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

BESIII Collaboration, M. Ablikim, M. N. Achasov +770

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…