Search for Long-Lived Axion-Like Particles Decaying into Diphotons in the ATLAS Detector

Irwin, Rebecca (2025) Search for Long-Lived Axion-Like Particles Decaying into Diphotons in the ATLAS Detector PhD thesis, University of Liverpool.

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Abstract

The limitations of the Standard Model (SM) motivate a wide array of Beyond the Standard Model (BSM) theories. Many predict Long Lived Particles (LLPs) with unconventional detector signatures. This thesis presents a Run 3 search for exotic Higgs decays in the channel h → Za, where the Axion-Like Particle (ALP) (a) decays into a pair of collimated, displaced photons. Such ALPs arise naturally in dark sector extensions of the SM and may be produced through the Higgs portal. To address the reconstruction challenges posed by displaced photon topologies, the photon pair in this analysis is reconstructed as a single jet. A Graph Neural Network (GNN) jet tagger is developed to distinguish signal-like jets from background. Two signal region categories are defined to ensure sensitivity across a broad range of ALP masses and lifetimes. A preliminary sensitivity study is performed under two systematic uncertainty scenarios. In parallel, a performance study of the ATLAS Semiconductor Tracker (SCT) is conducted using dedicated data taken between 2016 and 2024. Levels of noise, efficiency and size of clusters as deposited in the SCT modules were studied, demonstrating the stability of their performance under increasing pile-up and radiation conditions.

Item Type:	Thesis (PhD)
Uncontrolled Keywords:	ALP, ATLAS, Axion-Like Particle, CERN, Higgs, Large Hadron Collider, LHC, LLP, Long Lived Particle, Photon, SCT, Semiconductor Tracker
Divisions:	Faculty of Science & Engineering Faculty of Science & Engineering > School of Physical Sciences Faculty of Science & Engineering > School of Physical Sciences > Physics
Depositing User:	Symplectic Admin
Date Deposited:	19 May 2026 12:59
Last Modified:	20 May 2026 08:33
DOI:	10.17638/03198008
Supervisors:	D'Onofrio, Monica Meloni, Federico Rompotis, Nikolaos
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3198008
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