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Description
The Belle and Belle II experiments have collected samples of collision data at centre-of-mass energies near the $\Upsilon(4S)$ resonance. These data include low-multiplicity processes with constrained kinematics, which allow us to perform searches for dark sector particles in the mass range from a few MeV to 10 GeV. Using a 365/fb sample collected by Belle II, we search for inelastic dark matter and an associated dark higgs. We also search for a particle X that decays to the $\mu^+ \mu^-$ final state, in two models: (1) X is a new vector gauge boson Z' in a $L_\mu - L_\tau$ gauge model and (2) a new muonphilic scalar boson S.
Furthermore, we study rare B and D meson decays that could be relevant for dark-sector particle searches. By analyzing 711/fb of Belle data, we search for an axion-like particle (ALP) that decays to $\gamma\gamma$ in B meson decays, $B\to K^{(*)} a(\to \gamma\gamma)$. In addition, we will talk about rare B and D meson decays that decay to final states involving a pair of neutrinos.
