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

Abstract

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

13 pages, 9 figures