Long lived ALP decays

The Standard Model (SM) of Particle Physics describes fundamental particles and how they interact. It leaves several open questions and experiments are searching for new physics beyond the SM. Particles in the SM have lifetimes that expand an enormous range of magnitudes. The lack of discovery at the LHC of new physics beyond the SM has led to research looking for unconventional signatures which might have eluded searches until now. Particles beyond the Standard Model (BSM) could have lifetimes that are long compared to SM particles at the weak scale. These long-lived particles (LLPs) can decay far from the interaction vertex of the primary proton-proton collision when produced at the Large Hadron Collider (LHC). For a particle to be classified as an LLP it needs to decay a macroscopic, reconstructable distance from the p-p interaction point, or be quasi-stable on the scale of the relevant detector. Long-lived particles can be light or heavy, and can travel fast or slow, and decay into anything that is detectable. The experimental signatures of LLPs at the LHC are varied, and they are typically different from SM signals. An LLP signature can be identified through localised deposits of energy inside calorimeters without associated tracks, such as Axion-like particles (ALPs) in delayed photons. Another LLP signature is displaced vertices in the inner detector or muon spectrometer, such as ALPs in leptons. The unusual signatures of BSM LLPs offer great potential for the discovery of new physics at particle colliders.
ALPs are hypothetical light pseudo-Nambu-Goldstone bosons which do not necessarily address the strong CP problem and appear in the spontaneous breaking of a global symmetry. ALPs can interact with all particles of the SM. The search for ALPs varies depending on the mass of the ALP and its couplings to SM particles. Smaller couplings to SM particles result in the ALP having a long lifetime and therefore being macroscopically detectable. Larger couplings and thus a stronger bound with the SM result in the ALP decaying promptly.
ALPs can be produced resonantly at the LHC and can either decay inside the detector or be very long lived. The work of this thesis focuses on the cases in which the ALP is very weakly coupled to the standard model. This means the ALP decay is long lived, but not at the point that it is leaving the detector volume without interacting and decaying. If the ALP decays into leptons that are charged they could be found in the tracker or the electromagnetic or hadronic calorimeters, assuming the decay length is not large. If the ALP decays into photons they can only be seen in the electromagnetic calorimeter. Therefore, there are different techniques to look for an ALP in the Higgs decay chain depending on its lifetime and decay mode. Studies with a-yy decaying promptly are on-going so the results of this project will be complementary.
The analysis to be carried out for this thesis will use data already collected during the Run 2 of the LHC, as well as data to be collected in the Run 3, starting in summer 2022. For this new data-taking period, the ATLAS detector has been upgraded for muon and other detector components but the Inner Detector (ID) tracking system is unchanged. In each proton-proton collision the direction, momentum, and charge of the electrically charged particles produced is measured by the ID. Due to ageing and increasingly challenging beam conditions, careful studies are needed to ensure good functionalities during the new data-taking period. My work will focus on the Semiconductor Tracker (SCT), one of the components of the ID made of silicon strip detector modules. It is expected that the performance of the silicon modules might be affected by noise increase as a result of radiation damage in preparation to Run 3. I will work to ensure good functionality of the SCT during Run 3 through calibration and test and adjustment of relevant operating parameters to guarantee minimal