Observation of a resonance X(1835) in J/\psi --> \gamma \pi^+ \pi^- \eta^{\prime}

TL;DR

This paper reports the observation of a new resonance, X(1835), in J/psi decays, with measured mass and width, suggesting it is a novel state not matching known mesons and possibly related to par{p} threshold phenomena.

Contribution

First observation of the X(1835) resonance in J/psi decays with detailed measurements of its properties and significance, indicating a potentially new meson-like state.

Findings

Resonance X(1835) observed with 7.7 sigma significance.

Measured mass M = 1833.7 MeV/c^2 and width b = 67.7 MeV/c^2.

Properties suggest a new state, not matching known mesons.

Abstract

The decay channel J/\psi --> \gamma \pi^+ \pi^- \eta^{\prime} is analyzed using a sample of 5.8\times 10^7 J\psi events collected with the BESII detector. A resonance, the X(1835), is observed in the \pi^+ \pi^- \eta^{\prime} invariant mass spectrum with a statistical significance of 7.7 \sigma. A fit with a Breit-Wigner function yields a mass M = 1833.7 \pm 6.1(stat) \pm 2.7(syst) MeV/c^2, a width \Gamma = 67.7 \pm 20.3(stat) \pm 7.7(syst) MeV/c^2 and a product branching fraction B(J/\psi --> \gamma X) B(X --> \pi^+ \pi^- \eta^{\prime}) = (2.2 \pm 0.4(stat) \pm 0.4(syst)) \times 10^{-4}. The mass and width of the X(1835) are not compatible with any known meson resonance. Its properties are consistent with expectations for the state that produces the strong p\bar{p} mass threshold enhancement observed in the J\psi -->\gamma p \bar{p} process at BESII.