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 $$ s\to dv\overline{v} $$ s → dv v ¯ and s → dℓ+ℓ− transitions. Hyperon decays into neutrinos $$ \left({B}_1\to {B}_2v\overline{v}\right) $$ B 1 → B 2 v v ¯ 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 $$ {B}_1\to {B}_2v\overline{v} $$ B 1 → B 2 v v ¯ decays would lead to constraints on new physics in the purely axial vector $$ \overline{d}{\gamma}_{\mu }{\gamma}_5s $$ d ¯ γ μ γ 5 s current that are stronger than the present limits from their kaon siblings $$ K\to \pi \pi v\overline{v} $$ K → ππv v ¯ . 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 $$ \left(\overline{d}{\gamma}^{\mu }s\right)\left(\overline{\mathrm{\ell}}{\gamma}_{\mu }{\gamma}_5\mathrm{\ell}\right) $$ d ¯ γ μ s ℓ ¯ γ μ γ 5 ℓ and $$ \left(\overline{d}{\gamma}^{\mu }{\gamma}_5s\right)\left(\overline{\mathrm{\ell}}{\gamma}_{\mu}\mathrm{\ell}\right) $$ d ¯ γ μ γ 5 s ℓ ¯ γ μ ℓ 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.

Progress on the exploratory calculation of the rare Hyperon decay $\Sigma^+ \to p \ell^+ \ell^-$

“…The current SM prediction of these form factors comes from a collection of methods [10][11][12]. The real part of 𝑎 and 𝑏 can be constrained from experimental measurements of a similar decay with a real photon Σ + → 𝑝𝛾.…”

“…Finally, the imaginary parts of all 4 form factors are computed in Chiral Perturbation Theory (𝜒 PT ) using the optical theorem. Putting these together gives the SM predictions [10][11][12] of the branching fraction (B) and forward-backward asymmetry (𝐴 FB ) of the…”

Prospects for a lattice calculation of the rare decay Σ+ → pℓ+ℓ−

“…In the following, we will briefly review the phenomenological determination [3][4][5] of the branching ratio for Σ + → p + − leading to the result quoted in eq. (1.3).…”

“…The current state-of-the-art Standard Model theory predictions for this process [3][4][5] use a combination of dispersion relations, experimental input, various formulations of Baryon Chiral Perturbation Theory and model estimates (e.g. vector meson dominance) and arrive at a range of results [5] 1.6 × 10 −8 ≤ B Σ + → pµ + µ − ≤ 8.9 × 10 −8 .…”

Discovering new physics in rare kaon decays

“…Additionally there is a renewed phenomenological interest in these decays in Ref. [57], where they are discussed jointly with the rare kaon decays. We are currently preparing a theoretical publication demonstrating how to compute the rare hyperon amplitude from first principles using lattice QCD.…”