Search for a massless particle beyond the Standard Model in the $Σ^+\rightarrow p+{\rm invisible}$ decay

A massless particle beyond the Standard Model is searched for in the two-body decay $Σ^+\rightarrow p+{\rm invisible}$ using $(1.0087\pm0.0044)\times10^{10}$ $J/ψ$ events collected at a center-of-mass energy of $\sqrt{s}=3.097$ GeV with the BESIII detector at the BEPCII collider. No significant signal is observed, and the upper limit on the branching fraction $B(Σ^+\rightarrow p+{\rm invisible})$ is determined to be $3.2\times10^{-5}$ 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.