Search for a massless particle beyond the Standard Model in the Σ⁺ → p + invisible decay

A massless particle beyond the Standard Model is searched for in the two-body decay Σ⁺→p+invisible using (1.0087±0.0044)×10¹⁰ J/ψ events collected at a center-of-mass energy of s=3.097GeV with the BESIII detector at the BEPCII collider. No significant signal is observed, and the upper limit on the branching fraction B(Σ⁺→p+invisible) is determined to be 3.2×10⁻⁵ at the 90% confidence level. This is the first search for a flavor-changing neutral current process with missing energy in hyperon decays which plays an important role in constraining new physics models.