Search for long-lived charginos and $\tau$-sleptons using final states with a disappearing track in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

TL;DR

This study searches for long-lived supersymmetric particles like charginos and tau-sleptons using disappearing track signatures in 13 TeV proton-proton collision data from ATLAS, setting new mass limits based on no observed excess.

Contribution

It introduces a novel search strategy combining machine learning and data-driven background estimation to improve sensitivity to long-lived supersymmetric particles.

Findings

No significant excess observed in data.

Mass limits up to 880 GeV for charginos with ~1 ns lifetime.

Exclusion of tau-sleptons up to 320 GeV for similar lifetimes.

Abstract

This paper reports a search for decays of long-lived charginos or $\tau$-sleptons to final states containing a short disappearing track, a single high-energy jet, and missing transverse momentum. The search uses 137 fb$^{-1}$ of data from 13 TeV proton-proton collisions recorded by the ATLAS detector during Run 2 of the LHC. Multiple search regions are defined, all requiring the presence of a track reconstructed from either three or four measurements in the innermost layers of the ATLAS detector. Regions with tracks having only three measurements are further characterised by the absence or presence of a low-energy charged pion reconstructed using a dedicated algorithm, leveraging machine learning. Data-driven methods are used to estimate the background contributions in the search regions. No significant excesses are found and 95% CL lower limits are placed on the masses of charginos and $\tau$-sleptons in the lifetime range $0.01{-}10$ ns. Observed (expected) mass limits of up to 225 GeV (250 GeV) are set for pure-higgsino charginos in scenarios with lifetimes below 0.03 ns, where the electroweakino mass splitting is entirely due to loop corrections involving the Standard Model bosons, and up to 720 GeV (840 GeV) for charginos with a lifetime of around 1 ns. For wino production, charginos with masses up to 880 GeV (1020 GeV) are excluded for lifetimes of around 1 ns. For $\tau$-sleptons with lifetimes of around 1 ns, masses are excluded up to 320 GeV (390 GeV) in Constrained Minimal Supersymmetric Standard Model scenarios and 300 GeV (380 GeV) in Gauge-Mediated Supersymmetry-Breaking scenarios.