$ZZ \to \ell^{+}\ell^{-}\ell^{\prime +}\ell^{\prime -}$ cross-section measurements and search for anomalous triple gauge couplings in 13 TeV $pp$ collisions with the ATLAS detector

TL;DR

This ATLAS study measures ZZ production in the fully leptonic channel at 13 TeV using 36.1 fb^-1, providing fiducial and total cross sections and 20 differential distributions to test SM predictions. The analysis combines qq̄ and gg-initiated ZZ contributions, validates NNLO QCD and NLO EW calculations, and performs a neutral aTGC search via the leading-Z pT observable, deriving 95% CL limits. No deviations from the SM are observed, and the results strengthen constraints on neutral aTGCs and related EFT operators while offering precise differential ZZ information for future phenomenology and beyond-SM searches.

Abstract

Measurements of $ZZ$ production in the $\ell^{+}\ell^{-}\ell^{\prime +}\ell^{\prime -}$ channel in proton-proton collisions at 13 TeV center-of-mass energy at the Large Hadron Collider are presented. The data correspond to 36.1 $\mathrm{fb}^{-1}$ of collisions collected by the ATLAS experiment in 2015 and 2016. Here $\ell$ and $\ell'$ stand for electrons or muons. Integrated and differential $ZZ \to \ell^{+}\ell^{-}\ell^{\prime +}\ell^{\prime -}$ cross sections with $Z \to \ell^+\ell^-$ candidate masses in the range of 66 GeV to 116 GeV are measured in a fiducial phase space corresponding to the detector acceptance and corrected for detector effects. The differential cross sections are presented in bins of twenty observables, including several that describe the jet activity. The integrated cross section is also extrapolated to a total phase space and to all Standard-Model decays of $Z$ bosons with mass between 66 GeV and 116 GeV, resulting in a value of $17.3 \pm 0.9$ [$\pm 0.6$ (stat.) $\pm 0.5$ (syst.) $\pm 0.6$ (lumi.)] pb. The measurements are found to be in good agreement with the Standard-Model predictions. A search for neutral triple gauge couplings is performed using the transverse momentum distribution of the leading $Z$-boson candidate. No evidence for such couplings is found and exclusion limits are set on their parameters.

Figures (18)

• Figure 1: Examples of leading-order SM Feynman diagrams for $Z\xspace Z\xspace$ production in proton--proton collisions: (a) $\Pquark\APquark$-initiated, (b) $g\xspace g\xspace$-initiated, (c) electroweak $Z\xspace Z\xspace jj$ production, (d) electroweak $Z\xspace Z\xspace jj$ production via weak-boson scattering.
• Figure 2: Example Feynman diagram of $Z\xspace Z\xspace$ production containing an aTGC vertex, here indicated by a red dot, which is forbidden in the SM.
• Figure 3: Invariant mass of one selected $Z$ boson candidate dilepton vs. the other, in the selected data events before the $Z$ boson candidate mass requirement. All other selections have been applied. (a) shows the $Z$ boson candidates arranged by transverse momentum. (b) shows the $Z$ boson candidates arranged by proximity of their mass to the $Z$ boson pole mass. The solid rectangle shows the signal region. Dashed gray lines mark the $Z$ boson candidate mass requirements for each pair, 66 to 116 . Only data are shown.
• Figure 4: Measured distributions of the selected data events along with predictions in bins of (a) the four-lepton mass, (b) the four-lepton transverse momentum, (c) the transverse momentum of the leading $Z$ boson candidate, and (d) the multiplicity of jets selected according to the least restrictive criteria used in this analysis ($|\eta| < 4.5$ and $\pt > 30$). The main prediction uses the nominal setup. The prediction uncertainty includes the statistical and systematic components, all summed in quadrature. Different signal contributions and the background are shown, as is an alternative prediction that uses Powheg + to generate the $\Pquark\APquark$-initiated subprocess. In (a), (b), and (c), the last bin is shown using a different $x$-axis scale for better visualization. The scale change is indicated by the dashed vertical line.
• Figure 5: Comparison of measured integrated fiducial cross sections to a SM prediction based on an NNLO calculation from Matrix with the $g\xspace g\xspace$-initiated contribution multiplied by a global NLO correction factor of $1.67$. A global NLO EW correction factor of $0.95$ is applied, except to the $g\xspace g\xspace$-initiated loop-induced contribution, and the contribution of around 2.5% from EW-$Z\xspace Z\xspace jj$ generated with Sherpa is added. For the prediction, the QCD scale uncertainty is shown as one- and two-standard-deviation bands.