Search for exotic long-lived particles decaying into hadronic states in the calorimeter of the ATLAS detector at the Large Hadron Collider

Exotic long-lived particles feature in many extensions to the Standard Model and, as a result of their unique decay signatures, may have been overlooked by previous searches for new physics, which have usually focused on promptly-decaying particles. This thesis presents a search for pairs of long-lived neutral particles decaying to hadronic final states in the calorimeters of the ATLAS detector, using data collected during Run 2 of the Large Hadron Collider. Long-lived neutral particles with masses between 5 and 400 GeV were considered, produced from decays of heavy bosons with masses between 125 and 1000 GeV, and decaying into Standard Model fermions. Machine learning techniques were employed to identify the displaced decays, and a data-driven estimate of the remaining background was performed. Since no events were observed in the search region, limits were set on the production cross section times branching ratio, and the result was extrapolated as a function of the decay length of the long-lived particles. The decay lengths probed range between a few centimetres and a few tens of metres. The combination of the results of this search with those from a search for displaced decays in the muon spectrometer is also presented, and provides a summary of the results of searches for the simplified hidden- sector model. Constraints on three new physics models not originally studied in the search are also shown, resulting from the reinterpretation of this search with the RECAST framework. The original data analysis workflow was completely captured using virtualisation techniques, allowing for an accurate and efficient reinterpretation of the search result in terms of the new signal models. For all three signal models considered, the results from the reinterpretation were complementary to the existing limits from previous dedicated searches, especially at shorter proper decay lengths

ATLAS displaced jets: Solving the LLP re-interpretation challenge using RECAST

A recent ATLAS search for displaced jets in the hadronic calorimeter is preserved in RECAST and thereafter used to constrain three new physics models not studied in the original work. A Stealth SUSY model and a Higgs-portal baryogenesis model, both predicting long-lived particles and therefore displaced decays, are probed for proper decay lengths between a few cm and 500 m. A dark sector model predicting Higgs and heavy boson decays to collimated hadrons via long-lived dark photons is also probed. The original data analysis workflow was completely captured using virtualisation techniques, allowing for an accurate and efficient reinterpretation of the published result in terms of new signal models following the RECAST protocol

Snapshot and prospects for long-lived particle searches

BSM physics with long-lived particles (LLPs) can produce many complex topologies. The ATLAS experiment has a broad range of LLP searches in which the experimentally challenging signatures may require custom triggers and non-standard reconstruction algorithms. This talk will cover a selection of recent results, and discuss some prospects for LLP searches with the ATLAS experiment at the HL-LHC

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

International audienceParticles 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