Search for dark matter in association with a leptonically decaying $Z$ boson in the ATLAS detector at the Large Hadron Collider

This dissertation describes a search for the invisible decays of dark matter particles produced in association with a $Z$ boson, where the latter decays to a charged lepton pair. The dataset for this search includes 13.3 fb$^{-1}$ of collisions recorded in 2015 and 2016 at a centre-of-mass energy of 13 TeV in the ATLAS detector at the Large Hadron Collider in Geneva, Switzerland. The invisible particles manifest themselves as missing transverse momentum, or MET, in the detector, while the charged leptons of interest are electron ($e^+e^-$) or muon ($\mu^+\mu^-$) pairs. The models simulated for this study are vector mediated simplified models with Dirac fermionic dark matter particles with couplings $g_q$ = 0.25, $g_{\chi}$ = 1 and $g_{\ell} = 0$ . The main background to this analysis, $ZZ\rightarrow\ell^+\ell^-\nu\bar{\nu}$, is irreducible, as it shares the same signature as the signal. It is estimated with Monte Carlo simulations including contributions from both $qq\rightarrow ZZ$ and $gg\rightarrow ZZ$ production modes. Where possible, other backgrounds are estimated using data-driven techniques and reduced through various selection criteria. The final search is performed by looking for a deviation from the Standard Model background expectation in the MET distribution using two signal regions, $ee$ and $\mu\mu$. This is done using statistical tools to make a likelihood fit and set a 95$\%$ confidence level limit as no deviations are found. Limits are placed on the presented model of dark matter for mediator masses up to 400 GeV and for a range of dark matter masses from 1 to $\sim$ 200 GeV.