Search for the Slepton Cascade Decay using Final States with Opposite or Same Sign Three Leptons in the LHC-ATLAS experiment

The Standard Model of particle physics (SM) provides a remarkably successful framework for describing nearly all known phenomena in particle physics. Despite its successes, the SM leaves several fundamental issues unresolved, such as the fine-tuning problem associated with the Higgs boson mass, the nature of dark matter, and the muon g − 2 anomaly, among others. The Minimal Supersymmetric Standard Model (MSSM) is one of the most compelling extensions of the SM. Supersymmetry (SUSY) posits a symmetry between bosons and fermions and predicts the existence of new partner particles, referred to as “superpartners”, for every SM particle. Although no direct evidence for SUSY particles has been observed in collider experiments to date, ongoing searches at the Large Hadron Collider (LHC), which operates at high center-of- mass energies and accumulates large data sets, continue to probe the phase space for heavier and rarer SUSY particles. To account for the muon g − 2 anomaly within the SUSY framework, at least three SUSY particles must have sufficiently light masses, typically of the order of 100 GeV.
A promising model that addresses both the muon g −2 anomaly and a portion of the dark matter abundance involves a specific mass hierarchy where the light-flavored left-handed sleptons ( $\widetilde{l}_{L}; \widetilde{e}_{L} or \widetilde{\mu }_{L}$), along with a nearly degenerate sneutrino ($\widetilde{\nu}$), are the heaviest states, while the bino- dominated neutralino ( $\widetilde{\chi}_{0}^{3}$) occupies an intermediate mass range, and the Higgsino consists the lightest SUSY particle ( $\widetilde{\chi}_{0}^{1}$). Due to the small coupling between sleptons and the Higgsino, determined basically by the Yukawa coupling, sleptons predominantly undergo cascade decays into $\widetilde{\chi}_{0}^{1}$ via $\widetilde{\chi}_{0}^{3}$ in collider experiments. This characteristic decay pattern plays a crucial role in experimental searches for this model.

This thesis presents a search for the cascade decay of left-handed sleptons and sneutrinos using $140 fb^{1}$ of proton-proton collision data at $\sqrt{s} = 13 TeV$, collected by the ATLAS detector. The search focuses on events characterized by a final state with exactly three leptons. Two distinct analysis strategies are employed: one requiring a same-flavor opposite-sign lepton pair with a large transverse mass and significant missing transverse energy (SROS), and the other requiring three leptons with the same electric charge (SRSS). These strategies are optimized to enhance the sensitivity to targeted SUSY signals while suppressing the SM background contributions. The phase space probed by these strategies has not been explored in previous LHC analyses, making this search a novel contribution to SUSY studies. Strategies for estimating the dominant backgrounds have been developed and rigorously validated. In particular, a unique background estimation method is developed specifically for regions containing two or more muons in the SRSS category.

The observed event yields, after applying the optimized selection criteria, are carefully compared to the expected background and signal yields. This comparison, along with the analysis of the features of selected events, revealed no significant deviations from the predictions of the null signal hypothesis. Consequently, it is concluded that no evidence of a data excess indicative
of the targeted slepton decay scenario is found within the dataset. Exclusion limits are therefore established for the targeted slepton decay scenario. Slepton masses up to 450 (420) GeV are excluded at the 95% confidence level for a lightest SUSY particle mass of 100 (150) GeV.