Search for a dark leptophilic scalar produced in association with taupair in electron-positron annihilation at center-of-mass energies near 10.58 GeV

Speaker:  Diptaparna Biswas

Abstract: This thesis presents a search for a dark leptophilic scalar (phi_L) produced in association with taupair in electron-positron annihilation. This search will put strong constraints on new physics models that can explain the origin of dark matter beyond the current understanding of the particle content of the Standard Model. The dark scalar can decay into an electron-positron pair with invariant mass less than the di-muon mass, or into a pair of muon-antimuon above that. We search for the dark scalar in the mass range 30 MeV to 6.5 GeV. The strength of coupling between this leptophilic dark scalar and standard model leptons is proportional to the mass of the lepton it couples with. This makes the decay channel e+e- -> tau+ tau- phi_L to be the one with the highest sensitivity in the mass range being searched for. The goal of this analysis is to discover such a dark scalar particle or improve upon previous upper limits of the dark coupling constant as a function of the dark scalar mass using the data around Y(4S) resonance from the SVD2 dataset of the Belle experiment.

The Belle II detector, successor of this Belle detector, is built around the world’s highest luminosity electron-positron collider SuperKEKB, located at the KEK accelerator complex in Japan. In this thesis, a general overview of the SuperKEKB accelerator and the Belle II experiment is also presented. On the technical contribution part, my job as the deputy of operations and slowcontrol developer for the K-long and Muon (KLM) detector is discussed in this thesis. Also, my contribution in the upgrade of various parts of the Data Acquisition (DAQ) system of this experiment is described in detail.