Search for a long-lived spin-0 particle in b → s quark transitions at the Belle II experiment

Abstract

Phenomena such as dark matter can be connected to the standard model via additional spin-0 mediators. This doctoral thesis describes a search for a long-lived spin-0 particle S in rare B-meson decays mediated by b → s quark transitions. A dataset corresponding to an integrated luminosity of 189fb−1 of e+e− collisions collected at the Υ(4S) resonance energy by the Belle II experiment is analysed. The search is carried out in eight exclusive channels, the two modes of production B+ → K+S and B0 → K∗0 S with decays via S → e+e−/μ+μ−/π+π−/K+K−. Decays of the S to standard model particles are motivated if the decay to dark matter is kinematically not possible. Mediator masses between (0.025 − 4.78) GeV/c2 and lifetimes between (0.001 − 400) cm are probed. Long-lived particle reconstruction is studied and validated with K0S-mesons. Maximum likelihood fits to the reconstructed S mass distribution are S used to determine the signal yield. No evidence for the signal process is found. Model-independent upper limits are derived on the branching fractions of the signal processes. The upper limits extend down to the order of O(10−7). These are the first constraints on hadronic final states of the S produced in B → KS and the most stringent bounds from a direct search for S → e+e− at e+e− colliders. The results are interpreted in models that predict a dark Higgs-like scalar and an axionlike particle with fermion couplings. The model-dependent bounds are competitive with existing experimental constraints.