New physics in s → 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 (Formula presented.) transitions. Hyperon decays into neutrinos ((Formula presented.)) 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 ∼ 10−13 in the standard model. In the context of the low-energy effective theory, we find that the anticipated BESIII measurements of the (Formula presented.) decays would lead to constraints on new physics in the purely axial vector (Formula presented.) current that are stronger than the present limits from their kaon siblings (Formula presented.). 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 KL→ μ+μ− and K+→ π+μ+μ−. We conclude that the current kaon bounds are a few orders of magnitude better than those that could be obtained from Σ+→ pμ+μ− except for two scenarios with new physics in the ((Formula presented.))((Formula presented.)) and ((Formula presented.))((Formula presented.)) 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.