Measurements of the cross section and branching fraction of $Z\rightarrow 4\ell$ in 7 and 8 TeV pp collisions with the ATLAS detector (YRS)

This talk presents measurements of singly-resonant $Z$ decay to four leptons 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 $M_{\ell^+\ell^-}>4$ GeV and $76 < M_{4\ell} <106$ GeV are measured to be $114\pm27(stat)\pm7(syst)\pm2(lumi)$ at 7 TeV and $150\pm13(stat)\pm7(syst)\pm5(lumi)$ 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 $(4.2\pm0.4)\times10^{-6}$, consistent with the Standard Model prediction of $(4.37\pm0.03)\times10^{-6}$.