Long-Lived Particles at the Energy Frontier: The MATHUSLA Physics Case (1806.07396v2)

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 $\mu$m scale up to the Big Bang Nucleosynthesis limit of $\sim 10^7$m. Neutral LLPs with lifetimes above $\sim$ 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 ATLAS 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.

• The paper demonstrates MATHUSLA's novel approach to detecting long-lived particles with enhanced sensitivity compared to traditional LHC detectors.
• It utilizes a large decay volume and robust timing techniques to achieve near-zero background for rare event detection.
• The findings highlight potential breakthroughs in probing beyond-Standard Model physics and deepening our understanding of dark matter and Higgs stabilization.

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

The exploration of physics beyond the Standard Model (BSM) has been invigorated by the quest to discover Long-Lived Particles (LLPs), which are compelling candidates predicted by various extensions of the Standard Model (SM). The MATHUSLA (MAssive Timing Hodoscope for Ultra-Stable neutraL pArticles) project offers a strategic approach toward detecting such particles at the Large Hadron Collider (LHC), particularly in scenarios where traditional detectors exhibit limited sensitivity.

Theoretical Motivation and Challenges

LLPs are an expected result in many theories addressing fundamental puzzles like naturalness, dark matter, baryogenesis, and neutrino masses. These particles often have lifetimes spanning several orders of magnitude due to weak interactions with the SM, making them elusive to conventional detectors at LHC's collision point due to triggering and background issues. Particularly, neutral LLPs with long lifetimes pose a significant detection challenge, as they frequently evade the primary detectors' range. This scenario necessitates an innovative approach that extends beyond the traditional detection methodologies.

MATHUSLA: A Novel Detection Paradigm

MATHUSLA represents an innovative proposal to surmount these challenges. Located near surface levels above ATLAS or CMS, MATHUSLA aims to detect LLPs by reconstructing displaced vertices in a controlled low-background environment. It promises a significant enhancement in sensitivity—by several orders of magnitude compared to the existing detectors—particularly for the long-lifetime domain. The successful identification of such particles stands to open a transformative window into BSM physics.

Design and Background Rejection

MATHUSLA's design emphasizes a large decay volume with simple, modular detector instruments, aiming for efficient reconstruction of LLP decays with exceptional background rejection. This setup mitigates risks related to cosmic rays and LHC-generated muons through robust geometrical and timing discrimination strategies. Preliminary studies indicate that this design could operate under near-zero background conditions, a critical factor in detecting the rare and consequential signatures of LLPs.

Sensitivity and Reach

MATHUSLA's viability is underscored by its projected sensitivity to LLP production cross-sections around the order of femtobarns, matching production rates feasible at LHC, especially in scenarios involving the Higgs boson and other high-energy processes. As such, MATHUSLA stands positioned to probe a wide parameter space, particularly concerning neutral LLP signals, which remain challenging for ATLAS and CMS due to high background levels and complex event triggers.

Implications for BSM Physics and Beyond

The successful implementation of MATHUSLA carries significant theoretical and practical implications. Not only would it enhance the LHC's BSM reach, but it would also provide crucial data relevant to several pressing theoretical questions, such as the stabilization mechanisms of the Higgs mass and the nature of dark matter. Furthermore, the modular and scalable nature of the MATHUSLA concept implies adaptability to evolving experimental requirements and potential future collider environments.

Conclusion and Future Directions

MATHUSLA represents a pivotal step in expanding our detection capabilities for LLPs at the LHC. By progressing toward construction and deployment, MATHUSLA could substantially augment the discovery potential for LLPs, marking a significant advancement in uncovering novel facets of particle physics. As theoretical developments continue to shape our understanding of BSM physics, experimental tools like MATHUSLA are crucial, promising to either confirm or challenge our current models by offering insights into the deepest and most enigmatic corners of the particle world. Future investigations and technological enhancements could only serve to further cement MATHUSLA's role in this ongoing scientific endeavor.