with the prediction from the photon control sample, for simulated events with $M_\mathrm{jj}>750\,\text{Ge\spaceV}\xspace$. The upper left subfigure shows the distributions in the pseudorapidity $\eta$ of the most forward tagging jet and the upper right shows the smallest q/g discriminant of the two tagging jets. The lower left shows the pseudorapidity separation $\Delta\eta_\mathrm{jj}$ and the lower right the relative $p_{\mathrm{T}}$ balance of the tagging jets $\Delta^\text{rel}_{p_{\mathrm{T}}\xspace}$. The DY $\gamma\mathrm{jj}$ distribution contains the contribution from prompt and misidentified photons as estimated from simulation and it is compared to the simulated $\mathrm{DY}\,{Z}\mathrm{jj}$ sample in the top panel of each subfigure. The bottom panels show the ratio between the $\mathrm{DY}\,{Z}\mathrm{jj}$ distribution and the photon-based prediction, and includes the different sources of estimated total uncertainty in the background shape from the photon control sample. (See text for specification of impact of loose, tight and pure photons).