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.

20 pages, 6 figures