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

The ATLAS collaboration
Aaboud, Morad , Aad, Georges , Abbott, Brad , Abdinov, Ovsat , Abeloos, Baptiste , Abidi, Syed Haider , AbouZeid, Ossama , Abraham, Nicola , Abramowicz, Halina , Abreu, Henso
Eur.Phys.J.C 78 (2018) 199, 2018.

Abstract (data abstract)
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 proton-proton collision data at the center-of-mass energy of 13 TeV data sample corresponding to 36.1 $\mathrm{fb}^{-1}$ of integrated luminosity recorded in 2015 and 2016 by the ATLAS detector at the LHC. This analysis focuses on the decays $H^{\pm\pm}\to e^{\pm}e^{\pm}$, $H^{\pm\pm}\to e^{\pm}\mu^{\pm}$ and $H^{\pm\pm}\to \mu^{\pm}\mu^{\pm}$, fitting the dilepton mass spectra in several exclusive signal regions. No significant evidence of a signal was observed and corresponding limits on the production cross-section and consequently the lower limit on $m(H^{\pm \pm})$ were derived at $95\%$ confidence level. With $\ell={e,\mu}$, the observed lower limit on the mass of a doubly-charged Higgs only coupling to left-handed leptons varies from 770 GeV to 870 GeV (850 GeV expected) for $Br(H^{\pm\pm}\to \ell^{\pm}\ell^{\pm})=100\%$ and both the expected and observed mass limits are still above 450 GeV for $Br(H^{\pm\pm}\to \ell^{\pm}\ell^{\pm})=10\%$ for any combination of partial branching ratios.

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