Search for doubly charged Higgs boson production in multi-lepton final states using $139\,\text{fb}^{-1}$ of proton--proton collisions at $\sqrt{s}= 13\,\text{TeV}$ with the ATLAS detector

A search for pair production of doubly charged Higgs ($H^{\pm \pm}$) bosons, 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 139 fb$^{-1}$ of integrated luminosity recorded during the Run 2 of the Large Hadron Collider by the ATLAS detector. 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. Corresponding limits on the production cross-section and consequently a lower limit on $m(H^{\pm \pm})$ are derived at 95% confidence level. Under the assumption that the branching ratios to each of the possible leptonic final states are equal, $\mathcal{B}(H^{\pm \pm} \rightarrow e^\pm e^\pm) = \mathcal{B}(H^{\pm \pm} \rightarrow e^\pm \mu^\pm) = \mathcal{B}(H^{\pm \pm} \rightarrow \mu^\pm \mu^\pm) = \mathcal{B}(H^{\pm \pm} \rightarrow e^\pm \tau^\pm) = \mathcal{B}(H^{\pm \pm} \rightarrow \mu^\pm \tau^\pm) = \mathcal{B}(H^{\pm \pm} \rightarrow \tau^\pm \tau^\pm) = 1/6$, the observed lower limit on the mass of a doubly charged Higgs boson is 1080 GeV, which represents an improvement over previous limits.