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.