Search for invisible decays of dark photon at Belle II

The Standard Model of particle physics displays a good description of the nature of fundamental particles. However, there are some inconsistencies between theory and experimental evidence. One of the observations made through astronomy and cosmology but is not explained by the Standard Model is dark matter. Many theories have been made to explain dark matter, which includes the dark photon particle. The dark photon is a hypothetical particle thought to exist as a mediator between the Standard Model photon and dark matter particles. The dark photons can be produced at an electron-positron collider like the Belle II experiment, located at the KEK particle physics facility. The work presented shows the first attempts of the search for dark photons with invisible decays, where studies on background were conducted for low dark photon masses. This involved understanding the different backgrounds for dark photon events, the geometry and efficiency of the main sub-detectors, and making the final background estimates. In addition, discrepancies between data and Monte Carlo events for the background control sample were found due to detector imperfections, which are also summarised here.Science, Faculty ofPhysics and Astronomy, Department ofGraduat