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In this invited talk I discuss two recent applications of charmonium (Psi) decays to N Nbar m final states, where N is a nucleon and m is a light meson. There are several motivations for studying these decays: 1) They are useful for the study of N* spectroscopy; 2) they can be used to estimate cross sections for the associated charmonium production processes p pbar to Psi m, which PANDA plans to exploit in searches for charmonium hybrid exotics; and 3) they may allow the direct experimental measurement of NNm (meson-nucleon) strong couplings, which provide crucial input information for meson exchange models of the NN force. The latter two topics are considered in this talk, which will also compare results from a simple hadron pole model of these decays to recent experimental data.
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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 Jpsi -> 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.
We use the Paris nucleon-antinucleon optical potential for explanation of experimental data in the process $e^+e^- rightarrow pbar p$ near threshold. It turns out that final-state interaction due to Paris optical potential allows us to reproduce available experimental data. It follows from our consideration that the isoscalar form factor is much larger than the isovector one.
We observe the decay $psi(3686) to n bar{n}$ for the first time and measure $psi(3686) to p bar{p}$ with improved accuracy by using $1.07times 10^8$ $psi(3686)$ events collected with the BESIII detector. The measured branching fractions are $mathcal{B}(psi(3686) to n bar{n}) = (3.06 pm 0.06 pm 0.14)times 10^{-4}$ and $mathcal{B}(psi(3686) to p bar{p}) = (3.05 pm 0.02 pm 0.12) times 10^{-4}$. Here, the first uncertainties are statistical and the second ones systematic. With the hypothesis that the polar angular distributions of the neutron and proton in the center-of-mass system obey $1+alpha cos^2theta$, we determine the $alpha$ parameters to be $alpha_{nbar{n}} = 0.68 pm 0.12 pm 0.11$ and $alpha_{pbar{p}} = 1.03 pm 0.06 pm 0.03$ for $psi(3686)to nbar{n}$ and $psi(3686)to pbar{p}$, respectively.
Using recent BABAR, CMD-3 and SND data, the sum of $e^+e^- to 3(pi^+pi^-), 2(pi^+pi^-pi^0), pbar{p}, nbar{n}$ cross sections is obtained. Unlike $e^+e^- to 3(pi^+pi^-)$ and $e^+e^- to 2(pi^+pi^-pi^0)$ processes, no structures in total cross section are found near the $Nbar{N}$ threshold within the limits of measurement errors.
The energy dependence of the cross sections of $pbar p$, $nbar n$, and meson production in $e^+e^-$ annihilation in the vicinity of the $pbar p$ and $nbar n$ thresholds is studied. The proton-neutron mass difference and the $pbar p$ Coulomb interaction are taken into account. The values of the cross sections are very sensitive to the parameters of the optical potential. It is shown that the commonly accepted factorization approach for the account of the Coulomb interaction does not work well enough in the vicinity of the threshold due to the finite size of the optical potential well.
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