Test of CP Symmetry in Hyperon to Neutron Decays

The quantum entangled J/ψ→ς+ς¯- pairs from (1.0087±0.0044)×1010 J/ψ events taken by the BESIII detector are used to study the nonleptonic two-body weak decays ς+→nπ+ and ς¯-→n¯π-. The CP-odd weak decay parameters of the decays ς+→nπ+ (α+) and ς¯-→n¯π- (α¯-) are determined to be 0.0481±0.0031stat±0.0019syst and -0.0565±0.0047stat±0.0022syst, respectively. The decay parameter α¯- is measured for the first time, and the accuracy of α+ is improved by a factor of 4 compared to the previous results. The simultaneously determined decay parameters allow the first precision CP symmetry test for any hyperon decay with a neutron in the final state with the measurement of ACP=(α++α¯-)/(α+-α¯-)=-0.080±0.052stat±0.028syst. Assuming CP conservation, the average decay parameter is determined as «α+» =(α+-α¯-)/2=-0.0506±0.0026stat±0.0019syst, while the ratios α+/α0 and α¯-/α¯0 are -0.0490±0.0032stat±0.0021syst and -0.0571±0.0053stat±0.0032syst, where α0 and α¯0 are the decay parameters of the decays ς+→pπ0 and ς¯-→p¯π0, respectively.