Search for doubly-charged Higgs boson production in multi-lepton final states with the ATLAS detector using proton-proton collisions at $\sqrt{s}=13\,\mathrm{TeV}$

A search for doubly-charged Higgs bosons with pairs of prompt, isolated, highly energetic leptons with the same electric charge is presented. The search uses the $pp$ data sample corresponding to 36.1 fb$^{-1}$ of integrated luminosity collected in 2015 and 2016 by the ATLAS detector at the LHC at the centre-of-mass energy of 13 TeV. The search scans through various doubly-charged Higgs branching ratio ($Br$) hypotheses, where $Br(H^{\pm\pm}\rightarrow e^{\pm}e^{\pm})+Br(H^{\pm\pm}\rightarrow e^{\pm}\mu^{\pm})+Br(H^{\pm\pm}\rightarrow \mu^{\pm}\mu^{\pm}) \leq 100\%$, in several exclusive signal regions. No significant evidence of signal was observed and corresponding limits on the production cross-section and consequently the lower limit on $m(H^{\pm \pm})$ were derived at 95% CL. Defining $\ell={e,\mu}$, the observed lower limit on $H_{L}^{\pm\pm}$ mass varies from 770 GeV to 870 GeV (850 GeV expected) for $Br(H^{\pm\pm}\rightarrow \ell^{\pm}\ell^{\pm})=100\%$ and is still above 450 GeV, for both observed and expected, for $Br(H^{\pm\pm}\rightarrow \ell^{\pm}\ell^{\pm})=10\%$ for any combination of partial branching ratios.