Observation of a Near-Threshold Enhancement in the $p \bar{p}$ Mass Spectrum from Radiative $J / ψ \to γ p \bar{p}$ Decays

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We observe a narrow enhancement near

2 m_{p}

in the invariant mass spectrum of

p \bar{p}

pairs from radiative

J / ψ \to γ p \bar{p}

decays. No similar structure is seen in

J / ψ \to π^{0} p \bar{p}

decays. The results are based on an analysis of a

58 \times 10^{6}

event sample of

J / ψ

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

2 m_{p}

M = 1859_{- 10}^{+ 3} {(s t a t)}_{- 25}^{+ 5} (s y s t) M e V / c^{2}

and the total width is

Γ < 30 M e V / c^{2}

at the 90% confidence level. These mass and width values are not consistent with the properties of any known particle.

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Observation of a Near-Threshold Enhancement in thepp¯Mass Spectrum from RadiativeJ/ψ→γpp¯Decays