Evidence of $ZZγ$ production with the ATLAS detector

Abstract

This paper presents the first evidence of the simultaneous production of two $Z$ bosons and one photon with the ATLAS detector at the Large Hadron Collider. The measurement is performed using the full Run-2 dataset, recorded from 2015 to 2018, of proton-proton collisions at a center-of-mass energy of $13$ TeV, corresponding to an integrated luminosity of $140$ fb$^{-1}$. The fully leptonic final state with four leptons and one photon is analyzed, $pp\rightarrow ZZγ\rightarrow\ell^+\ell^-\ell'^+\ell'^-γ$ with $\ell, \ell' = e$ or $μ$. This final state is measured in a fiducial region where photon final-state radiation is minimized, and the photon has a transverse momentum of $p_{\mathrm{T}}^γ> 20$ GeV. Eight events are selected, with a background estimate of $0.92\pm0.15$. This results in an observed (expected) significance of the $ZZγ$ final state of $4.4σ$ ($4.4σ$). The measured cross-section for $pp\rightarrow ZZγ\rightarrow\ell^+\ell^-\ell'^+\ell'^-γ$ in the fiducial region is $σ_{ZZγ} = 0.144 _{-0.051}^{+0.064} \text{ (stat.)} _{-0.005}^{+0.007} \text{ (syst.) fb}$, in agreement with the predicted Standard Model one, $σ_{\textrm{fid.}}^{\textrm{SM}} = 0.143 _{-0.004}^{+0.007}$ fb.

Figures (2)

• Figure 1: Typical Feynman diagrams producing an $\ell\ell\ell\ell\gamma$ final state for (a) the triboson production, (b) the $Z$-Higgs production, and (c) the $ZZ$ production with final-state radiation
• Figure 2: Data are compared with the signal and background expectations in the SR. In (a) the event yields are shown in the $4e$, $4\mu$, $2e2\mu$ and $4\ell$ channel, where the $4\ell$ channel is the sum of the other three channels, as distinguished by the dashed line. Distribution of (b) the photon transverse momentum $p_{\mathrm{T}}^\gamma\xspace$, (c) the four-lepton-plus-photon invariant mass $m_{4\ell\gamma}$, and (d) the dilepton invariant mass $m_{\ell\ell}$, which includes entries from the leading and sub-leading $Z$ boson.