Winston Roberts

The composite nature of baryons manifests itself in the existence of a rich spectrum of excited states, in particular in the important mass region 1-2 GeV for the light-flavoured baryons. The properties of these resonances can be identified by systematic investigations using electromagnetic and strong probes, primarily with beams of electrons, photons, and pions. After decades of research, the fundamental degrees of freedom underlying the baryon excitation spectrum are still poorly understood. The search for hitherto undiscovered but predicted resonances continues at many laboratories around the world. Recent results from photo- and electroproduction experiments provide intriguing indications for new states and shed light on the structure of some of the known nucleon excitations. The continuing study of available data sets with consideration of new observables and improved analysis tools have also called into question some of the earlier findings in baryon spectroscopy. Other breakthrough measurements have been performed in the heavy-baryon sector, which has seen a fruitful period in recent years, in particular at the B factories and the Tevatron. First results from the LHC indicate rapid progress in the field of bottom baryons. In this review, we discuss the recent experimental progress and give an overview of theoretical approaches.

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.