Is d* a candidate for a hexaquark-dominated exotic state?

We confirm our previous prediction of a d＊ state with I（JP） = 0（3＋） [Phys. Rev. C 60, 045203（1999）] and report for the first time based on a microscopic calculation that d＊ has about 2/3 hidden color（CC）configurations and thus is a hexaquark-dominated exotic state. By performing a more elaborate dynamical coupledchannels investigation of the △△-CC system within the framework of the resonating group method（RGM） in a chiral quark model, we find that the d＊ state has a mass of about 2.38-2.42 Ge V, a root-mean-square radius（RMS） of0.76-0.88 fm, and a CC fraction of 66%-68%. The last may cause a rather narrow width for the d＊ which, together with the quantum numbers and our calculated mass, is consistent with the newly observed resonance-like structure（M ≈2380 Me V, Γ≈70 Me V） in double-pionic fusion reactions reported by the WASA-at-COSY Collaboration.

Dibaryons: Molecular versus compact hexaquarks

Hexaquarks constitute a natural extension of complex quark systems,just as tetra-and pentaquarks do.To this end,the current status of d^(*)(2380)in both experiment and theory is reviewed.Recent high-precision meas-urements in the nucleon-nucleon channel and analyses thereof have established d^(*)(2380)as an indisputable reson-ance in the long-sought dibaryon channel.Important features of this I(J^(P))=0(3^(+))state are its narrow width and deep binding relative to the 0(1232)0(1232)threshold.Its decay branchings favor theoretical calculations predict-ing a compact hexaquark nature of this state.We review the current status of experimental and theoretical studies on d^(*)(2380)as well as new physics aspects it may bring in future.In addition,we review the situation at the 0(1232)N and N^(*)(1440)N thresholds,where evidence for a number of resonances of presumably molecular nature has been found-similar to the situation in charmed and beauty sectors.Finally,we briefly discuss the situation of dibaryon searches in the flavored quark sectors.

Analysis of the tetraquark and hexaquark molecular states with the QCD sum rules

In this article,we construct the color-singlet-color-singlet type currents and the color-singlet-colorsinglet-color-singlet type currents to study the scalar D*■*,D*D*tetraquark molecular states and the vector D*D*■*,D*D*D*hexaquark molecular states with the QCD sum rules in details.In calculations,we choose the pertinent energy scales of the QCD spectral densities with the energy scale formula■for the tetraquark and hexaquark molecular states respectively in a consistent way.We obtain stable QCD sum rules for the scalar D*■*,D*D*tetraquark molecular states and the vector D*D*■*hexaquark molecular state,but cannot obtain stable QCD sum rules for the vector D*D*D*hexaquark molecular state.The connected(nonfactorizable)Feynman diagrams at the tree level(or the lowest order)and their induced diagrams via substituting the quark lines make positive contributions for the scalar D*D*tetraquark molecular state,but make negative or destructive contributions for the vector D*D*D*hexaquark molecular state.It is of no use or meaningless to distinguish the factorizable and nonfactorizable properties of the Feynman diagrams in the color space in the operator product expansion so as to interpret them in terms of the hadronic observables,we can only obtain information about the short-distance and long-distance contributions.

Observation of e^(+)e^(-)→pppñπ-+c.c.

Using data taken at 29 center-of-mass energies between 4.16 and 4.70 GeV with the BESⅢdetector at the Beijing Electron Positron Collider corresponding to a total integrated luminosity of approximately 18.8 fb^(-1),the process e^(+)e^(-)→pppñπ+c.c.is observed for the first time with a statistical significance of 11.5σ.The average Born cross sections in the energy ranges of(4.160,4.380)GeV,(4.400,4.600)GeV and(4.610,4.700)GeV are measured to be(21.5±5.7±1.2)fb,(46.3±10.6±2.5)fb and(59.0±9.4±3.2)fb,respectively,where the first uncertainties are statistical and the second are systematic.The line shapes of the pñ and ppπ^(-)invariant mass spectra are consistent with phase space distributions,indicating that no hexaquark or di-baryon state is observed.

Understanding the structure of d~*(2380) in chiral quark model

The structure and decay properties of d＊ have been detailedly investigated in both the chiral SU（3） quark model and the extended chiral SU（3） quark model that describe the energies of baryon ground states and the nucleon-nucleon （NN） scattering data satisfactorily. By performing a dynamical coupled-channels study of the system of △△ and hidden-color channel （CC） with quantum numbers l（JP） = 0（3^＋） in the framework of the resonating group method （RGM）, we find that the d＊ has a mass of about 2.38-2.42 GeV and a root-mean-square radius （RMS） of about 0.76-0.88 fm. The channel wave function is extracted by a projection of the RGM wave function onto the physical basis, and the fraction of CC component in the d＊ is found to be about 66%-68%, which indicates that the d＊ is a hexaquark-dominated exotic state. Based on this scenario the partial decay widths of d＊ → dπ^0π^0 and d＊ → dn^＋n^- are further explicitly evaluated and the total width is then obtained by use of the branching ratios extracted from the measured cross sections of other possible decay channels. Both the mass and the decay width of d＊ calculated in this work are compatible with the data （M ≈ 2380 MeV, F ≈ 70 MeV） reported by WASA-at-COSY Collaboration.