CERN Accelerating science

Thesis
Report number	CERN-THESIS-2024-270
Title	Upgrade of the ATLAS tracking detector and measurement of the Higgs boson production cross-section at 13.6 TeV
Related title	Amélioration du détecteur de traces (ITk) pour la phase haute luminosité d’ATLAS et mesure de la section efficace de production du boson de Higgs at 13.6 TeV
Author(s)	Nakkalil, Keerthi (Centre National de la Recherche Scientifique (FR))
Thesis note	PhD : Paris Cité U. : 2024
Thesis supervisor(s)	Bomben, Marco ; Marchiori, Giovanni
Note	Presented 03 Oct 2024
Subject category	Detectors and Experimental Techniques ; Particle Physics - Experiment
Accelerator/Facility, Experiment	CERN LHC ; ATLAS
Abstract	High-energy physics explores the fundamental particles of the universe and their interactions by creating and analyzing collisions at particle accelerators operating at the highest energies. The Large Hadron Collider (LHC), the world's most powerful accelerator, has achieved groundbreaking successes since it began operations. Most notably, during Run 1 (2010-2012) at center-of-mass energies of 7-8 TeV, the ATLAS and CMS collaborations discovered the Higgs boson in 2012, confirming the last missing piece of the Standard Model. This discovery opened new avenues for studying this fundamental particle. In Run~2 (2015-2018), the LHC increased its collision energy to 13 TeV, enabling precise measurements of the Higgs boson's properties, including its mass, and decay into fermions such as bottom quarks and tau leptons. Run~2 also provided insights into rare processes, such as the production of final states containing multiple top quarks and precise measurements of the top quark and W boson masses. Run~3 (2022-present) features even higher energy (13.6 TeV) and increased collision rates, aiming to explore rare processes, improve precision measurements, and search for physics beyond the Standard Model. This thesis presents the first measurement of the Higgs boson production cross-section at 13.6~TeV using $\approx 30 fb^{-1}$ of pp collision data collected in 2022 with the ATLAS detector. The past 16 years have been remarkable for high-energy physics, marked by the successes of LHC experiments in advancing our understanding of fundamental particles. Significant upgrades are planned to enhance and extend the LHC's discovery potential. The High-Luminosity LHC (HL-LHC) upgrade aims to achieve instantaneous luminosities five times greater than the LHC's nominal value, enabling the ATLAS and CMS experiments to collect ten times more data. This increase will allow precision tests of the Higgs boson's properties and improve sensitivity to new physics scenarios, ensuring effective operation of LHC until the 2040s. The current tracking detectors of the ATLAS and CMS experiments will be replaced with more radiation-hard, faster detectors of higher granularity. The inner detector of the ATLAS will be upgraded to an all-silicon tracker (ITk) featuring a new strip detector and pixel detector. The focus of this thesis is on the upgrade activities of the ATLAS tracking detectors, particularly addressing radiation damage challenges, a critical concern for the HL-LHC upgrade. Radiation damage affects the performance of silicon tracking detectors, with signal reduction being the most critical issue. Generating simulated data that accurately reflects performance evolution as luminosity and fluence accumulate is crucial. The ATLAS collaboration has developed algorithms to correct simulated Monte Carlo (MC) events for radiation damage effects, achieving impressive agreement between collision data and simulated events in Run 3. In preparation for the HL-LHC, a faster ATLAS MC production algorithm is imperative due to escalating collision rates, events, tracks, and particle hit rates, imposing stringent constraints on computing resources. This thesis presents a novel lightweight algorithm based on Look-Up Tables to model radiation damage effects. This algorithm has been studied for ITk planar pixel sensors and has the potential for extension to strip and 3D pixel sensors. Additionally, this thesis includes studies on realistically modeling the ITkPix readout chip within the ATLAS software framework, Athena, and pixel quad module development at the Laboratoire de Physique Nucléaire et des Hautes Énergies.

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