Speaker
Description
Motivated by multi-lepton anomalies reported in the literature, which suggest the possible existence of a new scalar resonance with a mass around 150~$\pm$~2 GeV, this analysis searches for resonances decaying into two photons produced in association with leptons. Phenomenological studies indicate a significant excess in the diphoton spectrum at $152\pm 2$ GeV, as well as deviations in final states with multiple leptons, moderate missing transverse energy, and bottom-quark jets. These observations can be interpreted within extensions of the Standard Model such as the 2HDM+S, in which a heavy Higgs boson decays into lighter scalar states that subsequently produce diphoton and multi-lepton signatures.
The analysis uses ATLAS Run 3 proton-proton collision data at $\sqrt{s}=13.6$ TeV, corresponding to an integrated luminosity of 164 fb$^{-1}$. While previous ATLAS diphoton resonance searches have explored a broad mass range, the region between 130 and 200 GeV remains relatively unexplored for diphoton final states produced in association with leptons. This analysis therefore targets this mass window, providing sensitivity to complementary signatures of potential new physics. For this conference, the analysis is restricted to leptonic signatures and considers three exclusive channels: events with zero electrons or muons and exactly one $\tau$ lepton (0$(e,\mu)$, 1$\tau$), events with exactly one electron and no $\tau$ leptons or muons (1$e$, 0$(\tau,\mu)$), and events with exactly one muon and no $\tau$ leptons or electrons (1$\mu$, 0$(\tau,e)$).
The analysis strategy is designed to remain as model independent as possible, with the goal of setting upper limits on the production cross section times branching ratio for narrow resonances in diphoton-plus-lepton final states, and represents a first step in a broader program that will extend to additional final states, including dilepton, lepton + $b$-jet, and missing transverse energy channels, providing a foundation for testing the reported anomalies using Run 3 ATLAS data and probing potential new scalar resonances in a previously underexplored region of phase space.