Long-Lived Particles at the Energy Frontier: The MATHUSLA Physics Case

Abstract

We examine the theoretical motivations for long-lived particle (LLP) signals at the LHC in a comprehensive survey of Standard Model (SM) extensions. LLPs are a common prediction of a wide range of theories that address unsolved fundamental mysteries such as naturalness, dark matter, baryogenesis and neutrino masses, and represent a natural and generic possibility for physics beyond the SM (BSM). In most cases the LLP lifetime can be treated as a free parameter from the µm scale up to the Big Bang Nucleosynthesis limit of ∼ 10 7 m. Neutral LLPs with lifetimes above ∼ 100m are particularly difficult to probe, as the sensitivity of the LHC main detectors is limited by challenging backgrounds, triggers, and small acceptances. MATHUSLA is a proposal for a minimally instrumented, large-volume surface detector near ATLAS or CMS. It would search for neutral LLPs produced in HL-LHC collisions by reconstructing displaced vertices (DVs) in a low-background environment, extending the sensitivity of the main detectors by orders of magnitude in the long-lifetime regime. In this white paper we study the LLP physics opportunities afforded by a MATHUSLA-like detector at the HL-LHC. We develop a model-independent approach to describe the sensitivity of MATHUSLA to BSM LLP signals, and compare it to DV and missing energy searches at AT-LAS or CMS. We then explore the BSM motivations for LLPs in considerable detail, presenting a large number of new sensitivity studies. While our discussion is especially oriented towards the long-lifetime regime at MATHUSLA, this survey underlines the importance of a varied LLP search program at the LHC in general. By synthesizing these results into a general discussion of the top-down and bottom-up motivations for LLP searches, it is our aim to demonstrate the exceptional strength and breadth of the physics case for the construction of the MATHUSLA detector.

R-parity Violation and Light Neutralinos at ANUBIS and MAPP

“…MAPP1 consists of two sub-detectors: MAPP-mCP to detect minimally charged particles and MAPP-LLP to search for neutral LLPs. We consider the latter with an approximate fiducial volume of ∼ 130 m 3 .…”

“…Using this approximation for the location of the ANUBIS detector, and the probability for it to be hit by a particle flying from the IP, we find a geometric coverage of the total solid angle of about 1.34%. 3…”

Minimal Neutral Naturalness Model

“…For charged top partners, they can be produced through the Drell-Yan process and then form bound states [13,[49][50][51]. For neutral top partners, it is more promising to detect them at the Large Hadron Collider (LHC) with the possible displaced vertex signature [6,52] from Higgs exotic decay.…”

Dark matter in very supersymmetric dark sectors

“…The lifetime for the LoSP is model dependent, but a similar range of lifetimes is expected. With the proposal of new experiments designed to discover long-lived particles such as MATHUSLA [43], FASER [44], CODEX-b [45], and AL3X [46], the LHC could be primed to see one displaced vertex while the further detector could see the second, longer, displaced vertex.…”