Observation of Two New N* Peaks in J/psi -> $p pi^- \bar n$ and $\bar pπ^+n$ Decays

Abstract

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