Observation of a Near-Threshold Enhancement in the $p\overline{p}$ Mass Spectrum from Radiative $J/\psi \to \gamma p\overline{p}$ Decays

We observe a narrow enhancement near $2m_p$ in the invariant mass spectrum of $p\overline{p}$ pairs from radiative $J/\psi \to \gamma p\overline{p}$ decays. No similar structure is seen in $J/\psi \to {\pi }^{0}p\overline{p}$ decays. The results are based on an analysis of a $58\times {10}^6$ event sample of $J/\psi$ decays accumulated with the BESII detector at the Beijing electron-positron collider. The enhancement can be fit with either an $S$- or $P$-wave Breit-Wigner resonance function. In the case of the $S$-wave fit, the peak mass is below $2m_p$ at $M={1859}_{-10}^{+3}{\mathrm{(}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{)}}_{-25}^{+5}\mathrm{(}\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{)}\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ and the total width is $\Gamma <30\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ at the 90% confidence level. These mass and width values are not consistent with the properties of any known particle.

Figure 1

The $p\overline{p}$ invariant mass distribution for the $J/\psi \to \gamma p\overline{p}$-enriched event sample.

Figure 2

The $M_{p\overline{p}}-2m_p$ distribution for (a) selected $J/\psi \to {\pi }^{0}p\overline{p}$ decays and (b) MC $J/\psi \to {\pi }^{0}p\overline{p}$ events that satisfy the $\gamma p\overline{p}$ selection criteria. The smooth curves are the result of fits described in the text.

Figure 3

(a) The near threshold $M_{p\overline{p}}-2m_p$ distribution for the $\gamma p\overline{p}$ event sample. The solid curve is the result of the fit described in the text; the dashed curve shows the fitted background function. The dotted curve indicates how the acceptance varies with $p\overline{p}$ invariant mass. (b) The $M_{p\overline{p}}-2m_p$ distribution with events weighted by $q_0/q$.

Figure 4

The background-subtracted, acceptance-corrected $|\cos {\theta }_{\gamma }|$ distribution for $J/\psi \to \gamma p\overline{p}$-enriched events with $M_{p\overline{p}}\le 1.9\mathrm{G}\mathrm{e}\mathrm{V}/c^2$. The solid curve is a fit to a $1+{cos}^2{\theta }_{\gamma }$ shape for the region $|\cos {\theta }_{\gamma }|\le 0.8$; the dashed curve is the result of a fit to ${sin}^2{\theta }_{\gamma }$.