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UBC Theses and Dissertations

Search for axion like particles with the BaBar detector and photon hadron separation using Zernike moments at Belle II Hershenhorn, Alon


Even though the Standard Model of particle physics is a very successful model, we know that it is incomplete. For example, it does not explain why the world we see around us is made of matter rather than anti-matter and it does not incorporate the force of gravity. One extension of the Standard Model is the introduction of Axion Like Particles, ALPs. ALPs appear in string theories and supersymmetry and they might explain some astrophysical anomalies. ALPs can be produced in electron-positron colliders and detected in the specialized detectors built around their interaction point, like the PEP-II collider and the BaBar detector at the Stanford Linear Accelerator Center. This work presents an un-blinded search for an ALP that couples exclusively to photons in 5% of the BaBar data. We search for an excess in the invariant mass distribution of ALP candidates over a smooth background. The results are consistent with the data being composed only of Standard Model background. 90% credible interval upper limits are set on the ALP production cross section and coupling constant. These limits exclude previously unexplored regions of the phase space in the mass range 0.29 GeV/c² to 5 GeV/c². In searches involving photons, it is important to be able to efficiently detect them while rejecting other types of particles. Many high energy particle detectors detect photons in electromagnetic calorimeters that are made up of many cells. A photon interacting with the calorimeter typically leaves a different energy distribution in the cells than some other particle types, hadrons, for example. Discriminating variables for photons, based on Zernike moments, are developed in order to improve the photon identification at Belle II. One of the new variables is found to be the best at identifying photons among all other such variables used at Belle II for photons with energies in the energy range most relevant to e+e− → BB events.

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