Ted Barnes

In this invited presentation I review some recent developments in the theory of charmonium that appear likely to be of importance for future experimental studies in this field. The specific areas considered are double charmonium production, LQCD studies of charmonium, recent results for hadron loops, c-cbar production cross sections at PANDA, charm molecules, and two recent developments, "charmiscelleny".

In this paper we consider a sequential ``meson emission'' mechanism for charmonium decays of the type $\Psi${}$\rightarrow${}NN\ifmmode\bar\else\textasciimacron\fi{}m, where $\Psi${} is a generic charmonium state, N is a nucleon, and m is a light meson. This decay mechanism, which may not be dominant in general, assumes that an NN\ifmmode\bar\else\textasciimacron\fi{} pair is created during charmonium annihilation, and the light meson m is emitted from the outgoing nucleon or antinucleon line. A straightforward generalization of this model can incorporate intermediate N* resonances. We derive Dalitz plot event densities for the cases $\Psi${}=$\eta${}c, J/$\psi${}, $\chi${}c0, $\chi${}c1, and $\psi${}$'${}; and m=$\pi${}0, f0, and $\omega${} (and implicitly, any 0-+, 0++, or 1-- final light meson). It may be possible to separate the contribution of this decay mechanism to the full decay amplitude through characteristic event densities. For the decay subset $\Psi${}$\rightarrow${}pp\ifmmode\bar\else\textasciimacron\fi{}$\pi${}0 the two model parameters are known, so we are able to predict absolute numerical partial widths for $\Gamma${}($\Psi${}$\rightarrow${}pp\ifmmode\bar\else\textasciimacron\fi{}$\pi${}0). In the specific case J/$\psi${}$\rightarrow${}pp\ifmmode\bar\else\textasciimacron\fi{}$\pi${}0 the predicted partial width and Mp$\pi${} event distribution are intriguingly close to experiment. We also consider the possibility of scalar meson and glueball searches in $\Psi${}$\rightarrow${}pp\ifmmode\bar\else\textasciimacron\fi{}f0. If the meson emission contributions to $\Psi${}$\rightarrow${}NN\ifmmode\bar\else\textasciimacron\fi{}m decays can be isolated and quantified, they can be used to estimate meson-nucleon strong couplings {gNNm}, which are typically poorly known, and are a crucial input in meson exchange models of the NN interaction. The determination of gNN$\pi${} from J/$\psi${}$\rightarrow${}pp\ifmmode\bar\else\textasciimacron\fi{}$\pi${}0 and the (poorly known) gNN$\omega${} and the anomalous ``strong magnetic'' coupling $\kappa${}NN$\omega${} from J/$\psi${}$\rightarrow${}pp\ifmmode\bar\else\textasciimacron\fi{}$\omega${} are considered as examples.

In this paper we consider a sequential "meson emission" mechanism for charmonium decays of the type Psi -> N Nbar m, where Psi is a generic charmonium state, N is a nucleon and m is a light meson. This decay mechanism, which may not be dominant in general, assumes that an NNbar pair is created during charmonium annihilation, and the light meson m is emitted from the outgoing nucleon or antinucleon line. A straightforward generalization of this model can incorporate intermediate N* resonances. We derive Dalitz plot event densities for the cases Psi = eta_c, J/psi, chi_c0, chi_c1} and psi' and m = pi0, f0 and omega (and implicitly, any 0^{-+}, 0^{++} or 1^{--} final light meson). It may be possible to separate the contribution of this decay mechanism to the full decay amplitude through characteristic event densities. For the decay subset Psi -> p pbar pi0 the two model parameters are known, so we are able to predict absolute numerical partial widths for Gamma(Psi -> p pbar pi0). In the specific case J/psi -> p pbar pi0 the predicted partial width and M_{p pi0} event distribution are intriguingly close to experiment. We also consider the possibility of scalar meson and glueball searches in Psi -> p pbar f0. If the meson emission contributions to Psi -> N Nbar m decays can be isolated and quantified, they can be used to estimate meson-nucleon strong couplings {g_NNm}, which are typically poorly known, and are a crucial input in meson exchange models of the NN interaction. The determination of g_NNpi from J\psi -> p pbar pi0 and the (poorly known) g_NNomega and the anomalous "strong magnetic" coupling kappa_{NNomega} from J/psi -> p pbar omega are considered as examples.

The couplings of charmonia and charmonium hybrids (generically Psi) to ppbar are of great interest in view of future plans to study these states using an antiproton storage ring at GSI. These low to moderate energy Psi-ppbar couplings are not well understood theoretically, and currently must be determined from experiment. In this letter we note that the two independent Dirac (gamma_mu) and Pauli (sigma_munu) ppbar couplings of the J/psi and psi' can be constrained by the angular distribution of e+e- -> (J/psi, psi') -> ppbar on resonance. A comparison of our theoretical results to recent unpolarized data allows estimates of the ppbar couplings; in the better determined J/psi case the data is inconsistent with a pure Dirac (gamma_mu) coupling, and can be explained by the presence of a sigma_munu term. This Pauli coupling may significantly affect the cross section of the PANDA process p pbar -> pi0 J/psi near threshold. There is a phase ambiguity that makes it impossible to uniquely determine the magnitudes and relative phase of the Dirac and Pauli couplings from the unpolarized angular distributions alone; we show in detail how this can be resolved through a study of the polarized reactions.

In this paper we summarize our recent results for low energy associated charmonium production cross sections, using 1) crossing symmetry, and 2) an explicit hadronic model. These predictions are of relevance to the planned charmonium and charmonium hybrid production experiment PANDA at GSI.

The QCD mechanisms underlying the exclusive strong decays and hadronic production amplitudes of charmonium remain poorly understood, despite decades of study and an increasingly detaled body of experimental information. One set of hadronic channels of special interest are those that include baryon-antibaryon states. These are being investigated experimentally at BES and CLEO-c in terms of their baryon resonance content, and are also of interest for the future PANDA experiment, in which charmonium and charmonium hybrids will be produced in p-pbar annihilation in association with light mesons. In this paper we develop a simple initial-state light meson emission model of the near-threshold associated charmonium production processes p pbar -> pi0 ccbar, and evaluate the differential and total cross sections for these reactions in this model. (Here we consider the ccbar states eta_c, J/psi, psi', chi_0 and chi_1.) The predicted near-threshold cross section for p pbar -> pi0 J/psi is found to be numerically similar to two previous theoretical estimates, and is roughly comparable to the (sparse) existing data for this process. The theoretical charmonium angular distributions predicted by this model are far from isotropic, which may be of interest for PANDA detector design studies.