Observation of ψ (3686) →n n ̄ and improved measurement of ψ (3686) →p p ̄

Abstract

We observe the process psi(3686) -> p (p) over bar eta' for the first time, with a statistical significance higher than 10 sigma, and measure the branching fraction of J/psi -> p (p) over bar eta' with an improved accuracy compared to earlier studies. The measurements are based on 4.48 x 10(8) psi(3686) and 1.31 x 10(9) J/psi events collected by the BESIII detector operating at the BEPCII. The branching fractions are determined to be B(psi(3686) -> p (p) over bar eta') = (1.10 +/- 0.10 +/- 0.08) x 10(-5) and B(J/psi -> p (p) over bar eta' = (1.26 +/- 0.02 +/- 0.07) x 10(-4), where the first uncertainties are statistical and the second ones systematic. Additionally, the eta - eta' mixing angle is determined to be -24 degrees +/- 11 degrees based on psi(3686) -> p (p) over bar eta', and -24 degrees +/- 9 degrees based on J/psi -> p (p) over bar eta', respectively.