Measurements of Four-Lepton Production at the $Z$ Resonance in $pp$ Collisions at $\sqrt{s}=7$ and 8 TeV with ATLAS

Measurements of four-lepton ($4\ell$, $\ell =e,\mu$) production cross sections at the $Z$ resonance in $pp$ collisions at the LHC with the ATLAS detector are presented. For dilepton and four-lepton invariant mass regions $m_{{\ell }^{+}{\ell }^{-}}>5\mathrm{GeV}$ and $80<m_{4\ell }<100\mathrm{GeV}$, the measured cross sections are $76\pm 18\text{(stat)}\pm 4\text{(syst)}\pm 1.4\text{(lumi)}\mathrm{fb}$ and $107\pm 9\text{(stat)}\pm 4\text{(syst)}\pm 3.0\text{(lumi)}\mathrm{fb}$ at $\sqrt{s}=7$ and 8 TeV, respectively. By subtracting the nonresonant $4\ell$ production contributions and normalizing with $Z\to {\mu }^{+}{\mu }^{-}$ events, the branching fraction for the $Z$ boson decay to $4\ell$ is determined to be $(3.20\pm 0.25\text{(stat)}\pm 0.13\text{(syst)})\times 10^{-6}$, consistent with the standard model prediction.

Figure 1

Examples of (a) $s$-channel and (b) $t$-channel Feynman diagrams for $4\ell$ production in $pp$ collisions.

Figure 2

Data and MC invariant mass distributions of (a) the leading lepton pair, $m_{12}$, (b) the subleading lepton pair, $m_{34}$ , and (c) the four-lepton system, $m_{4\ell }$. All selections are applied except in (c) there is no $m_{4\ell }$ requirement. The background contributes $<1\%$ of the total expected signal (invisible in the plots).