Σ+→pℓ+ℓ− decays within the standard model and beyond

Motivated by the LHCb measurement of the hyperon decay mode Σ+→pμ+μ− and prospects for improvement, we revisit the estimates for the rate and muon forward-backward asymmetry within the standard model and beyond. The standard model prediction has a fourfold ambiguity, and we suggest ways to resolve it with other measurements, including possible studies of Σ+→pe+e− in the BESIII and LHCb experiments. We use the recent BESIII measurements of Σ+→pγ and Σ+→Nπ to reduce the uncertainty in the long-distance contribution to Σ+→pμ+μ−. Beyond the standard model, we consider a general effective Hamiltonian at low energy with ten operators whose Wilson coefficients parametrize the new physics. We derive expressions for the Σ+→pμ+μ− rate and the associated muon forward-backward asymmetry in terms of these coefficients. Finally, we present the constraints on these Wilson coefficients that result from both kaon and hyperon decays and emphasize their complementarity. Published by the American Physical Society 2025