A High-Granularity Timing Detector for the Phase-II upgrade of the ATLAS Calorimeter system: detector concept, description and R&D and beam test results

The increase of the number of interactions (pile-up) at the High Luminosity LHC (HL-LHC) will have a severe impact on the ATLAS detector reconstruction and trigger performance. The end-cap and forward regions, where the liquid Argon (LAr) calorimeter has coarser granularity and the inner tracker has poorer momentum resolution, will be particularly affected. A High Granularity Timing Detector (HGTD) is proposed in front of the LAr end-cap calorimeters for pile-up mitigation and for luminosity measurement. It will cover the pseudo-rapidity range 2.4 < |eta| < 4.0. Two silicon sensors double-sided layers on each end-cap will provide precision timing information for minimum ionising particles (MIPs) in order to assign each particle to the correct vertex. Readout cells have a size of 1.3 mm x 1.3 mm, leading to a highly granular detector with over 3.6 millions of channels. The Low Gain Avalanche Detectors (LGAD) technology has been chosen for the HGTD detector as it provides a compact sensor with large signal over ratio leading to excellent timing performance. The requirements and overall specifications of the HGTD will be presented. LGAD R&D campaigns were carried out to study the sensors and their radiation hardness. Latest laboratory and test beam results will be presented.