First observation of $\psi (2S)\to p\overline{n}{\pi }^{-}+c.c.$

Using $14\times {10}^6$ $\psi (2S)$ events collected with the Beijing Spectrometer (BESII) at the Beijing Electron-Positron Collider, the branching fractions of $\psi (2S)$ decays to $p\overline{n}{\pi }^{-}$ and $\overline{p}n{\pi }^{+}$ and the branching fractions of the main background channels $\psi (2S)\to p\overline{n}{\pi }^{-}{\pi }^0$, $\psi (2S)\to \gamma {\chi }_{c0}\to \gamma p\overline{n}{\pi }^{-}$, $\psi (2S)\to \gamma {\chi }_{c2}\to \gamma p\overline{n}{\pi }^{-}$, and $\psi (2S)\to \gamma {\chi }_{cJ}\to \gamma p\overline{n}{\pi }^{-}{\pi }^0$ are determined. The contributions of the $N^{*}$ resonances in $\psi (2S)\to p\overline{n}{\pi }^{-}+c.c.$ are also discussed.

Figure 1

Missing mass distributions of (a) $p{\pi }^{-}$ and (b) $\overline{p}{\pi }^{+}$ of $\psi (2S)\to p\overline{n}{\pi }^{-}+c.c.$ candidate events. The squares with error bars are data, the blank histograms are the fit, the shaded histograms are normalized backgrounds from $\psi (2S)\to \gamma {\chi }_{cJ}$, ${\chi }_{cJ}\to p\overline{n}{\pi }^{-}+c.c.$, $\psi (2S)\to p\overline{n}{\pi }^{-}{\pi }^0+c.c.$, $\psi (2S)\to \gamma {\chi }_{cJ}$, ${\chi }_{cJ}\to p\overline{n}{\pi }^{-}{\pi }^0+c.c.$, and the dashed curves are background shapes from the fit.

Figure 2

The $\gamma \gamma$ invariant mass distribution for selected $\psi (2S)\to \gamma p\overline{n}{\pi }^{-}\gamma \gamma$ candidate events.

Figure 3

The $p\overline{n}{\pi }^{-}\gamma \gamma$ invariant mass distribution for selected $\psi (2S)\to \gamma p\overline{n}{\pi }^{-}{\pi }^0$ candidate events. The shaded histogram is the sum of $\psi (2S)\to {\pi }^0{\pi }^{0}J/\psi$, $J/\psi \to p\overline{n}{\pi }^{-}$ and $\psi (2S)\to p\overline{n}{\pi }^{-}{\pi }^0$ backgrounds which have been normalized to data according to their branching fractions.

Figure 4

The $\gamma \gamma$ invariant mass distribution of the selected $\psi (2S)\to \gamma \gamma p\overline{n}{\pi }^{-}$ candidate events. The squares with error bars are data, and the histogram is the fit.

Figure 5

The $p\overline{n}{\pi }^{-}$ invariant mass distribution of the selected $\psi (2S)\to \gamma p\overline{n}{\pi }^{-}$ candidate events. The squares with error bars are data, the histogram is the fit, and the dashed curve is the background shape from the fit.

Figure 6

Dalitz plots for (a) $\psi (2S)\to p\overline{n}{\pi }^{-}$ and (b) $\psi (2S)\to \overline{p}n{\pi }^{+}$. Here we require $|m_{p\pi }^{\mathrm{recoil}}-0.938|<0.1\mathrm{GeV}/c^2$.

Figure 7

$p{\pi }^{-}$ (or $\overline{p}{\pi }^{+}$) and $\overline{n}{\pi }^{-}$ (or $n{\pi }^{+}$) invariant mass distributions for $\psi (2S)\to p\overline{n}{\pi }^{-}+c.c.$

Figure 8

Data corrected by MC simulated efficiency and phase space versus $p{\pi }^{-}$ (or $\overline{p}{\pi }^{+}$) and $\overline{n}{\pi }^{-}$ (or $n{\pi }^{+}$) invariant mass for $\psi (2S)\to p\overline{n}{\pi }^{-}+c.c.$ candidate events.