ATLAS measurements of the 7 and 8 TeV cross sections for $Z\rightarrow 4\ell$ in pp collisions

This note presents measurements of singly-resonant $Z$ decay to four leptons ($e^+e^-e^+e^-$, $\mu^+\mu^-\mu^+\mu^-$, and $e^+e^- \mu^+\mu^-$ ) in proton-proton collisions at 7 TeV and 8 TeV with the ATLAS detector. The single resonant peak from $Z \rightarrow 4\ell$ provides a practical test of the detector response to a four-lepton resonance similar to the Higgs boson decay $H \rightarrow ZZ^* \rightarrow 4\ell$. Data samples of proton-proton collisions with integrated luminosities of 4.6 fb$^{-1}$ at $\sqrt{s} = 7$ TeV and 20.7 fb$^{-1}$ at $\sqrt{s} = 8$ TeV are used to select 21 and 141 candidate events, respectively. The background contamination is estimated to be less than 1%. The cross sections in the phase-space defined by dilepton and four-lepton invariant masses of $M_{\ell^+\ell^-}\ge 4$ GeV and $76 \le M_{4\ell} \le 106$ GeV are measured to be $114 \pm 27\text{ (stat)} \pm 7\text{ (syst)} \pm 2\text{ (lumi)}$ fb at 7 TeV and $150 \pm 13\text{ (stat)} \pm 7\text{ (syst)} \pm 5\text{ (lumi)}$ fb at 8 TeV. A branching fraction for the $Z$ boson decay to $4\ell$ in the above phase-space is derived from the cross-section measurements to be $(4.2\pm 0.4)\times 10^{-6}$, consistent with the Standard Model prediction of $(4.37 \pm 0.03) \times 10^{-6}$.