The increase of the particle flux (pile-up)at the HL-LHC with luminosities of L≃ 7.5×1034 cm−2s−1 will have a severe impact on the ATLAS detector reconstruction and trigger performance. The end-cap and forward region where the liquid Argon 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 endcap calorimeters for pile-up mitigation and for luminosity measurement. It will cover the pseudo-rapidity range from 2.4 to 4. Two Silicon sensors double sided layers will provide precision timing information for MIPs with a resolution better than 30ps per track in order to assign each particle to the correct vertex. Readout cells have a size of 1.3mm×1.3mm, leading to a highly granular detector with 3 M of channels.
Low Gain Avalanche Detectors (LGAD) technology has been chosen as it provides enough gain to reach the large signal over noise ratio needed.