Measurement of the $ZZ$ production cross section in $pp$ collisions at $\sqrt{s}$ = 8 TeV using the $ZZ\to\ell^{-}\ell^{+}\ell^{\prime -}\ell^{\prime +}$ and $ZZ\to\ell^{-}\ell^{+}ν\barν$ channels with the ATLAS detector

TL;DR

The ATLAS ZZ measurement at $\sqrt{s}=8$ TeV uses $20.3\,\mathrm{fb}^{-1}$ to extract fiducial and total cross sections in $ZZ\to4\ell$ and $ZZ\to 2\ell2\nu$ channels, obtaining $\sigma_{pp\to ZZ}^{\mathrm{tot}} = 7.3\,{\pm}0.4\,(stat){\pm}0.3\,(syst){\,+0.2}_{-0.1}\,\mathrm{(lumi)}$ pb, consistent with the SM prediction $6.6^{+0.7}_{-0.6}$ pb. Differential cross sections are unfolded for several kinematic observables, showing overall agreement with SM predictions from NLO QCD (PowhegBox+Alioli) and LO gluon-initiated processes (gg2VV), with EW corrections reducing fiducial/total yields by a few percent. The analysis also sets limits on anomalous neutral triple gauge couplings in an effective Lagrangian framework, finding no evidence for new physics and tightening prior constraints. These results provide precise tests of electroweak dynamics and inform background modeling for Higgs and beyond-SM searches at the LHC. The combination of leptonic final states, forward-electron acceptance, and robust background control yields high-precision cross sections and stringent aTGC limits relevant for future collider studies.

Abstract

A measurement of the $ZZ$ production in the $\ell^{-}\ell^{+}\ell^{\prime -}\ell^{\prime +}$ and $\ell^{-}\ell^{+}ν\barν$ channels $(\ell = e, μ)$ in proton--proton collisions at $\sqrt{s} = 8$ TeV at the Large Hadron Collider at CERN, using data corresponding to an integrated luminosity of 20.3 fb$^{-1}$ collected by the ATLAS experiment in 2012 is presented. The fiducial cross sections for $ZZ\to\ell^{-}\ell^{+}\ell^{\prime -}\ell^{\prime +}$ and $ZZ\to \ell^{-}\ell^{+}ν\barν$ are measured in selected phase-space regions. The total cross section for $ZZ$ events produced with both $Z$ bosons in the mass range 66 to 116 GeV is measured from the combination of the two channels to be $7.3\pm0.4\textrm{(stat)}\pm0.3\textrm{(syst)}\pm0.2\textrm{(lumi)}$ pb, which is consistent with the Standard Model prediction of $6.6^{+0.7}_{-0.6}$ pb. The differential cross sections in bins of various kinematic variables are presented. The differential event yield as a function of the transverse momentum of the leading $Z$ boson is used to set limits on anomalous neutral triple gauge boson couplings in $ZZ$ production.

Figures (9)

• Figure 1: Lowest-order Feynman diagrams for $ZZ$ production. The \ref{['fig:ZZtchannel']}$t$-channel and \ref{['fig:ZZuchannel']}$u$-channel diagrams contribute to $ZZ$ production cross section, while the \ref{['fig:ZZschannel']}$s$-channel diagram is not present in the SM, as it contains a neutral $ZZZ$ or $ZZ\gamma$ vertex. Examples of one-loop contributions to $ZZ$ production via gluon pairs are shown in \ref{['fig:ZZgg1']}, \ref{['fig:ZZgg2']} and \ref{['fig:ZZggH']}.
• Figure 2: Kinematic distributions for $ZZ\to\ell^{-}\ell^{+}\ell^{\prime\, -}\ell^{\prime\, +}$ candidates in all four-lepton final states: \ref{['fig:evt_yield_signal_zz4l_leading_zmass_4l']}$m_{\ell^{-}\ell^{+}}^{\mathrm{lead}}$, \ref{['fig:evt_yield_leading_zpt_4l']}$\pT^{Z_{\mathrm{lead}}}$, \ref{['fig:evt_yield_signal_zz4l_zzmass_4l']}$m_{\ell^{-}\ell^{+}\ell^{\prime\, -}\ell^{\prime\, -}}$ and \ref{['fig:evt_yield_signal_zz4l_zzpt_4l']}$\pT^{ZZ}$. The points represent the observed data and the histograms show the expected number of ZZ signal events and the background estimate. The shaded band shows the combined statistical and systematic uncertainties in the prediction and the background. No selection on the leading lepton pair mass is required for \ref{['fig:evt_yield_signal_zz4l_leading_zmass_4l']}, while the full selection is applied for the other distributions.
• Figure 3: The mass of the leading lepton pair versus the mass of the subleading lepton pair. The events observed in the data are shown as solid circles and the $ZZ\to\ell^{-}\ell^{+}\ell^{\prime\, -}\ell^{\prime\, +}$ signal prediction from simulation, normalized to the luminosity of the data, as pink boxes. The size of each box is proportional to the number of events in each bin. The region enclosed in the solid red box indicates the signal region defined by the requirements on the lepton pair masses for $ZZ$ events.
• Figure 4: Kinematic distributions for $ZZ\to \ell^{-}\ell^{+}\nu\bar{\nu}$ candidates in both lepton final states: \ref{['fig:evt_yield_papernoratio_2l2nu_DDperfPlot_ll_mz']}$m_{\ell^{-}\ell^{+}}$, \ref{['fig:evt_yield_papernoratio_2l2nu_DDperfPlot_ll_ptz']}$p_{\mathrm{T}}^{Z}$, \ref{['fig:evt_yield_papernoratio_2l2nu_DDperfPlot_ll_mt']}$m_{\mathrm{T}}^{ZZ}$ and \ref{['fig:papernoratio_2l2nu_DDperfPlot_ll_dphi_unfold']}$\Delta\phi(\ell^{+},\ell^{-})$. The points represent the observed data and the histograms show the expected number of ZZ signal events and the background estimate. The shaded band shows the combined statistical and systematic uncertainties in the prediction and the background. The last bin in \ref{['fig:evt_yield_papernoratio_2l2nu_DDperfPlot_ll_ptz']} and \ref{['fig:evt_yield_papernoratio_2l2nu_DDperfPlot_ll_mt']} distributions, contains the overflow events.
• Figure 5: The ratio of the measured $ZZ$ cross sections in the fiducial phase space to the SM prediction from PowhegBox and gg2VV in each of the five decay modes considered. The ratio between the total combined cross section and the SM prediction is also shown. The inner grey error bars on the data points represent the statistical uncertainties, while the outer black error bars represent the total uncertainties. The green and yellow bands represent the $1\sigma$ and $2\sigma$ uncertainties, respectively, associated with the SM prediction.