Lepton flavor violation in exclusive $b\to d\ell_i\ell_j$ and $b\to s\ell_i\ell_j$ decay modes

Abstract

We discuss the exclusive lepton flavor violating (LFV) decays modes based on $b\to d\ell_i\ell_j$ and $b\to s\ell_i\ell_j$ by considering the ground state mesons and baryons. After spelling out the expressions for such decay rates in a low energy effective theory which includes generic contributions arising from physics beyond the Standard Model (BSM), we show that the experimental bounds on meson decays can be used to bound the corresponding modes involving baryons. We find, for example, $\mathcal{B}(\Lambda_b\to \Lambda\mu\tau)\lesssim 4\times 10^{-5}$. We also consider two specific models and constrain the relevant LFV couplings by using the low energy observables. In the first model we assume the Higgs mediated LFV and find the resulting decay rates to be too small to be experimentally detectable. We also emphasize that the regions favored by the bounds $\mathcal{B}(h\to\mu\tau)^\mathrm{Atlas}$ and $\mathcal{B}(h\to e\tau)^\mathrm{Atlas}$ are not compatible with $\mathcal{B}(\mu\to e\gamma)^\mathrm{MEG}$ to $1\sigma$. In the second model we assume LFV mediated by a heavy $Z'$ boson and find that the corresponding $b$-hadron branching fractions can be $\mathcal{O}(10^{-6})$, thus possibly within experimental reach at LHCb and Belle~II.