Performance Studies of the Low Gain Avalanche Detectors for the ATLAS High Granularity Timing Detector in Beam Tests

The significant increase of pileup interactions is one of the main experimental challenges for the physics program during the High Luminosity (HL) LHC operation phase. As part of the ATLAS upgrade program, the High Granularity Timing Detector (HGTD) is designed for pile-up effects mitigation in the forward region and for bunch per bunch luminosity measurements. The HGTD, based on Low Gain Avalanche Detector (LGAD) technology and covering the pseudorapidity region between 2.4 and 4.0, will provide high precision timing information to distinguish between collisions occurring close in space but well-separated in time. Apart from being radiation resistant, LGAD sensors should deliver 30 ps time resolution per track for a minimum-ionising particle at the start of lifetime, increasing to 75 ps at the end of HL-LHC operation. The performances of several irradiated LGADs from different vendors studied with the test beams in 2021-2022 at CERN SPS and DESY will be presented. This study covers the promising results in terms of collected charge, time resolution and hit efficiency of LGADs. A time resolution of < 50 ps is observed in most cases for highly irradiated sensors (2.5e15 neq/cm2).