Measurement of the pp to ZZ production cross section and constraints on anomalous triple gauge couplings in four-lepton final states at sqrt(s) = 8 TeV

TL;DR

This CMS study measures inclusive ZZ production in pp collisions at sqrt(s)=8 TeV using the ZZ→llll' (l=e,μ; l′=e,μ,τ) final states with 19.6 fb^-1. It reports a total cross section σ(pp→ZZ) = 7.7 ± 0.5 (stat) +0.5−0.4 (syst) ±0.4 (theo) ±0.2 (lumi) pb for 60 < mZ < 120 GeV, with differential distributions consistent with SM predictions. The analysis constrains anomalous ZZZ and ZZγ triple gauge couplings through a 95% CL study of the four-lepton mass spectrum, yielding tight one-dimensional limits on f4^Z, f5^Z, f4^γ, and f5^γ values. The results, based on a large CMS data sample at 8 TeV, strengthen SM validation in the electroweak sector and substantially tighten limits on potential new physics in gauge boson self-interactions.

Abstract

A measurement of inclusive ZZ production cross section and constraints on anomalous triple gauge couplings in proton-proton collisions at sqrt(s) = 8 TeV are presented. A data sample, corresponding to an integrated luminosity of 19.6 inverse femtobarns was collected with the CMS experiment at the LHC. The measurements are performed in the leptonic decay modes ZZ to lll'l', where l = e, mu and l' = e, mu, tau. The measured total cross section, sigma(pp to ZZ) = 7.7 +/- 0.5 (stat.) +0.5-0.4 (syst.) +/- 0.4 (theo.) +/- 0.2 (lum.) pb for both Z bosons produced in the mass range 60 < m[Z] < 120 GeV, is consistent with standard model predictions. Differential cross sections are measured and well described by the theoretical predictions. The invariant mass distribution of the four-lepton system is used to set limits on anomalous ZZZ and ZZ gamma couplings at the 95% confidence level: -0.004 < f[4,Z] < 0.004, -0.004 < f[5,Z] < 0.004, -0.005 < f[4,gamma] < 0.005, and -0.005 < f[5,gamma] < 0.005.

Figures (6)

• Figure 1: Distribution of the reconstructed four-lepton mass for the (upper left) $4\mathrm{e}$, (upper right) $4{\mu}$, (lower left) $2\mathrm{e}2{\mu}$, and (lower right) combined $\ell\ell\tau\tau$ decay channels. The data sample corresponds to an integrated luminosity of 19.6$\,\text{fb}^\text{$-$1}$. Points represent the data, the shaded histograms labeled ${Z}{Z}$ represent the powheg+gg2zz+pythia predictions for ${Z}\xspace{Z}\xspace$ signal, the histograms labeled $\mathrm{W}{Z}/{Z}$+jets show the background, which is estimated from data, as described in the text.
• Figure 2: (upper left) Distribution of the reconstructed four-lepton mass for the sum of the $4\mathrm{e}$, $4{\mu}$, and $2\mathrm{e}2{\mu}$ decay channels. (upper right) Reconstructed ${Z}_1$ mass. The correlation between the reconstructed ${Z}\xspace_1$ and ${Z}\xspace_2$ masses for the (lower left) combined $4\mathrm{e}$, $4{\mu}$, and $2\mathrm{e}2{\mu}$ final states and (lower right) for $\ell\ell\tau\tau$ final states. Points represent the data, the shaded histograms labeled ${Z}{Z}$ represent the powheg+gg2zz+pythia predictions for ${Z}\xspace{Z}\xspace$ signal, the histograms labeled $\mathrm{W}{Z}/{Z}$+jets show background, which is estimated from data, as described in the text.
• Figure 3: Differential cross sections normalized to the fiducial cross section for the combined 4$\mathrm{e}$, 4${\mu}$, and 2$\mathrm{e}$2${\mu}$ decay channels as a function of $p_{\mathrm{T}}\xspace$ for (upper left) the highest $p_{\mathrm{T}}$ lepton in the event, (upper right) the ${Z}_1$, and (lower left) the ${Z}{Z}$ system. Figure (lower right) shows the normalized ${d}\sigma/{d}{m_{{Z}{Z}}}$ distribution. Points represent the data, and the shaded histograms labeled ${Z}{Z}$ represent the powheg+gg2zz+pythia predictions for ${Z}\xspace{Z}\xspace$ signal, while the solid curves correspond to results of the mcfm calculations. The bottom part of each subfigure represents the ratio of the measured cross section to the expected one from powheg+gg2zz+pythia (black crosses with solid symbols) and mcfm (red crosses). The shaded areas on all the plots represent the full uncertainties calculated as the quadrature sum of the statistical and systematic uncertainties, whereas the crosses represent the statistical uncertainties only.
• Figure 4: Differential cross section normalized to the fiducial cross section for the combined 4$\mathrm{e}$, 4${\mu}$, and 2$\mathrm{e}$2${\mu}$ decay channels as a function of (top) azimuthal separation of the two ${Z}$ bosons and (bottom) $\Delta R$ between the ${Z}$-bosons. Points represent the data, and the shaded histograms labeled ${Z}{Z}$ represent the powheg+gg2zz+pythia predictions for ${Z}\xspace{Z}\xspace$ signal, while the solid curves correspond to results of the mcfm calculations. The bottom part of each subfigure represents the ratio of the measured cross section to the expected one from powheg+gg2zz+pythia (black crosses with solid symbols) and mcfm (red crosses). The shaded areas on all the plots represent the full uncertainties calculated as the quadrature sum of the statistical and systematic uncertainties, whereas the crosses represent the statistical uncertainties only.
• Figure 5: Distribution of the four-lepton reconstructed mass for the combined $4\mathrm{e}$, 4${\mu}$, and $2\mathrm{e}2{\mu}$ channels. Points represent the data, the shaded histogram labeled ${Z}{Z}$ represents the powheg+gg2zz+pythia predictions for ${Z}\xspace{Z}\xspace$ signal, the histograms labeled $\mathrm{W}{Z}/{Z}$+jets shows background, which is estimated from data, as described in the text. The dashed and dotted histograms indicate the results of the sherpa simulation for the SM ($f_4^{{Z}} = 0$) and in the presence of an ATGC ($f_4^{{Z}}=0.015$) with all the other anomalous couplings set to zero. The last bin includes all entries with masses above 1000$\,\text{Ge\spaceV}$.