Search for slow charged long-lived particles with the ATLAS detector at the LHC.

This thesis presents a search for beyond the Standard Model slow, charged long-lived particles produced in proton–proton collisions at the Large Hadron Collider with the ATLAS detector. The search uses Run 2 (2015–2018) and Run 3 (2022–2024) data, corresponding to integrated luminosities of 140 fb−1 at √s= 13 TeV and 169 fb−1 at √s= 13.6 TeV. The analysis relies on independent measurements of the particle mass from the ionisation in the Inner Detector and the Muon Spectrometer and the time-of-flight in the Muon Spectrometer and the Hadronic Calorimeter. This work focuses on the combination of the ionisation information from the Inner Detector and the time-of-flight from the Muon Spectrometer. The time-of-flight measurement requires a dedicated reconstruction chain, provided by the “Slow Muon” algorithm. The performance of the algorithm and the developments implemented in recent software releases are discussed. A framework for tube-level calibration of the Muon Spectrometer barrel was developed, and the corresponding corrections are discussed. A new analysis channel was introduced to extend the sensitivity of the search using the full analysis statistics. The background estimation is performed using the data-driven method. The studies documented in this work demonstrate that particle travelling with the speed significantly smaller than the speed of light can be measured with high efficiency. The developed Muon Spectrometer time-of-flight calibration method allows to use the obtained particle speed measurement with a precision of 4%. The estimated sensitivity indicates that the search is expected to exclude the pair-produced detector-stable stau with masses up to 700 GeV.