Studies of Z boson decays to four leptons in pp collisions at $\sqrt{s} = 8 \text{ TeV}$ with the ATLAS detector at the LHC

This thesis presents measurements of the fiducial and total cross-section of the $Z \to \ell^+ \ell^- \ell^+ \ell^-$ process ($\ell = e^{\pm},\mu^{\pm}$) at $\sqrt{s} = 8 \text{ TeV}$ using the ATLAS 2012 dataset ($\mathcal{L} = 20.3 \text{ fb}^{-1}$). This process provides a practical cross-check of detector response to a four-lepton resonance in a regime similar to that of the newly discovered Higgs boson in the $H \to ZZ^{(*)} \to \ell^+ \ell^- \ell^+ \ell^-$ channel. The total $Z \to \ell^+ \ell^- \ell^+ \ell^-$ cross-section in a phase space defined by $80 \text{ GeV} < M_{4\ell} < 100 \text{ GeV}$ and $M_{\ell^+\ell^-} \geq 4 \text{ GeV}$ is measured as $\sigma_{\text{PS}} = 149 \pm 15 \text{ (stat.)} \pm 15 \text{ (syst.)} \pm 4 \text{ (lumi.) fb}$. The branching ratio, $BR(Z \to \ell^+ \ell^- \ell^+ \ell^-)$, is measured as $(4.6 \pm 0.6) \times 10^{-6}$, which agrees with the Standard Model prediction of $(4.50 \pm 0.03) \times 10^{-6}$. A proposal for extending the phase space is presented and assessed, and analytical fits to the $Z \to \ell^+ \ell^- \ell^+ \ell^-$ peak in this extended regime are presented.