Proposals for probing BSM in unexplored corners of Top physics and Next-to-Minimal Supersymmetric Standard Model (NMSSM)

Abstract

Despite its triumphs, the Standard Model (SM) fails to address many experimental observations and theoretical puzzles. Explanation of these necessitates postulation of new physics beyond those in the SM (BSM). The thesis includes different approaches to unveil the nature of BSM physics, i.e. particles and/or interactions, ranging from searching for deviations in interactions of known particles like the top quark, which could hint at new forces or couplings in various measurements of the particle properties at high-energy colliders, to direct searches at the collider for the hypothetical particles occurring in some of the possible extensions of the SM, postulated to address its shortcomings.

Top quark holds a particular interest in many SM extensions since its mass is near the electroweak symmetry-breaking scale. A tagging method is developed in a model-independent framework to identify boosted top quarks ($t \to b \tau \nu_\tau$). The method utilizes jet substructure techniques to obtain $b$- and $\tau$- like subjets inside a top fatjet. The strategy efficiently reject the dominant backgrounds, utilizing several kinematic variables constructed from the $b$- and $\tau$- tagged subjets. Distinguishing ability of polarization sensitive observables like energy fractions, angular correlation of the subjets is investigated which can reveal information about the chiral structure of top production including BSM interactions. Since our procedure do not focus on reconstructing the top mass, the analysis can be extended to identify any heavy BSM particle leading to final state containing $b$ and $\tau$.

Experimental searches have largely constrained the parameter space with prompt BSM particles leading to conventional signatures, while long-lived particles (LLPs) offer intriguing collider signatures like displaced vertices (DVs), usually uncommon to the SM. We explore possibility of LLPs in one of the well-established supersymmetric extensions of the SM, the next-to-minimal supersymmetric model (NMSSM), comprises of an additional singlet superfield along with two Higgs doublets. In the phenomenologically allowed parameter space of the NMSSM, the bino-like next-to Lightest Supersymmetric Particles (NLSPs) can undergo long-lived three-body decays to singlino-like LSP, in the limit of $m_{\text{NLSP}} - m_{\text{LSP}} < m_Z$. We present a strategy involving identification of DVs in NLSP decays through a track-based analysis. Properties of DV tracks can efficiently reject the backgrounds from SM processes and additional instrumental effects arising at detectors like CMS or ATLAS. Our analysis yields an excellent signal significance for the high luminosity Large Hadron Collider (HL-LHC) environment, suggesting a novel way to probe regions of the electroweakino parameter space that are otherwise challenging.

In our final exploration, we performed a more general scan covering a large range of LSP mass up to $\sim$ 1 TeV to identify the region of NMSSM parameter space where singlino-dominated LSP satisfies the experimentally measured upper limit of relic density. We focus on the region $m_{LSP} \lesssim 50$ GeV where relic density compliance is achieved through DM annihilation primarily via the s-channel exchange of light singlet Higgs. These light singlets are quite distinct possibilities for the NMSSM compared to other supersymmetric models and proceed to have collimated decays to $b\bar{b}/\tau^+\tau^-$, etc. We performed a collider analysis to explore the future potential of probing triple-boson final states at HL-LHC, involving a light Higgs boson. This type of experimental signature presents a possibility of exploring the nature of DM in a corner of NMSSM, where direct detection experiments may not have a significant reach in years to come.