A measurement of the $ZZ \rightarrow \ell^{-}\ell^{+}\nu\bar{\nu}$ cross section at 8 TeV and limits on anomalous neutral triple gauge couplings with the ATLAS detector

This thesis presents a measurement of the $ZZ$ diboson production cross section using the dataset from proton-proton collisions at $\sqrt{s} = 8$ TeV collected in 2012 by the ATLAS experiment at the Large Hadron Collider at CERN, corresponding to a total integrated luminosity of $20.3$ fb$^{-1}$. The ATLAS detector and its component subsystems are described, with particular focus on the subsystems which have the largest impact on the analysis. Events are selected by requiring one pair of opposite-sign, same-flavour leptons and large missing transverse momentum, consistent with on-shell $ZZ$ production. Two separate measurements of the $pp \rightarrow ZZ$ cross section are made, first in a restricted ("fiducial") phase space in each of the decay modes $ZZ \rightarrow e^{-}e^{+}\nu\bar{\nu}$ and $ZZ \rightarrow \mu^{-}\mu^{+}\nu\bar{\nu}$ to give: \begin{equation*} \begin{aligned} \sigma_{pp \rightarrow ZZ}^{\mathrm{fid}} & = 4.99\, ^{+0.76}_{-0.71}\;(\mathrm{stat})\, ^{+0.45}_{-0.38}\;(\mathrm{syst})\,^{+0.20}_{-0.15}\;(\mathrm{lumi})\;\, \mathrm{fb} \,\,\,\,\,\, (ZZ \rightarrow e^{-}e^{+}\nu\bar{\nu}) \, , \\ \sigma_{pp \rightarrow ZZ}^{\mathrm{fid}} & = 4.67\, ^{+0.70}_{-0.65}\;(\mathrm{stat})\, ^{+0.45}_{-0.35}\;(\mathrm{syst})\,^{+0.19}_{-0.15}\;(\mathrm{lumi})\;\, \mathrm{fb} \,\,\,\,\,\, (ZZ \rightarrow \mu^{-}\mu^{+}\nu\bar{\nu}).\\ \end{aligned} \end{equation*} A theoretical cross section is calculated using the PowhegBox and gg2VV generators. Using the detector acceptance and reconstruction efficiency extracted in each decay mode, the measurements are then extrapolated to a more inclusive phase space requiring both $Z$ bosons to be in the mass range $66 < m_{\ell{-}\ell^{+}} < 116$ GeV to give two independent measurements of the $pp \rightarrow ZZ$ cross section: \begin{equation*} \begin{aligned} \sigma_{pp \rightarrow ZZ}^{\mathrm{extrapolated}} & = 8.99\, ^{+1.37}_{-1.28}\;(\mathrm{stat})\, ^{+0.95}_{-0.78}\;(\mathrm{syst})\,^{+0.36}_{-0.28}\;(\mathrm{lumi}) \;\, \mathrm{pb} \,\,\,\,\,\, (ZZ \rightarrow e^{-}e^{+}\nu\bar{\nu}) \, , \\ \sigma_{pp \rightarrow ZZ}^{\mathrm{extrapolated}} & = 8.68\, ^{+1.29}_{-1.21}\;(\mathrm{stat})\, ^{+0.94}_{-0.73}\;(\mathrm{syst})\,^{+0.35}_{-0.27}\;(\mathrm{lumi})\;\, \mathrm{pb} \,\,\,\,\,\, (ZZ \rightarrow \mu^{-}\mu^{+}\nu\bar{\nu}). \end{aligned} \end{equation*} Another measurement of the extrapolated cross section is made that incorporates events in two $ZZ \rightarrow \ell^{-}\ell^{+}\nu\bar{\nu}$ ($e^{-}e^{+}\nu\bar{\nu}$, $\mu^{-}\mu^{+}\nu\bar{\nu}$) and three $ZZ \rightarrow \ell^{-}\ell^{+}\ell^{\prime\, -}\ell^{\prime\, +}$ ($e^{-}e^{+}e^{-}e^{+}$, $e^{-}e^{+}\mu^{-}\mu^{+}$, $\mu^{-}\mu^{+}\mu^{-}\mu^{+}$) decay modes to give \begin{equation*} \sigma_{pp \rightarrow ZZ}^{\mathrm{extrapolated}} = 7.30 \,^{+0.38}_{-0.37}\;(\mathrm{stat})\,^{+0.28}_{-0.25}\;(\mathrm{syst})\,^{+0.23}_{-0.20}\;(\mathrm{lumi})\;\, \mathrm{pb}\, . \end{equation*} Finally, limits are set on anomalous neutral $ZZV$ couplings ($V = Z,\gamma$), within an effective Lagrangian framework. The expected signal yield as a function of coupling strength is obtained via a procedure which reweights events produced with aTGCs to events produced by Standard Model processes. Expected and observed limits on these parameters are set at 95% confidence-level, with observed limits in the range \begin{equation*} \begin{aligned} & -0.0047 < f_{4}^{\gamma} < 0.0046,\, -0.0040 < f_{4}^{Z} < 0.0040 \, , \\ & -0.0048 < f_{5}^{\gamma} < 0.0047,\, -0.0040 < f_{5}^{Z} < 0.0041 \,. \end{aligned} \end{equation*}