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

To boost the performance of the Large Hadron Collider (LHC) and increase the potential of discoveries after 2027, the High-Luminosity LHC (HL-LHC) project has been announced, with the aim of increasing the luminosity by a factor of 10 above the design value of the LHC. This will lead to an increase of the pile-up interactions and will negatively impact the reconstruction of objects in the ATLAS detector, as well as its triggering performance. In particular, at the forward region, where the inner detector has a lower momentum resolution and the electromagnetic calorimeter has a coarser granularity. Therefore, a high granularity timing detector (HGTD) has been proposed to mitigate the pile-up effect; complementing the inner tracker (ITk) and providing a luminosity measurement, it will be located in front of the LAr end-cap calorimeter. The HGTD will cover the pseudo-rapidity region between 2.4 and 4.0, with two layers of double-sided silicon sensors that will provide a time resolution of 30 ps per track. Each readout cell has a cross Section of 1.3 mm $\times1.3$ mm, ensuring a high granular detector with 3.7 million channels. To achieve the required high signal-to-noise ratio and provide sufficient gain, Low Gain Avalanche Detector (LGAD) technology was chosen. The general specifications and requirements of LGAD will be outlined, as well as the technical design and status of the project. The ongoing research and development effort to study the sensors, readout ASIC, and other components, supported by laboratory and test beam results, will also be presented, in addition to some physics and performance results.