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

TL;DR

This study reports the first observation of the decay $oldsymbol{\psi(3686) o nar{n}}$ and provides an improved measurement of $oldsymbol{\psi(3686) o par{p}}$ branching fractions using a large data sample from BESIII.

Contribution

The paper presents the first observation of $oldsymbol{\psi(3686) o nar{n}}$ decay and refines the measurement of $oldsymbol{\psi(3686) o par{p}}$ branching ratios with detailed angular distribution analysis.

Findings

First observation of $oldsymbol{\psi(3686) o nar{n}}$ decay.

Measured branching fractions: $oldsymbol{(3.06 imes 10^{-4})}$ for $nar{n}$ and $oldsymbol{(3.05 imes 10^{-4})}$ for $par{p}$.

Determined $oldsymbol{\alpha}$ parameters for angular distributions: $oldsymbol{0.68}$ for $nar{n}$ and $oldsymbol{1.03}$ for $par{p}$.

Abstract

We observe the decay $\psi(3686) \to n \bar{n}$ for the first time and measure $\psi(3686) \to p \bar{p}$ with improved accuracy by using $1.07\times 10^8$ $\psi(3686)$ events collected with the BESIII detector. The measured branching fractions are $\mathcal{B}(\psi(3686) \to n \bar{n}) = (3.06 \pm 0.06 \pm 0.14)\times 10^{-4}$ and $\mathcal{B}(\psi(3686) \to p \bar{p}) = (3.05 \pm 0.02 \pm 0.12) \times 10^{-4}$. 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 $1+\alpha \cos^2\theta$, we determine the $\alpha$ parameters to be $\alpha_{n\bar{n}} = 0.68 \pm 0.12 \pm 0.11$ and $\alpha_{p\bar{p}} = 1.03 \pm 0.06 \pm 0.03$ for $\psi(3686)\to n\bar{n}$ and $\psi(3686)\to p\bar{p}$, respectively.