Measurement of the 4l Cross Section at the Z Resonance and Determination of the Branching Fraction of Z->4l in pp Collisions at sqrt(s) = 7 and 8 TeV with ATLAS

Abstract

Measurements of four-lepton (4$\ell$, $\ell=e,μ$) 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 region $m_{\ell^+\ell^-} > 5$ GeV and $80 < m_{4\ell} < 100$ GeV, the measured cross sections are $76 \pm 18 \text { (stat) } \pm 4 \text { (syst) } \pm 1.4 \text { (lumi) }$ fb and $107 \pm 9 \text{ (stat) } \pm 4 \text{ (syst) } \pm 3.0 \text { (lumi) }$ fb at $\sqrt s$ = 7 and 8 TeV, respectively. By subtracting the non-resonant 4$\ell$ production contributions and normalizing with $Z\rightarrow μ^+μ^-$ events, the branching fraction for the $Z$ boson decay to $4\ell$ is determined to be $\left( 3.20 \pm 0.25\text{ (stat)} \pm 0.13\text{ (syst)} \right) \times 10^{-6}$, consistent with the Standard Model prediction.

Figures (2)

• Figure 1: Examples of (a) $s$-channel and (b) $t$-channel Feynman diagrams for 4$\ell$ production in $pp$ collisions.
• Figure 2: Invariant mass distributions of (a) the leading lepton pair, $m_{12}$, (b) the subleading lepton pair, $m_{34}$ , and (c) the four-lepton system, $m_{4\ell}$. The MC simulation expectation for a combination of all channels is compared to $\sqrt{s}$ = 7 and 8 Te V data. 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).