| 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. |
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