Search for doubly-charged Higgs boson in multi-lepton final states using 36.1 fb$^{-1}$ with ATLAS at $\sqrt{s}$ = 13 TeV

A search for new high mass resonances which decay to two high-$p_{\rm T}$ leptons with same-sign charge is presented. The results reported here use the \textit{pp} collision data sample corresponding to 36.5 fb$^{-1}$ of integrated luminosity collected in 2015 and 2016 by the ATLAS detector at the LHC with a centre-of-mass energy of 13 TeV. We choose the pair production of the doubly charged Higgs boson (DCH) via the Drell-Yan process as a benchmark model, because we regard it as the main production mode for this new particle at the LHC. The decay mode of the DCH was assumed to be exclusively into leptons. \\ Leptonic final states are very interesting since they provide a good sensitivity and small systematic uncertainties. In addition, requiring same-sign leptonic final states reduces the background contamination from Standard Model processes while providing a large sensitivity to BSM phenomena, such as $H^{\pm\pm}$ production. This BSM particle is motivated in the picture of left-right symmetric models, which are appealing because they are able to give an explanation on two points left unanswered by the Standard Model: parity violation and neutrino mass origins. The hypothesis assumes the existence of a higher energy scale symmetry of the type $SU(2)_L\times SU(2)_R \times U(1)_{B-L}$, subsequently broken into the Standard Model electroweak symmetry $SU(2)_L \times U(1)_Y$. The Higgs scalar sector needs to be extended by the addition of a Higgs triplet. The initial symmetry is spontaneously broken by the non-vanishing vacuum expectation value acquired by the new Higgs field, which generates both Dirac and Majorana neutrino mass terms.\\ A model independent search will also be performed on distinct event categories, defined by the number of same-sign lepton pairs. This search will focus on signal regions definition and on the strategy for the estimation of the main backgrounds: mis-identified (fake) prompt leptons, originating from either hadronic jets or secondary weak hadron decays, and electrons with mis-identified charge. The search will be performed in all light lepton flavor channels, allowing also the doubly charged Higgs to decay to mixed lepton flavor final states via lepton number violation. A limit on the mass of the doubly charged Higgs boson, for both left-handed and right-handed particle states, will be set.