Low Gain Avalanche Detectors for the ATLAS High Granularity Timing Detector

The Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN), Geneva, will interrupt its operation in 2026, after the current Run 3, to be upgraded to the High Luminosity LHC (HL-LHC), providing $p-p$ collisions with a center-of-mass energy $\sqrt{s}=$\SI{14}{\tera\electronvolt} and a luminosity of 7.5 $\times$ 10$^{34}$~cm$^{-2}$s$^{-1}$. ATLAS (A Toroidal LHC ApparatuS), one of the two general purpose experiments at the LHC, will have to be upgraded in order to meet the requirements given by the larger luminosity. Among the several upgrades of the ATLAS sub-detectors, the current Inner Detector will be fully replaced by the Inner Tracker (ITk), based on silicon detectors with a fine granularity and improved radiation hardness. A new detector, the High Granularity Timing Detector (HGTD), will provide precise (\SIrange{30}{50}{\pico\second}) track time information in the forward region. The combination of the ITk and HGTD measurements will make possible the separation of vertices close in space but separated in time, improving the ATLAS reconstruction performance. In the context of this thesis, a preliminary simulation study was carried out in order to estimate the reduction of the background to Vector Boson Fusion (VBF) analyses using the HGTD. In the framework of this thesis, the Low Gain Avalanche Detector (LGAD) technology, with which the HGTD detector will be instrumented, has been studied. These silicon sensors consist of planar n-on-p detectors with an extra highly doped p-type layer designed to increase the sensor gain through impact ionization. Thin LGAD sensors have shown a time resolution of about \SI{30}{\pico\second} on the detection of minimum ionization particles before irradiation. Studies of LGAD sensors produced by CNM with different approaches (dopants) both before and after irradiation to the HGTD required fluence were performed. Many of these exploratory studies were done for the first time in the context of this thesis.