Technical Proposal: A High-Granularity Timing Detector for the ATLAS Phase-II Upgrade

The large increase of pileup is one of the main experimental challenges for the HL-LHC physics program. A new powerful way to mitigate the effects of the pileup is to use high-precision timing information 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-ionising particles, this device will significantly improve the performance in the forward region. The high-precision timing information greatly improves the track-to-vertex association, leading to a performance similar to that in the central region for both jet and lepton reconstruction, as well as the tagging of heavy-flavour 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 HGTD offers unique capabilities for the online and offline luminosity determination.