A High-Granularity Timing Detector for the Phase-II upgrade of the ATLAS Detector System

The expected increase of the particle flux at the high luminosity phase of the LHC with instantaneous luminosities up to L=7.5×1034 cm^-2s^-1 will have a severe impact on pile-up. The pile-up is expected to increase on average to 200 interactions per bunch crossing. The reconstruction performance for especially jets and transverse missing energy will be severely degraded in the end-cap and forward region of the ATLAS detector. A High Granularity Timing Detector (HGTD) is proposed in front of the liquid Argon end-cap calorimeters of ATLAS for pile-up mitigation in the offline reconstruction. An additional use of the detector as a luminometer is proposed. This device covers the pseudo-rapidity range of 2.4 to about 4. Four layers of Silicon sensors are foreseen to provide precision timing information with a time resolution of the order of 30 pico-seconds per minimum ionizing particle in order to assign the energy deposits in the calorimeter to different proton-proton collision vertices. Each readout sensor has a transverse size of only a few mm, leading to a highly granular detector with several million readout channels. The expected improvements in performance are relevant for physics processes, i.e, vector-boson fusion and vector-boson scattering processes, and for physics signatures with large missing transverse energy. The chosen silicon sensor technology is Low Gain Avalanche Detectors (LGAD). In this document, starting from the physics motivations of the High Granularity Timing Detector, the proposed detector layout and Front End readout, laboratory and beam test characterization of sensors and the results of radiation tests will be discussed.