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.
Highlights
Document editors: James Beacham, Brian Shuve Particles in the Standard Model (SM) have lifetimes spanning an enormous range of magnitudes, from the Z boson (τ ∼ 2 × 10−25 s) through to the proton (τ 1034 years) and electron, as shown in figure 1.models beyond the SM (BSM) typically predict new particles with a variety of lifetimes
Since we do not expect collimated sprays of final-state particles to be a sensible way of organizing information in the event in all cases, for our purposes here we focus on observables that can be defined globally
Such objects are constructed requiring information from all parts of the detector including hits close to the IP, calorimeter deposits known to be signatures of particles originating from the IP, and muons with tracks that traverse the entirety of the detector, moving out from the IP
Summary
Document editors: James Beacham, Brian Shuve Particles in the Standard Model (SM) have lifetimes spanning an enormous range of magnitudes, from the Z boson (τ ∼ 2 × 10−25 s) through to the proton (τ 1034 years) and electron (stable), as shown in figure 1.models beyond the SM (BSM) typically predict new particles with a variety of lifetimes. Despite a wide and seemingly comprehensive research program—both existing and, in this document, proposed to be expanded—for LLPs at the LHC, in cases of ultra-low-mass particles, ultra-long lifetimes, or unusual LLP charges, it is hard or impossible to trigger on and/or reconstruct such events in the main ATLAS, CMS, and LHCb detectors. This has led to new proposals for dedicated experiments to look for LLPs in new regimes that are otherwise inaccessible at the LHC. It is of great interest to our community to be able to reinterpret the LLP experimental results for new models which may be developed in the future182
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