Observation of an anomalous line shape of the ${\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ mass spectrum near the $p\bar{p}$ mass threshold in $J/\psi \to \gamma {\eta }^{\prime }{\pi }^{+}{\pi }^{-}$

Abstract

Using $1.09\times 10^9$ $J/\psi$ events collected by the BESIII experiment in 2012, we study the $J/\psi \to \gamma {\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ process and observe a significant abrupt change in the slope of the ${\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ invariant mass distribution at the proton-antiproton ($p\bar{p}$) mass threshold. We use two models to characterize the ${\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ line shape around $1.85\text{GeV}/c^2$: one which explicitly incorporates the opening of a decay threshold in the mass spectrum (Flatté formula), and another which is the coherent sum of two resonant amplitudes. Both fits show almost equally good agreement with data, and suggest the existence of either a broad state around $1.85\text{GeV}/c^2$ with strong couplings to $p\bar{p}$ final states or a narrow state just below the $p\bar{p}$ mass threshold. Although we cannot distinguish between the fits, either one supports the existence of a $p\bar{p}$ molecule-like state or bound state with greater than $7\sigma$ significance.

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• The BESIII Physics Programme
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• Numerical calculation of the decay widths, the decay constants, and the decay energy spectra of the resonances of the delta-shell potential
• Light unflavor vector meson spectroscopy around the mass range of $2.4\sim 3$ GeV and possible experimental evidence
• Potential higher radial excitations in the light pseudoscalar meson family
• Study of $\eta (1475)$ and $X(1835)$ in radiative $J/\psi$ decays to $\gamma \varphi$
• Observation of the Anomalous Shape of $X(1840)$ in $J/\psi \to \gamma 3({\pi }^{+}{\pi }^{-})$ Indicating a Second Resonance Near $p\bar{p}$ Threshold
• Revisiting the pseudoscalar meson and glueball mixing and key issues in the search for pseudscalar glueball state
• Interaction of real and virtual $p\bar{p}$ pairs in $J/\psi \to p\bar{p}\gamma (\rho ,\omega )$ decays
• $J/\psi \to \gamma {\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ and the structure observed around the $\bar{p}p$ threshold
• Gluonic nature of the newly observed state $X(2600)$
• Radiative production and decays of the exotic ${\eta }_1^{\prime }(1855)$ and its siblings
• Observation of $J/\psi$ Electromagnetic Dalitz Decays to $X(1835)$, $X(2120)$ and $X(2370)$
• Study of light tetraquark spectroscopy
• Structure around $p\bar{p}$ threshold in $J/\psi$ radiative decays
• Production of $D^{(\ast )}{\bar{D}}^{(\ast )}$ near the thresholds in $e^{+}{e}^{-}$ annihilation
• Measurement of the branching fraction of $J/\psi \to \omega {\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ and search for $J/\psi \to \omega X(1835),X(1835)\to {\eta }^{\prime }{\pi }^{+}{\pi }^{-}$ decay
• Common explanation of the behavior of some $e^{+}{e}^{-}$ annihilation processes around $\sqrt{s}$ = 1.9 GeV
• Study of two-photon decays of pseudoscalar mesons via $J/\psi$ radiative decays
• Reexamination of antinucleon-nucleon interactions in covariant chiral effective field theory
• Coupled-channel analysis of the near-threshold $e^{+}{e}^{-}\to N\bar{N}$ cross sections
• Baryons and baryoniums in the perspective of QCD sum rules