Search for type-II seesaw mechanism processes with same charge leptons in the final states with the ATLAS detector

A search for pair production of doubly charged Higgs bosons ($H^{\pm \pm}$), each decaying into a pair of prompt, isolated, and 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 an integrated luminosity of 139 fb$^{-1}$ recorded by the ATLAS detector during Run Two of the Large Hadron Collider (LHC). This analysis focuses on same-charge leptonic decays, $H^{\pm \pm} \rightarrow \ell^{\pm} \ell^{\prime \pm}$, where $\ell, \ell^{\prime} = e, \mu, \tau$, in two-, three- and four-lepton channels but only considers final states, which include electrons or muons. No evidence of a signal is observed. Assuming that the branching ratios to each possible leptonic final state are equal, $\mathcal{B}(H^{\pm \pm} \rightarrow e^{\pm} e^{\pm} / e^{\pm} \mu^{\pm} / \mu^{\pm} \mu^{\pm} / e^{\pm} \tau^{\pm} / \mu^{\pm} \tau^{\pm} / \tau^{\pm} \tau^{\pm}) = 1/6$, the corresponding upper limits on the pair production cross-section of doubly charged Higgs bosons are derived as a function of its mass, $m_{H^{\pm \pm}}$, at a 95% confidence level. The limits, which are the strongest produced by the ATLAS Collaboration to date, are obtained for the left-right symmetric type-II seesaw model. Additionally, this work provides the first direct test of the Zee-Babu neutrino mass model at the LHC.