New cc̄ structure around 3764 MeV at BES and predictions from unquenching the confinement spectrum

The newly discovered signal in the cc̄ spectrum by the BES collaboration is analysed on the basis of previous results of the Resonance-Spectrum-Expansion model for mesonmeson scattering. An explanation for the relatively large fraction of non-DD̄ modes for the ψ(1D)(3770) resonance is discussed. Very recently the BES Collaboration observed an anomalous line-shape in the range of 3.650 GeV to 3.872 GeV, by studying the ee → hadrons total cross sections [1]. The experimental result indicates that the anomalous line-shape can be interpreted as two possible enhancement structures, at about 3.779 GeV and 3.764 GeV, respectively. The former probably corresponds to the well-established ψ(1D)(3770), whereas the latter seems to be a new structure [1]. We will refer to the new structure by the name X(3764) in this work. In the referred BES preprint, the existence of conflicting results with respect to the branching fraction for non-DD̄ hadronic decays of the ψ(1D)(3770) is emphasized. On the one hand, the total branching fraction for exclusive non-DD̄ modes is measured to be less than 2% [2,3]. But on the other hand, for inclusive non-DD̄ decay modes, values of about 15% are found [4, 5]. The aim of the present short paper is to illustrate that it is not difficult to explain the X(3764), within a canonical cc̄ framework, provided we take into account the unquenching of the pure confinement spectrum. Over the past three decades, several strategies for unquenching qq̄ states have been developed, based on different mechanisms for confinement [6–11]. The essential point of unquenching the confinement spectrum is the inclusion of meson loops in the quarkantiquark description, or, equivalently, the inclusion of quark loops in meson-meson scattering, which results in resonance widths [12], central resonance positions different from the masses of