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| Home > Publications database > Search for long-lived Axion-like Particlesproduced in Higgs Boson Decays with theATLAS Detector |
| Home > Publications database > Search for long-lived Axion-like Particlesproduced in Higgs Boson Decays with theATLAS Detector |
| Master Thesis | PUBDB-2025-04781 |
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2024
Abstract: In the endeavour to understand the fundamental components of the universe, the search for newparticles beyond the Standard Model of particle physics is a pivotal task. Axion-like particles (ALPs)are among the prime targets of this search, arising in various theoretical frameworks, includingquantum chromodynamics, supersymmetric extensions of the Standard Model of particle physics orstring theory. They are predicted to have weak interactions with standard matter, making them primecandidates for a dark matter constituent or a mediator between the Standard Model and dark sectors.Strong evidence for the existence of dark matter is seen in a variety of astrophysical observations,like the rotation curves of galaxies, the cosmic microwave background radiation or the large-scalestructure of the universe. However, the nature of dark matter remains unknown and there are noconstituent particles provided by the Standard Model.With the discovery of the Higgs boson in 2012 by the ATLAS and CMS Experiments at CERN, theStandard Model of particle physics has been completed. The Higgs boson plays an essential role in theStandard Model, providing mass to elementary particles through the Higgs mechanism. However, theHiggs boson is not the final piece of the puzzle and raises new questions about the hierarchy problemor the stability of the Higgs potential. The Higgs boson coupling to new particles beyond the StandardModel is well motivated by many theories, like Higgs portal models or Two-Higgs-Doublet models.In the case of axion-like particles coupled via the Higgs portal to the Standard Model, they can belong-lived and decay into photons, providing a unique signature in high-energy physics experiments.The long lifetime of the ALPs, however, poses a challenge for their detection and reconstruction, asthey can travel macroscopic distances before decaying. Standard photon reconstruction fails in thiscase and dedicated reconstruction techniques are required to identify and reconstruct the ALP decayproducts.In this thesis a search for axion-like particles produced in Higgs boson decays and subsequentlydecaying into pairs of photons with the ATLAS detector is presented. The consequences of the longALP lifetime are discussed and the effectiveness of an event selection optimized for promptly decayingALPs is evaluated. Two different reconstruction algorithms based on low-level detector informationusing different machine learning techniques are developed to improve the identification of ALP decayphotons and compared to each other. The performance of both algorithms is evaluated using simulateddata and the sensitivity of the search is estimated for different ALP masses and lifetimes.
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