$$b \rightarrow s\ell \ell $$ Decays at Belle

Abstract

The observable \(R_{K}\) which is the ratio of branching fractions for \(B \rightarrow K\mu \mu \) to \(B \rightarrow Kee\), tests lepton flavor universality (LFU) in the standard model (SM), and hence constitutes an important probe for new physics (NP). We report herein a sensitivity study of \(R_{K}\) in \(B \rightarrow K\ell \ell \) and of the equivalent \(R_{K}(J/\psi )\) in \(B\rightarrow KJ/\psi (\rightarrow \ell \ell )\). The latter is measured with Belle’s full data sample of \(772 \times 10^{6}\) \(B\bar{B}\) pairs and the result is consistent with unity. In a variety of NP models, lepton flavor violation (LFV) comes together with LFU violation. We also report on searches for LFV in \(B \rightarrow K \mu e\) and \(B \rightarrow K e \mu \) modes. Belle has recently measured LFV \(B^{0}\rightarrow K^{*0}\ell \ell '\) and the most stringent upper limit is found.