Search for doubly charged Higgs boson production in multi-lepton final states with the ATLAS detector using proton–proton collisions at ^√s = 13 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 a proton–proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to 36.1 fb⁻¹ of integrated luminosity recorded in 2015 and 2016 by the ATLAS detector at the LHC. This analysis focuses on the decays H^±± → e^±e^±, H^±± → e^±µ^± and H^±± → µ^±µ^±, fitting the dilepton mass spectra in several exclusive signal regions. No significant evidence of a signal is observed and corresponding limits on the production cross-section and consequently a lower limit on m(H^±±) are derived at 95% confidence level. With `<sup>±</sup>`^± = e^±e^±/µ^±µ^±/e^±µ^±, the observed lower limit on the mass of a doubly charged Higgs boson only coupling to left-handed leptons varies from 770 GeV to 870 GeV (850 GeV expected) for B(H^±± → `<sup>±</sup>`^±) = 100% and both the expected and observed mass limits are above 450 GeV for B(H^±± → `<sup>±</sup>`^±) = 10% and any combination of partial branching ratios.