Study of $J/\psi\to p\bar{p}$ and $J/\psi\to n\bar{n}$

TL;DR

This study measures the decay rates and angular distributions of $J/ar{ ext{psi}}$ decays to proton-antiproton and neutron-antineutron pairs using a large dataset, providing insights into the decay mechanisms.

Contribution

It provides precise measurements of branching fractions and angular distribution parameters for $J/ar{ ext{psi}}$ decays, revealing a large phase angle between decay amplitudes.

Findings

Branching fraction for $J/\psi\to p\bar{p}$ is $(2.112\pm0.004\pm0.031)\times10^{-3}$.

Branching fraction for $J/\psi\to n\bar{n}$ is $(2.07\pm0.01\pm0.17)\times10^{-3}$.

Angular distribution parameter $\alpha$ is approximately 0.595 for $p\bar{p}$ and 0.50 for $n\bar{n}$ decays.

Abstract

The decays $J/\psi\to p\bar{p}$ and $J/\psi\to n\bar{n}$ have been investigated with a sample of 225.2 million $J/\psi$ events collected with the BESIII detector at the BEPCII $e^+e^-$ collider. The branching fractions are determined to be $\mathcal{B}(J/\psi\to p\bar{p})=(2.112\pm0.004\pm0.031)\times10^{-3}$ and $\mathcal{B}(J/\psi\to n\bar{n})=(2.07\pm0.01\pm0.17)\times10^{-3}$. Distributions of the angle $\theta$ between the proton or anti-neutron and the beam direction are well described by the form $1+\alpha\cos^2\theta$, and we find $\alpha=0.595\pm0.012\pm0.015$ for $J/\psi\to p\bar{p}$ and $\alpha=0.50\pm0.04\pm0.21$ for $J/\psi\to n\bar{n}$. Our branching-fraction results suggest a large phase angle between the strong and electromagnetic amplitudes describing the $J/\psi\to N\bar{N}$ decay.