Search for a massless particle beyond the Standard Model in the Σ → + Invisible Decay

A massless particle beyond the Standard Model is searched for in the two-body decay ${\Sigma }^{+}\to p+\mathrm{invisible}$ using $\left(1.0087±0.0044\right)×{10}^{10}$ $J/\psi $ events collected at a center-of-mass energy of $\sqrt{s}=3.097\phantom{\rule{0.25em}{0ex}}\text{GeV}$ with the BESIII detector at the BEPCII collider. No significant signal is observed, and the upper limit on the branching fraction $B\left({\Sigma }^{+}\to p+\mathrm{invisible}\right)$ 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.