Search for a massless particle beyond the Standard Model in the $\Xi^0\to\Lambda + \text{invisible}$ decay

Abstract

A search for a massless beyond-standard-model particle is performed in the decay $\Xi^{0}\to\Lambda+\text{invisible}$ using $(1.0087 \pm 0.0044)\times 10^{10}$ $J/\psi$ events collected with the BESIII detector at the BEPCII collider. No significant signal is observed and the upper limit on the branching fraction $\mathcal{B}(\Xi^{0}\to\Lambda+\text{invisible})$ is set to be $2.3 \times 10^{-4}$ at the $90\%$ confidence level. This is the first search for a flavor-changing neutral current process with missing energy in $\Xi^0$ decays. Throughout this paper, charge-conjugate processes are always implied.