Search for compressed electroweak SUSY production in final states with two $τ$-leptons in $\sqrt{s} = 13$ and $13.6$ TeV $pp$ collisions with the ATLAS detector

Abstract

Searches for the production of $τ$-sleptons ($\tildeτ$), charginos ($\tildeχ^{\pm}_1$), and neutralinos ($\tildeχ^0_2$) in events with at least one hadronically decaying $τ$-lepton are presented, targeting compressed mass spectra scenarios. For charginos and neutralinos, the cascade decays producing either a $τ$-slepton or a $τ$-sneutrino are considered. Two final selections are studied, requiring either one hadronically decaying $τ$-lepton and one other charged lepton, or two or more hadronically decaying $τ$-leptons. To enhance sensitivity to the compressed mass spectra, production modes with initial-state radiation (ISR) jets are considered. The searches use 140 $\mathrm{fb}^{-1}$ and 53 $\mathrm{fb}^{-1}$ of proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider during 2015-2018 at a centre-of-mass energy of $\sqrt{s} = 13$ TeV and 2022-2023 at a centre-of-mass energy of $\sqrt{s} = 13.6$ TeV, respectively. The searches provide sensitivity to the compressed mass spectra scenarios with light $τ$-sleptons. The results are generally consistent with the Standard Model, although several deviations with local significances of $2.1-2.4 σ$ are observed. These deviations weaken the observed exclusion limits. For direct $τ$-slepton production, in the ISR channel alone the limits are set for $Δm(\tildeτ, \tildeχ^0_1) = 30$ GeV for $τ$-slepton masses up to $115$ GeV at 95% confidence level. Given the observed data, no exclusion limits are set for direct $τ$-sleptons production when statistically combining all considered channels. For electroweakino production, limits are set for $Δm(\tildeχ^0_2, \tildeχ^0_1) = 30$ GeV for $\tildeχ^0_2$ masses up to $200$ GeV. For pure chargino pair-production, limits are set for chargino masses up to $175$ GeV for the same mass splitting.