Abstract
The expected increase of the particle flux at the high-luminosity phase of the LHC (HL-LHC) with instantaneous luminosities up to L = 7.5× 1034 cm−1s−1 will have a severe impact on the ATLAS detector performance. The pile-up is expected to increase on average to 200 interactions per bunch crossing. The reconstruction and trigger performance for electrons, photons as well as jets and transverse missing energy will be severely degraded in the end-cap and forward region, where the liquid Argon based electromagnetic calorimeter has coarser granularity and the inner tracker has poorer momentum resolution compared to the central region. A High Granularity Timing Detector (HGTD) is proposed in front of the liquid Argon end-cap calorimeters for pile-up mitigation and for bunch per bunch luminosity measurements. This device should cover the pseudo-rapidity range of 2.4 to about 4.0. Two Silicon sensors double-sided layers are foreseen to provide a precision timing information for minimum ionizing particle with a time resolution better than 50 pico-seconds per hit (i.e. 30 pico-seconds per track) in order to assign the particle to the correct vertex. Each readout cell has a transverse size of 1.3 mm×1.3 mm leading to a highly granular detector with about 3 millions of readout electronics channels. Low-Gain Avalanche Detector (LGAD) technology has been chosen as it provides an internal gain good enough to reach large signal over noise ratio needed for excellent time resolution. Extensive LGAD research and development (R&D) campaigns are carried out to investigate the suitability of this new technology as timing sensors for HGTD. The related readout ASIC is also being studied extensively.
Highlights
In the new condition of HL-LHC pileup will be one of the main challenges
The readout of the sensors will be done with an application-specific integrated circuit (ASIC) chip, ALTIROC, that will be bump-bonded to the LGAD sensor
The electronics jitter for a charge of about 10 fC is required to be smaller than 25 ps, i.e smaller than the dispersion induced by the Landau fluctuations on the energy deposit which limits the time resolution to 25 ps at large sensor gain
Summary
A major challenge for the tracking detectors is to efficiently reconstruct the charged particles created in the primary interactions and correctly assign them to the production vertices. This requires the resolution of the longitudinal track impact parameter (z0), provided by the Inner Tracker (ITk), to be much smaller than the inverse of the average pileup density (0.6 mm). A powerful new way to address the challenging pileup conditions of HL-LHC is to exploit the time spread of the collisions in each bunch crossing to distinguish between tracks originating in collisions occurring very close in space but well-separated in time. The HGTD detector has to withstand a 1 MeV neutron equivalent particle fluence of 5.1 × 1015 neq/cm, assuming one replacement of the inner part (3.1 < |η| < 4.0) after half of the total integrated luminosity of 4000 fb−1
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