The increase of the particle flux (pile-up) at the HL-LHC with instantaneous luminosities up to
𝐿 ≃ 7.5 × 1034cm−1
s
−2 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) will be installed in front of the LAr
end-cap calorimeters for pile-up mitigation and luminosity measurement. The HGTD is a novel
detector introduced to assist the new all-silicon Inner Tracker (ITk) in the pseudo-rapidity range
from 2.4 to 4.0, 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 × 1.3 mm,
leading to a highly granular detector with 3.7 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 the other components, supported by laboratory and test beam
results, will also be presented.