A step towards Supersymmetry? Searching for dark matter at the LHC with the ATLAS detector, using missing transverse energy and leptons

Thesis

This thesis focuses on the search for new physics using the ATLAS detector. The nature of dark matter remains one of the fundamental open questions in modern physics, and the absence of a viable dark matter candidate within the Standard Model (SM) is a significant limitation of the theory. Supersymmetry (SUSY) offers a compelling extension of the SM, providing a natural dark matter candidate while also addressing several other shortcomings. The central content of this thesis covers searches for the direct pair production of charged light-flavour supersymmetric sleptons (selectrons or smuons) each decaying into a stable neutralino (χ˜⁰₁) and an associated Standard Model lepton. The thesis concludes with a study targeting improvements for Run 3, in the trigger used to collect the data for the searches.

The analysis targets the challenging “corridor” region where the mass difference, ∆m, between the slepton and the lightest neutralino is small. This region has historically been difficult to probe, and the analysis aims to close the persistent gap in sensitivity to models with ∆m ≲ m(W). Events are selected by requiring a high-energy jet, significant missing transverse momentum, and two same-flavour opposite-sign leptons (e or µ). The study uses proton–proton collision data collected at √s = 13 TeV with the ATLAS detector, corresponding to an integrated luminosity of 140 fb⁻¹. A series of kinematic selections is applied, including multiple boosted decision trees optimised for different values of ∆m. This approach delivers expected sensitivity across the full ∆m corridor for the first time.

The results are generally consistent with Standard Model expectations, where the largest deviations are observed with local significances of 2.0 σ in the selectron search and 2.4 σ in the smuon search. Discovery significances are calculated for specific SUSY signal models. For the selectron search, a maximum discovery significance of 1.9σ is obtained for a model with selectron mass of 375 GeV, with a neutralino mass of 335 GeV. For the smuon search, a maximum discovery significance of 2.1σ is obtained for a model with smuon mass of 150 GeV, with a neutralino mass of 140 GeV. Although these excesses weaken the observed exclusion reach in some regions of parameter space, the previously unaddressed gap in sensitivity in this corridor region is substantially reduced. At 95% confidence level, constraints are set on simplified models of selectron and smuon pair production, with selectron (smuon) masses up to 300 (350) GeV excluded for ∆m values between 2 GeV and 100 GeV.

Authors: Ruggiero, A

• DOI: 10.5287/ora-9eo7v8a9p
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: particle physics; dark matter; supersymmetry; ATLAS
• Subjects: Physics