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

The High-Luminosity Large Hadron Collider (HL-LHC) will run instantaneous luminosities up to 7.5 × 10 34 cm -2 s -1 , i.e. the number of simultaneous collision vertices per bunch crossing increases significantly, which gives a severe impact on the detector performance.Therefore, a novel High Granularity Timing Detector (HGTD) based on Low Gain Avalanche Diode (LGAD) technology, approved in 2019, will be installed in front of the LAr end-cap calorimeters for pile-up mitigation (sixfold) and luminosity measurements.The HGTD consists of two double-sided LGAD instrumented layers on each side, covering a pseudo-rapidity range from 2.4 to 4.0, providing precise timing measurements for MIP particles with a resolution of 30 ps per track in order to assign each particle to the correct vertex.The HGTD LGAD is a 50 µm thick n-on-p planar pixel sensor with a p + multiplication layer between, each pixel is 1.3×1.3mm 2 big resulting in a capacitance of ∼4 pF.Such thin active volume with a modest few tens internal gain results in a high drift velocity that facilities fast timing measurements, and thus been chosen.Sensors will be readout by a customised front-end readout chip, ATLAS LGAD Timing Integrated Readout Chip (ALTIROC), designed in 130 nm CMOS technology.The total sensor surface is about 6.4 m 2 yielding approximately 3.6 million readout channels.This article presents the general specifications and project status of HGTD, and provides an overview of the latest research and development status, particularly for the critical components.