Overview of the ATLAS High Granularity Timing Detector: project status and results

The increase of the particle flux at the HL-LHC scenario, with instantaneous luminosities up to $7.5\times 10^{34}$ cm$^{-1}$s$^{-2}$, will have a severe impact on the ATLAS detector reconstruction and trigger performance. This is particularly challenging in the detector end-cap and forward region, where the liquid Argon calorimeter has coarser granularity and the inner tracker has poorer momentum resolution. A High Granularity Timing Detector (HGTD) will be installed in front of the LAr end-cap calorimeters to help with pile-up mitigation, as well as to provide a measurement of the luminosity. Thus, the HGTD is introduced to assist the new all-silicon Inner Tracker (ITk) in the pseudo-rapidity $2.4 <|\eta |< 4.0$ range, adding the capability to measure charged-particle trajectories in time, as well as space. Two silicon-sensor double-sided layers will provide precision timing information for minimum-ionising particles with a resolution as good as 30 ps per track, to help disentangle tracks from different vertices in the same bunch crossing. Readout cells have a size of 1.3 mm $\times $ 1.3 mm, leading to a highly granular detector with 3.6 million channels. Low Gain Avalanche Detectors (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 design and the project status. The on-going R$\& $D effort, carried out to study the sensors, the readout ASIC, and other components, supported by laboratory and test beam results, will also be presented