Abstract
The expected increase of the particle flux at the high luminosity phase of the LHC with instantaneous luminosities up to $\mathrm {L}= 7.5 \times 10 ^{34}$ cm $^{-2} \mathrm {s}^{-1}$ 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.
Highlights
Under such a high density of interactions, distinguishing the individual vertices, finding the primary vertex and matching the tracks to the vertices only using the spacial resolution of the Inner Tracker (ITk) of ATLAS becomes a challenge
With radiation levels reaching up to 9×1015 neq/cm2 and a Total Ionizing Dose (TID) of 9 MGy after 4000 fb−1 including safety factors, a replacement of the inner-radius (R < 300 mm) modules is planned at the half-time of the HL-LHC
The fast STOP signal enters the delay line and the amount of cells needed to delay it so that it becomes in-time with the START signal corresponds to the time measurement
Summary
In 2026, LHC will enter its High Luminosity phase, where the instantaneous luminosity will increase from 2-3×1034 cm−2s−1 to 7.5×1034 cm−2s−1 [1]. In HL-LHC conditions, the local average vertex density is expected to be approximately 1.6 vertices/mm reaching up to 3 vertices/mm in certain occasions. Under such a high density of interactions, distinguishing the individual vertices, finding the primary vertex and matching the tracks to the vertices only using the spacial resolution of the Inner Tracker (ITk) of ATLAS becomes a challenge. As a consequence the accuracy of the reconstruction of all physics objects is expected to be degraded This effect is especially pronounced in the forward region, where the particle density is the largest. Due to its high granularity, it is expected to provide good linearity between the number of hits and the number of interactions, while adjustable time windows can be used to study afterglow effects
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