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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We examine the potential of future long-lived particle experiments to probe dark QCD models that feature Emerging Jets. The core of this analysis focuses on the transverse detectors AL3X, ANUBIS, CODEX-b, and MATHUSLA as they cover the most relevant parameter space, though the highly forward experiments MAPP, FORMOSA, and FASER are also explored. Geometric coverage of the detectors is calculated and used to determine the number of signal events and kinematic distribution measured for a collection of different benchmark models. This is used to map out the discovery potential of the Emerging Jets parameter space. Although all experiments demonstrate some reach, AL3X, ANUBIS, and MATHUSLA stand out as the most promising for exploring the dark QCD Emerging Jets parameter space.

The current standard model (SM) of particle physics is known to be an incomplete model of the Universe. Experiments have given strong evidence for the existence of dark matter. Other alterations to the SM could include the presence of extra forces and/or non-standard cosmologies. In our current era, we have many experimental