Search for Long-Lived Particles at High Luminosity Large Hadron Collider and Beyond

Abstract

Despite extensive searches, clear indications of new physics beyond the Standard Model (BSM) remain elusive. Traditionally, experimental searches at the Large Hadron Collider (LHC) during Phase I and phenomenological studies within BSM physics have focused on promptly decaying particles. More recently, however, the focus has shifted toward particles with macroscopic decay lengths, known as Long-Lived Particles (LLPs). These particles, predicted by several well-motivated BSM theories, such as R-Parity Violating (RPV) supersymmetry (SUSY), hidden valley models, and dark matter models, have become a compelling focal point for BSM physics searches. This thesis, focusing on the search for LLPs at the High Luminosity Large Hadron Collider (HL-LHC) and beyond, is divided into three parts, detailed as follows:

Firstly, considering the anticipated increase in pile-up events at the HL-LHC, the effective triggering of LLPs at the earliest analysis stage is critical for efficient searches. Our studies suggest that conventional Level-1 (L1) triggers, designed primarily for prompt particles, may be inadequate in the high pile-up scenario of the HL-LHC, necessitating dedicated L1 triggers for LLPs. We propose dedicated L1 triggers that utilize track data to identify LLP decaying to jets. We explore the prospect of utilizing Minimum Ionizing Particle (MIP) timing detector information at the HL-LHC for triggering LLP events with displaced jets. Our investigation extends to the utilization of Electromagnetic Calorimeter (ECAL) timing information to develop dedicated triggers for displaced jets. Our findings show that combining track and timing data available at L1 significantly enhances LLP detection capabilities at the trigger level over a wide range of decay lengths. Finally, we introduce a novel machine-learning strategy using a message-passing graph autoencoder employing edge convolution to refine L1 track-based triggering, achieving high signal acceptance rates for light LLPs.

The second part investigates the RPV SUSY model within the HL-LHC framework, focusing on the pair production of electroweakinos $\chi_2^0$ and $\chi_1^{\pm}$. We analyze a scenario where these decay into a Higgs and W boson, respectively, along with a long-lived Lightest Supersymmetric Particle (LSP), $\chi_1^0$, resulting in both prompt and displaced signatures. Integrating tracker, ECAL, and MIP Timing Detector (MTD) data and utilizing triggers developed in the first part of the thesis, we establish improved limits on the masses of electroweakinos, exceeding current experimental projections for LLPs that decay very late in the detector.

In the final part, we address the challenges and prospects of future collider experiments to detect light LLPs in the forward direction by proposing FOREHUNT, a novel forward detector for the 100 TeV FCC-hh. Tailored to detect light LLPs from B-meson decays and heavy neutral leptons, we propose several detector configurations optimized for such signatures. We compare the signal acceptance rate with other dedicated LLP detectors like MATHUSLA, CODEX-b, and DELIGHT. Our findings indicate that for LLPs with a decay length shorter than 10 m, FOREHUNT emerges as the premier detector choice.