Observation of Two New N* Peaks in J/psi -> $pp{i}^{-}\bar{n}$ and $\bar{p}\ddot{I}{€}^{+}n$ Decays

Abstract

The $\ddot{I}€N$ system in decays of $J/\ddot{I}ˆ\to \bar{N}N\ddot{I}€$ is limited to be isospin 1/2 by isospin conservation. This provides a big advantage in studying $N^{\ast }\to \ddot{I}€N$ compared with $\ddot{I}€N$ and ${\hat{I}}^{3}N$ experiments which mix isospin 1/2 and 3/2 for the $\ddot{I}€N$ system. Using 58 million $J/\ddot{I}ˆ$ decays collected with the Beijing Electron Positron Collider, more than 100 thousand $J/\ddot{I}ˆ\to p\ddot{I}{€}^{-}\bar{n}+c.c.$ events are obtained. Besides two well known $N^{\ast }$ peaks at 1500 MeV and 1670 MeV, there are two new, clear $N^{\ast }$ peaks in the $p\ddot{I}€$ invariant mass spectrum around 1360 MeV and 2030 MeV. They are the first direct observation of the $N^{\ast }(1440)$ peak and a long-sought "missing" $N^{\ast }$ peak above 2 GeV in the $\ddot{I}€N$ invariant mass spectrum. A simple Breit-Wigner fit gives the mass and width for the $N^{\ast }(1440)$ peak as $1358\pm 6\pm 16$ MeV and $179\pm 26\pm 50$ MeV, and for the new $N^{\ast }$ peak above 2 GeV as $2068\pm 3_{-40}^{+15}$ MeV and $165\pm 14\pm 40$ MeV, respectively.