This thesis presents the Forward Search Experiment (FASER) and its role in exploring physics beyond the Standard Model by detecting long-lived particles produced in the far-forward region of the ATLAS Interaction Point at the LHC. The work focuses on the search for axion-like particles (ALPs), which are motivated by dark matter and hidden sector frameworks. Three benchmark ALP scenarios are studied, involving couplings to SU(2)_Lgauge bosons, photons, and gluons.

The thesis provides a detailed overview of the FASER detector, including its trigger system, data acquisition, and recent upgrades. Calibration and performance studies using 2021 test beam data are presented, with an emphasis on simulation validation and preshower detector characterization.

Using 57.7 fb⁻¹ of Run 3 data collected in 2022–2023, the work presented in this thesis sets world-leading exclusion limits on ALPs in the mass range 100 < m_a < 250 MeV and coupling range 3 × 10⁻⁵ < g_{aW W} < 5 × 10⁻⁴ GeV⁻¹, along with complementary limits for photon- and gluon-coupled ALPs. Contributions include background modeling, estimation of preshower systematic uncertainties, and statistical interpretation of the results. Additional simulation studies involving detector timing are included to support future analyses.

Furthermore, an upgrade of the detector’s preshower system to a monolithic pixel detector was completed, including testing and commissioning efforts, aspects which are documented in this thesis. The system is currently installed and taking data.

The thesis concludes with an analysis of a 2024 test beam campaign using the upgraded preshower system, focusing on event matching, detector alignment, energy correlation studies, and calorimeter resolution improvements. These studies reinforce FASER’s capability to probe low-mass, weakly coupled particles and its continued significance in the search for BSM physics.

Title	Published in	Year	Views	Downloads

Searches for Long-Lived Particles with Photonic Final States in the FASER Experiment at the LHC: Results and Upgrades		2025	98	97
First Measurement of the Muon Neutrino Interaction Cross Section and Flux as a Function of Energy at the LHC with FASER	Physical review letters	2025	47	38

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Noshin Tarannum, Noshin

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