A High-Granularity Timing Detector (HGTD) for the ATLAS Phase-II upgrade

The increase of the particle flux (pile-up) at the HL-LHC with luminosities of $L\approx 7.5 \times 10^{34}$cm$^{−2}$s$^{−1}$ 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 end-cap calorimeters for pile-up mitigation and for luminosity measurement. It will cover the pseudo-rapidity range from 2.4 to 4.0. Two double sided layers of Silicon sensors will provide precision timing information for MIPs with a resolution better than 30 ps per track in order to assign each particle to the correct vertex. Readout cells have a size of $1.3$ mm $\times$ $1.3$ mm, leading to a highly granular detector with $\sim$ 3 million channels. Low Gain Avalanche Detector (LGAD) technology has been chosen as it provides enough gain to reach the large signal over noise ratio needed. The requirements and overall specifications of the HGTD will be presented as well as the technical proposal. LGAD R&D campaigns are carried out to study the sensors, the related ASICs, and the radiation hardness. Laboratory and test beam results will be presented.