Observation of $ψ(3686) \to n\bar{n}$ and improved measurement of $ψ(3686) \to p \bar{p}$

Abstract

We observe the decay ψ(3686)→nn¯ for the first time and measure ψ(3686)→pp¯ with improved accuracy by using 1.07×108ψ(3686) events collected with the BESIII detector. The measured branching fractions are B(ψ(3686)→nn¯)=(3.06±0.06±0.14)×10−4 and B(ψ(3686)→pp¯)=(3.05±0.02±0.12)×10−4. Here, the first uncertainties are statistical and the second ones systematic. With the hypothesis that the polar angular distributions of the neutron and proton in the center-of-mass system obey 1+αcos2⁡θ, we determine the α parameters to be αnn¯=0.68±0.12±0.11 and αpp¯=1.03±0.06±0.03 for ψ(3686)→nn¯ and ψ(3686)→pp¯, respectively.