We incorporate the color-screening effect due to light quark pair creation into the heavy quarkantiquark potential, and investigate the effects of screened potential on the spectrum of higher charmonium. We calculate the masses, electromagnetic decays, and E1 transitions of charmonium states in the screened potential model, and propose possible assignments for the newly discovered charmonium or charmonium-like "X, Y, Z" states. We find the masses of higher charmonia with screened potential are considerably lower than those with unscreened potential. The χc2(2P ) mass agrees well with that of the Z(3930), and the mass of ψ(4415) is compatible with ψ(5S) rather than ψ(4S). In particular, the discovered four Y states in the ISR process, i.e., Y (4008), Y (4260), Y (4320/4360), Y (4660) may be assigned as the ψ(3S), ψ(4S), ψ(3D), ψ(6S) states respectively. The X(3940) and X(4160) found in the double charmonium production in e + e − annihilation may be assigned as the ηc(3S) and χc0(3P ) states. Based on the calculated E1 transition widths for χc1(2P ) → γJ/ψ and χc1(2P ) → γψ(2S) and other results, we argue that the X(3872) may be a χc1(2P ) dominated charmonium state with some admixture of the D 0D * 0 component. Possible problems encountered in these assignments and comparisons with other interpretations for these X, Y, Z states are discussed in detail. We emphasize that more theoretical and experimental investigations are urgently needed to clarify these assignments and other interpretations.

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Using the same quenched limit as input, we compare the charmonium spectra predicted by two different models, i.e., the coupled-channel model and the screened potential model in the mass region below 4 GeV, in which the contributions from decay channels involving P-wave (as well as even higher excited) D mesons can be neglected. We find that the two models have similar global features in describing the charmonium spectrum since they approximately embody the same effect of the vacuum polarization of dynamical light quark pairs. Adopting these models will be helpful to clarify the nature of the newly discovered charmonium or charmoniumlike states; and the coupled-channel model is more adept in investigating the influences of open-charm thresholds on the charmonium spectrum. In particular, we show the S-wave decay coupling effect on lowering the c1 ð2PÞ mass toward the D " D Ã threshold, in support of the assignment of the Xð3872Þ as a c1 ð2PÞ-dominated charmonium state.