A High-Granularity Timing Detector in ATLAS: Performance at the HL-LHC

The large increase of pileup is one of the main experimental challenges for the HL-LHC physics program. A powerful new way to address this challenge is to exploit the time spread of the interactions to distinguish between collisions occurring very close in space but well separated in time. A High-Granularity Timing Detector, based on low gain avalanche detector technology, is proposed for the ATLAS Phase-II upgrade. Covering the pseudorapidity region between 2.4 and 4.0, with a timing resolution of 30 ps for minimum-ionizing particles, this device will significantly improve the performance in the forward region. High-precision timing greatly improves the track-to-vertex association, leading to a performance similar to that in the central region for the reconstruction of both jets and leptons, as well as for the tagging of heavy-flavor jets. These improvements in object reconstruction performance translate into important sensitivity gains and enhance the reach of the HL-LHC physics program. In addition, the High-Granularity Timing Detector offers unique capabilities for the online and offline luminosity determination.