Searching for long-lived particles beyond the Standard Model at the Large Hadron Collider

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.European Cooperation in Science and Technologyhttps://doi.org/10.13039/501100000921Javna Agencija za Raziskovalno Dejavnost RShttps://doi.org/10.13039/501100004329Ministry of Education and Science of the Russian Federationhttps://doi.org/10.13039/501100003443Fondo Nacional de Desarrollo Científico y Tecnológicohttps://doi.org/10.13039/501100002850Deutsche Forschungsgemeinschafthttps://doi.org/10.13039/501100001659Agence Nationale de la Recherchehttps://doi.org/10.13039/501100001665Consellería de Cultura, Educación e Ordenación Universitaria, Xunta de Galiciahttps://doi.org/10.13039/501100008425Ministerio de Economía y Competitividadhttps://doi.org/10.13039/501100003329Simons Foundationhttps://doi.org/10.13039/100000893National Science Foundationhttps://doi.org/10.13039/100000001U.S. Department of Energyhttps://doi.org/10.13039/100000015Ministerstwo Nauki i Szkolnictwa Wyższegohttps://doi.org/10.13039/501100004569Science and Technology Facilities Councilhttps://doi.org/10.13039/501100000271Horizon 2020 Framework Programmehttps://doi.org/10.13039/100010661Generalitat Valencianahttps://doi.org/10.13039/501100003359Narodowe Centrum Naukihttps://doi.org/10.13039/501100004281Ministry of Science and Technology, Taiwanhttps://doi.org/10.13039/501100004663Fundação de Amparo à Pesquisa do Estado de São Paulohttps://doi.org/10.13039/501100001807Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschunghttps://doi.org/10.13039/501100001711Peer Reviewe