Search for Axion-Like Particles produced in e+e− collisions and photon energy resolution studies at Belle II

Abstract

Despite the great successes achieved by the Standard Model (SM) in explain- ing and predicting the behavior and existence of particles, multiple phenom- ena are yet to be given a satisfying explanation. Amongst these is Dark Matter (DM), a kind of matter that would permeate the whole Universe and that so far has been observed only via its gravitational interactions. One possible extension of the SM, which may contribute to solve the mystery of DM and/or explain some astrophysical anomalies, are Axion-Like Particles (ALPs). The model taken into consideration in this thesis is of an ALP interacting with SM photons with a coupling strength gaγγ and having mass ma. This thesis describes a search for the direct production of such ALP via the process e+e− → γa(a → γγ), in the mass range 0.2 < ma < 9.7 GeV/c2. This search is performed using 0.445 fb−1 of data collected in 2018 by the Belle II detector. No evidence for ALPs is found, and a 95%-confidence-level upper limit is set on the coupling constant gaγγ at the level of 10−3 GeV−1. These limits are the strongest to date for 0.2 < ma < 1 GeV/c2. Given that the final state of the e+e− → γa(a → γγ) process is fully neutral, being made up by three photons, a proper kinematic fit with neutral particles may be a powerful tool to improve signal resolution. To achieve such a kinematic fit, a precise knowledge of the photon covariance matrix is needed. Such matrix is obtained from the results of photon resolution studies, whose status and results are presented in this thesis.