Observation of an Anomalous Line Shape of the η′π+π- Mass Spectrum near the p p Mass Threshold in J /ψ →γη′π+π-

M. Ablikim, M. N. Achasov, S. Ahmed, X. C. Ai, O. Albayrak, M. Albrecht, D. J. Ambrose, A. Amoroso, F. F. An, Q. An, J. Z. Bai, R. Baldini Ferroli, Y. Ban, D. W. Bennett, J. V. Bennett, N. Berger, M. Bertani, D. Bettoni, J. M. Bian, F. BianchiE. Boger, I. Boyko, R. A. Briere, H. Cai, X. Cai, O. Cakir, A. Calcaterra, G. F. Cao, S. A. Cetin, J. F. Chang, G. Chelkov, G. Chen, H. S. Chen, H. Y. Chen, J. C. Chen, M. L. Chen, S. Chen, S. J. Chen, X. Chen, X. R. Chen, Y. B. Chen, H. P. Cheng, X. K. Chu, G. Cibinetto, H. L. Dai, J. P. Dai, A. Dbeyssi, D. Dedovich, H. Muramatsu, R. Poling, BESIII Collaboration

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Abstract

Using 1.09×109 J/ψ events collected by the BESIII experiment in 2012, we study the J/ψ→γη′π+π- process and observe a significant abrupt change in the slope of the η′π+π- invariant mass distribution at the proton-antiproton (pp) mass threshold. We use two models to characterize the η′π+π- line shape around 1.85 GeV/c2: one that explicitly incorporates the opening of a decay threshold in the mass spectrum (Flatté formula), and another that 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 GeV/c2 with strong couplings to the pp final states or a narrow state just below the pp mass threshold. Although we cannot distinguish between the fits, either one supports the existence of a pp moleculelike state or bound state with greater than 7σ significance.

Original languageEnglish (US)
Article number042002
JournalPhysical review letters
Volume117
Issue number4
DOIs
StatePublished - Jul 21 2016

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