Searching for long-lived particles beyond the Standard Model at the Large Hadron Collider (1903.04497v1)

Abstract: Particles beyond the Standard Model (SM) can generically have lifetimes that are long compared to SM particles at the weak scale. When produced at experiments such as the Large Hadron Collider (LHC) at CERN, these long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton-proton collision. Such LLP signatures are distinct from those of promptly decaying particles that are targeted by the majority of searches for new physics at the LHC, often requiring customized techniques to identify, for example, significantly displaced decay vertices, tracks with atypical properties, and short track segments. Given their non-standard nature, a comprehensive overview of LLP signatures at the LHC is beneficial to ensure that possible avenues of the discovery of new physics are not overlooked. Here we report on the joint work of a community of theorists and experimentalists with the ATLAS, CMS, and LHCb experiments --- as well as those working on dedicated experiments such as MoEDAL, milliQan, MATHUSLA, CODEX-b, and FASER --- to survey the current state of LLP searches at the LHC, and to chart a path for the development of LLP searches into the future, both in the upcoming Run 3 and at the High-Luminosity LHC. The work is organized around the current and future potential capabilities of LHC experiments to generally discover new LLPs, and takes a signature-based approach to surveying classes of models that give rise to LLPs rather than emphasizing any particular theory motivation. We develop a set of simplified models; assess the coverage of current searches; document known, often unexpected backgrounds; explore the capabilities of proposed detector upgrades; provide recommendations for the presentation of search results; and look towards the newest frontiers, namely high-multiplicity "dark showers", highlighting opportunities for expanding the LHC reach for these signals.

• The paper details innovative search strategies for long-lived particles at the LHC, emphasizing displaced vertices and specialized triggers.
• It presents systematic coverage using simplified models to identify detection gaps, particularly in low mass and low momentum LLP scenarios.
• The study outlines promising detector upgrades and advanced timing systems that bridge theoretical predictions with experimental capabilities.

Overview of Long-Lived Particle Searches in Contemporary Collider Experiments

The academic paper presented here provides a meticulous survey of the experimental search strategies and developments for long-lived particles (LLPs) at the Large Hadron Collider (LHC). It delineates the interplay between theoretical work and experimental setups, providing insights into the existing methodologies for detecting LLPs, their signatures, backgrounds, the results from current searches, and the additional potential of planned upgrades and dedicated detectors.

Key Concepts and Strategies for LLP Searches

LLPs, unlike promptly decaying particles traditionally targeted in many new physics searches, possess unique signatures due to their extended lifetimes before decay. This necessitates bespoke search methodologies that differ significantly from those for promptly decaying particles, such as:

• Displaced Vertices and Tracks: These are indicators of decays occurring away from the interaction vertex, often crucial in identifying LLPs.
• Dedicated Triggers and Reconstruction: Special triggers that can capture unusual decay patterns and utilize displacement information are essential. Reconstruction algorithms optimized for detecting displaced decay patterns also play a critical role.

The paper discusses frameworks utilizing simplified models to provide a systematic cover for LLP signatures and understanding the gaps in existing experimental design. Such models are essential not only to assess coverage of experimental searches but also to guide the focus on possible undetected physics signals.

Existing Coverage and Experimental Improvements

The searches discussed tackle various decay scenarios: all-hadronic, leptonic, semi-leptonic, photonic, and more exotic signatures like disappearing tracks and out-of-time signals. Significant existing efforts across LHC collaborations, such as ATLAS, CMS, and LHCb, each bring unique capabilities to the LLP hunting ground. However, limitations in coverage, particularly for low-mass and low-momentum LLPs, persist due to inadequate triggering or high background noise in current methodologies.

The expected strategic upgrades, particularly in tracking and timing detectors, highlight promising future prospects. The incorporation of technologies like precision timing detectors, enhanced calorimeter granularity, and sophisticated track reconstruction algorithms promises substantial advancements in LLP detection capability.

Predictions, Extrapolation, and Reinterpretation

The paper also provides a pathway towards extending current findings (“gaps in coverage”) to optimize the physics reach of future runs of the LHC and associated experimental programs. These insights emphasize the importance of triggering on associated particles to infer LLP presence—crucial for detecting lower-mass LLPs, for which existing hard scatter triggers may fall short.

Reinterpretation remains a technical and computational challenge in detecting LLPs against multifaceted backgrounds and interpreting existing searches in the light of various models, beyond their originally considered frameworks.

Future Directions and Implications

The implications of these experimental searches are profound—beyond increasing the particle detection gamut at the LHC, they could potentially bridge gaps in our understanding of both confirmed and speculative elements of particle physics, like dark matter or hidden sectors. Moreover, the strategic roadmap provided for the theoretical development and the proposed experimental innovations for detecting LLPs suggest a significant shift towards broader, more encompassing new physics searches in the coming HL-LHC era.

Conclusion

This comprehensive paper serves as a critical repository of past, present, and future directions in the search for long-lived particles at the LHC. By amalgamating theoretical insights with practical detector innovations, it elucidates pathways towards future discoveries that have the potential to significantly advance our understanding of high energy physics. The advocated improvements and dedicated searches further underscore the necessity for ongoing collaboration between theorists and experimentalists to fully exploit the potential of the LHC and its successors.