Departmental Doctoral Oral Examination (Thesis Title: “Search for Axion Like Particles with the BaBar detector”)

Abstract:
Even though the Standard Model of particle physics is a very successful model, we know that it is incomplete. There is physics beyond the Standard Model. One extension of the Standard Model is the introduction of Axion Like Particles, ALPs. 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 a search for an ALP that couple 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% 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/c^2 to 5 GeV/c^2 .
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.