Study of $J/\ddot{I}ˆ\to p\bar{p}$ and $J/\ddot{I}ˆ\to n\bar{n}$

Abstract

The decays $J/\ddot{I}ˆ\to p\bar{p}$ and $J/\ddot{I}ˆ\to n\bar{n}$ have been investigated with a sample of 225.2 million $J/\ddot{I}ˆ$ events collected with the BESIII detector at the BEPCII $e^{+}{e}^{-}$ collider. The branching fractions are determined to be $\mathcal{B}(J/\ddot{I}ˆ\to p\bar{p})=(2.112\pm 0.004\pm 0.031)\times 10^{-3}$ and $\mathcal{B}(J/\ddot{I}ˆ\to n\bar{n})=(2.07\pm 0.01\pm 0.17)\times 10^{-3}$. Distributions of the angle $\hat{I}¸$ between the proton or anti-neutron and the beam direction are well described by the form $1+\hat{I}\pm {\cos }^2\hat{I}¸$, and we find $\hat{I}\pm =0.595\pm 0.012\pm 0.015$ for $J/\ddot{I}ˆ\to p\bar{p}$ and $\hat{I}\pm =0.50\pm 0.04\pm 0.21$ for $J/\ddot{I}ˆ\to n\bar{n}$. Our branching-fraction results suggest a large phase angle between the strong and electromagnetic amplitudes describing the $J/\ddot{I}ˆ\to N\bar{N}$ decay.

16 pages, 13 figures, the 2nd version, submitted to PRD