Prompt and Displaced Signatures of Physics Beyond the Standard Model

Abstract

The quest to understand our Universe’s fundamental particles and their interactions has led us to the Standard Model (SM) of particle physics. Despite successfully explaining the weak, electromagnetic, and strong interactions, the SM fails to explain many experimental observations and theoretical questions. The solutions to these problems require new physics beyond the SM (BSM), motivating the hunt for any indication of BSM physics. We study various probes of BSM physics, ranging from deviations of precision measurements from SM predictions which capture indirect hints of new physics, to searching directly for BSM particles at colliders or dark matter experiments. We explore signatures from prompt decay of BSM particles having very small lifetimes and the exotic array of signatures arising from long-lived BSM particles (LLPs). While the former has been the primary focus of most BSM searches, the latter possibility has recently gained attention, demanding a more careful examination to ensure we are not missing any part of the BSM parameter space.

The discovered Higgs boson is a leading portal connecting new physics particles to the SM particles, and this motivates our studies in the first part of the thesis. We begin by exploring the parameter space of the phenomenological Minimal Supersymmetric Standard Model (pMSSM), a well-motivated BSM model, with a neutralino thermal dark matter (DM) contributing to the invisible Higgs boson decay (M_DM ≤ M_h/2). We consider both positive and negative values of the higgsino mass parameter (µ) and track down the region of parameter space consistent with the current collider and astrophysical constraints. Our investigation shows that the recent experimental results put this scenario under severe tension.

Experimental searches have largely constrained the parameter space with prompt BSM particles leading to conventional signatures in many popular BSM theories. We, therefore, shift to non-conventional displaced scenarios in our subsequent study of long-lived mediator particles being pair produced from the decay of the Higgs boson (M_LLP ≤ M_h/2) and their subsequent decay into standard model particles. We compute the projected sensitivity of using the muon spectrometer of the CMS detector at the high luminosity version of the Large Hadron Collider (HL-LHC) experiment for different production modes of the Higgs boson and various decay modes of the mediator particle, along with dedicated detectors for LLP searches like CODEX-b and MATHUSLA. Subsequently, we study the prospects at the hadronic future circular collider (FCC-hh), expected to reach a centre-of-mass energy of 100 TeV. We propose the DELIGHT detectors for dedicated LLP searches at the FCC-hh and study their sensitivity.

The second part includes studies focusing on different kinds of LLP signatures using various simplified BSM scenarios as benchmarks. We demonstrate how the structure of collider detectors creates a distinction between the energy deposition pattern of prompt jets and displaced jets, where the latter comes from an LLP decay. Given the diverse applications of machine learning (ML) techniques and their promising results in various BSM searches, we study the usefulness of a convolutional neural network (CNN) in learning these differences and discriminating displaced jets from prompt jets, playing an essential role in LLP searches. Finally, we end on an optimistic note, which is motivated by the question of what can be said about the properties of the LLPs, like their mass and especially lifetime, once they are discovered. We discuss the challenges and remedies in estimating the mass and lifetime of an LLP, for a wide range of signatures, provided a few such LLP events are observed at HL-LHC.