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 (cid:48)± where ℓ, ℓ (cid:48) = 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.

A search is performed for delayed and nonpointing photons originating from the displaced decay of a neutral long-lived particle (LLP). The analysis uses the full Run 2 data set of proton-proton collisions delivered by the LHC at a center-of-mass energy of $\sqrt{s}=13$ TeV between 2015 and 2018 and recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb$^{-1}$. The capabilities of the ATLAS electromagnetic calorimeter are exploited to precisely measure the arrival times and trajectories of photons. The results are interpreted in a scenario where the LLPs are pair-produced in exotic decays of the 125 GeV Higgs boson, and each LLP subsequently decays into a photon and a particle that escapes direct detection, giving rise to missing transverse momentum. No significant excess is observed above the expectation due to Standard Model background processes. The results are used to set upper limits on the branching ratio of the exotic decay of the Higgs boson. A model-independent limit is also set on the production of photons with large values of displacement and time delay.