Observation of Two New $N^{*}$ Peaks in $J/\psi \to p{\pi }^{-}\overline{n}$ and $\overline{p}{\pi }^{+}n$ Decays

The decay $J/\psi \to \overline{N}N\pi$ provides an effective isospin $1/2$ filter for the $\pi N$ system due to isospin conservation. Using $58\times {10}^6$ $J/\psi$ decays collected with the Beijing Electromagnetic Spectrometer at the Beijing Electron Positron Collider, more than 100 thousand $J/\psi \to p{\pi }^{-}\overline{n}+\mathrm{c}.\mathrm{c}.$ events are obtained. Besides the two well-known $N^{*}$ peaks at around $1500\mathrm{MeV}/c^2$ and $1670\mathrm{MeV}/c^2$, there are two new, clear $N^{*}$ peaks in the $p\pi$ invariant mass spectrum around $1360\mathrm{MeV}/c^2$ and $2030\mathrm{MeV}/c^2$ with statistical significance of $11\sigma$ and $13\sigma$, respectively. We identify these as the first direct observation of the $N^{*}(1440)$ peak and a long-sought missing $N^{*}$ peak above $2\mathrm{GeV}/c^2$ in the $\pi N$ invariant mass spectrum.

Figure 1

The $p\pi$ invariant mass spectrum before the $M_{p\pi }>1.15\mathrm{GeV}/c^2$ requirement.

Figure 2

The missing mass distribution for $J/\psi \to p{\pi }^{-}\overline{n}$ with the fitted background shown by the shaded area.

Figure 3

Dalitz plots of $M_{n\pi }^2$ versus $M_{p\pi }^2$ for $J/\psi \to p{\pi }^{-}\overline{n}$ (left) and $\overline{p}{\pi }^{+}n$ (right).

Figure 4

The selection efficiency versus invariant mass for $p\pi$ (upper plot) and $\overline{n}\pi$ (lower plot) in the decay $J/\psi \to p{\pi }^{-}\overline{n}$.

Figure 5

The $p{\pi }^{-}$ and $\overline{p}{\pi }^{+}$ invariant mass spectra for $J/\psi \to p{\pi }^{-}\overline{n}$ (left) and $\overline{p}{\pi }^{+}n$ (right), compared with phase space.

Figure 6

Data divided by Monte Carlo phase space versus $p\pi$ invariant mass for $J/\psi \to p{\pi }^{-}\overline{n}$ (solid circles) and $J/\psi \to \overline{p}{\pi }^{+}n$ (open squares), compared with our fit (solid curve). The contributions of each resonance peak are shown by the dot-dashed lines in the same figure. The dashed line is the contribution of background terms including the nucleon pole term.