Search for a massless particle beyond the Standard Model in the ${\mathrm{\Sigma }}^{+}\to p+\mathrm{i}\mathrm{n}\mathrm{v}\mathrm{i}\mathrm{s}\mathrm{i}\mathrm{b}\mathrm{l}\mathrm{e}$ decay

A massless particle beyond the Standard Model is searched for in the two-body decay ${\mathrm{\Sigma }}^{+}\to p+\mathrm{i}\mathrm{n}\mathrm{v}\mathrm{i}\mathrm{s}\mathrm{i}\mathrm{b}\mathrm{l}\mathrm{e}$ using $(1.0087\pm 0.0044)\times {10}^{10}$ $J\mathrm{/}\psi$ 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 $B({\mathrm{\Sigma }}^{+}\to p+\mathrm{i}\mathrm{n}\mathrm{v}\mathrm{i}\mathrm{s}\mathrm{i}\mathrm{b}\mathrm{l}\mathrm{e})$ is determined to be $3.2\times {10}^{-5}$ 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.