Search for a massless particle beyond the Standard Model in the Ξ0 → Λ + invisible decay

A bstract A search for a massless beyond-standard-model particle is performed in the decay Ξ 0 → Λ + invisible using (1.0087 ± 0.0044) × 10 10 J / ψ events collected with the BESIII detector at the BEPCII collider. No significant signal is observed and the upper limit on the branching fraction ℬ(Ξ 0 → Λ + invisible) is set to be 2.3 × 10 − 4 at the 90% confidence level. This is the first search for a flavor-changing neutral current process with missing energy in Ξ 0 decays. Throughout this paper, charge-conjugate processes are always implied.