Search for doubly charged Higgs boson production in multi-lepton final states using 139 fb⁻¹ of proton-proton collisions at √s = 13 TeV with the ATLAS detector

A search for pair production of doubly charged Higgs bosons (H^±±), each decaying into a pair 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 an integrated luminosity of 139 fb⁻¹ recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. This analysis focuses on same-charge leptonic decays, H^±±→ℓ^±ℓ′^± where ℓ,ℓ′=e,μ,τ, 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^±±) are derived at 95% confidence level. Assuming that the branching ratios to each of the possible leptonic final states are equal, B(H^±±→e^±e^±)=B(H^±±→e^±μ^±)=B(H^±±→μ^±μ^±)=B(H^±±→e^±τ^±)=B(H^±±→μ^±τ^±)=B(H^±±→τ^±τ^±)=1/6, the observed lower limit on the mass of a doubly charged Higgs boson is 1080 GeV within the left-right symmetric type-II seesaw model, which is an improvement over previous limits. Additionally, a lower limit of m(H^±±) = 900 GeV is obtained in the context of the Zee-Babu neutrino mass model.