New physics in $s\to d$ semileptonic transitions: rare hyperon vs. kaon decays

Abstract

We investigate the potential of rare hyperon decays to probe the short distance structure in the $s\to d\hat{I}½\bar{\hat{I}}½$ and $s\to d{\ell }^{+}{\ell }^{-}$ transitions. Hyperon decays into neutrinos ($B_1\to B_2\hat{I}½\bar{\hat{I}}½$) can be reliably predicted by using form factors determined in baryon chiral perturbation theory. Their decay rates are sensitive to different short-distance operators, as compared to their kaon counterparts, and the corresponding branching fractions are in the range of $10^{-14}\sim 10^{-13}$ in the standard model. In the context of the low-energy effective theory, we find that the anticipated BESIII measurements of the $B_1\to B_2\hat{I}½\bar{\hat{I}}½$ decays would lead to constraints on new physics in the purely axial vector $\bar{d}{\hat{I}}_{\hat{I}}^3¼{\hat{I}}_5^{3}s$ current that are stronger than the present limits from their kaon siblings $K\to \ddot{I}€\ddot{I}€\hat{I}½\bar{\hat{I}}½$. On the other hand, although hyperon decays into charged leptons are dominated by long-distance hadronic contributions, angular observable such as the leptonic forward-backward asymmetry is sensitive to the interference between long- and short-distance contributions. We discuss the sensitivity to new physics of a potential measurement of this observable in comparison with observables in the kaon decays $K_L\to \hat{I}{¼}^{+}\hat{I}{¼}^{-}$ and $K^{+}\to \ddot{I}{€}^{+}\hat{I}{¼}^{+}\hat{I}{¼}^{-}$. We conclude that the current kaon bounds are a few orders of magnitude better than those that could be obtained from $\hat{I}{\pounds }^{+}\to p\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ except for two scenarios with new physics in the $(\bar d γ^μs)(\bar\ellγ_μγ_5\ell)$ and $(\bar d γ^μγ_5s)(\bar\ellγ_μ\ell)$ currents. Finally, we point out that the loop effects from renormalization group evolution are important in this context, when relating the low-energy effective field theory to new physics models in the UV.

27 pages, 1 figure, 5 tables. Matches published version