Meson-meson correlations in baryon-baryon and antibaryon-baryon interactions
Recent work of the Jülich group about the role of meson-meson correlations in baryon-baryon and antibaryon-baryon interactions is reviewed.
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arXiv subjects
K. Holinde
Publications and source records attributed to K. Holinde.
Correlated $ππ$ and $K\bar K$ exchange in the baryon-baryon interaction
A dynamical model for correlated two-pion and two-kaon exchange in the baryon- baryon interaction is presented, both in the scalar-isoscalar ($σ$) and the vector-isovector ($ρ$) channel. The correlations between the two pseudoscalar mesons are taken into account by means of $ππ- K\bar K$ amplitudes derived from a meson-exchange model, which is in line with the empirical $ππ$ data. It is found that correlated $K\bar K$ exchange plays an important role in the $σ$-channel for baryon-baryon states with non- vanishing strangeness. The strength of correlated $ππ$ plus $K\bar K$ exchange in the $σ$-channel decreases with the strangeness of the baryon- baryon system becoming more negative. The results for correlated $ππ$- exchange in the vector-isovector channel deviate from what is expected in the naive SU(3) picture for genuine $ρ$-exchange. Shortcomings of a simplified description in terms of sharp mass $σ$- and $ρ$-exchange are pointed out.
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Correlated $\bbox{πρ}$ Exchange in the $\bbox{NN}$ Interaction
We evaluate the contribution to the nucleon-nucleon interaction due to correlated $πρ$ exchange in the $π$, $ω$, and $A_1$/$H_1$ channels by means of dispersion-theoretic methods based on a realistic meson exchange model for the interaction between $π$ and $ρ$ mesons. These processes have substantial effects: In the pionic channel it counterbalances the suppression generated by a soft $πNN$ form factor of monopole type with a cutoff mass of about 1 GeV; in the $ω$-channel it provides nearly half of the empirical repulsion, leaving little room for explicit quark-gluon effects.
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$πNN$ and $πNΔ$ formfactors determined from a microscopic model for $πN$ scattering
We determine the $πNN$ and $πNΔ$ formfactors from the $P_{11}$ resp. $P_{33}$ partial wave of $πN$ scattering by dressing corresponding bare vertices with the help of $πN$ non--pole contributions. The underlying model is based on meson exchange, and involves nucleon and delta--isobar pole and crossed--pole terms together with correlated $ππ$--exchange in the $J^P=0^+$ ($σ$) and $1^-$ ($ρ$) channel. The results are very similar for $πNN$ and $πNΔ$ and can be roughly parametrized by a monopole with cutoff mass $>$ 500 MeV, with some variation due to model dependencies. Thus the formfactors are much less soft than derived before for the $πNN$ case by Saito and Afnan using the same procedure but different $πN$ interaction models.
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Role of correlated two-pion exchange in $K^+ N$ scattering
A dynamical model for S-- and P--wave correlated $2 π$ (and $K \bar K$) exchange between a kaon and a nucleon is presented, starting from corresponding $N \bar N \rightarrow K \bar K$ amplitudes in the pseudophysical region, which have been constructed from nucleon, $Δ$--isobar and hyperon ($Λ$, $Σ$) exchange Born terms and a realistic meson exchange model of the $ππ\rightarrow K \bar K$ and $K \bar K \rightarrow K \bar K$ amplitude. The contribution in the s--channel is then obtained by performing a dispersion relation over the unitarity cut. In the $ρ$--channel, considerable ambiguities exist, depending on how the dispersion integral is performed. Our model, supplemented by short range interaction terms, is able to describe empirical $K^+ N$ data below pion production threshold in a satisfactory way.
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Short range correlations in the pion s-wave self-energy of pionic atoms
We evaluate the contribution of second order terms to the pion-nucleus s-wave optical potential of pionic atoms generated by short range nuclear correlation. The corrections are sizeable because they involve the isoscalar s-wave $πN$ amplitude for half off-shell situations where the amplitude is considerably larger than the on-shell one. In addition, the s-wave optical potential is reanalyzed by looking at all the different conventional contributions together lowest order, Pauli corrected rescattering term, second order absorptive effects, terms from the interaction of pions with the virtual pion cloud (chiral corrections) and correlation effects. Different off-shell extrapolations for the $πN$ amplitude are used and it is found that, although some individual terms are sensitive to the extrapolation, the sum of them is rather insensitive. The results are compared with empirical values from best fits to the data and are found to be compatible, within theoretical and empirical uncertainties. The results do not rule out further contributions but they put stringent constraints on their strength.
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Combined Description of $\bf{\overline{N}N}$ Scattering and Annihilation With A Hadronic Model
A model for the nucleon-antinucleon interaction is presented which is based on meson-baryon dynamics. The elastic part is the $G$-parity transform of the Bonn $NN$ potential. Annihilation into two mesons is described in terms of microscopic baryon-exchange processes including all possible combinations of $π,η,ρ,ω,a_0,f_0,a_1,f_1,a_2,f_2,K,K^*$. The remaining annihilation part is taken into account by a phenomenological energy- and state independent optical potential of Gaussian form. The model enables a simultaneous description of nucleon-antinucleon scattering and annihilation phenomena with fair quality.
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Role of Meson-Meson Correlation Effects in the $\NNb\toρπ$ Annihilation Process
Meson-meson correlation effects are investigated in the $\NNb\toρπ$ annihilation process using a realistic meson-exchange model for the $ρπ$ interaction determined previously, together with a conventional baryon-exchange transition model and a consistent $\NNb$ interaction. For $\NNb$ $S$-states, they have a drastic effect and bring the relative (${^1S_0}/{^3S_1}$) branching ratio up to the experimental value, thus resolving the long-standing so-called ``$ρπ$'' puzzle. For $\NNb$ $P$-states, their effect is of minor importance, and discrepancies remain for those ratios involving annihilation from the $\NNb({^3P_J})$ state to $ρπ(l'=2)$.
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Hadron-Hadron Interactions
The present status of the chiral approach to the $NN$ interaction as proposed by Weinberg is discussed. The important role of correlation effects and explicit vector meson ($ρ$, $ω$) exchange in the dynamics of baryon-baryon interactions is demonstrated. As an example, the inclusion of the exchange of a correlated pair of $π$ and $ρ$ meson between two nucleons appears to be mandatory in order to resolve a long-standing puzzle concerning the formfactor at the pion-nucleon-nucleon vertex.
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A dynamical model for correlated two-pion-exchange in the pion-nucleon interaction
A microscopic model for the $N\bar N\toππ$ process is presented in the meson exchange framework, which in the pseudophysical region agrees with available quasiempirical information. The scalar ($σ$) and vector ($ρ$) piece of correlated two--pion exchange in the pion--nucleon interaction is then derived via dispersion integrals over the unitarity cut. Inherent ambiguities in the method and implications for the description of pion--nucleon scattering data are discussed.
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On the structure of the scalar mesons $f_0(975)$ and $a_0(980)$
We investigate the structure of the scalar mesons $f_0(975)$ and $a_0(980)$ within realistic meson-exchange models of the $ππ$ and $πη$ interactions. Starting from a modified version of the Jülich model for $ππ$ scattering we perform an analysis of the pole structure of the resulting scattering amplitude and find, in contrast to existing models, a somewhat large mass for the $f_0(975)$ ($m_{f_0}=1015$ MeV, $Γ_{f_0}=30$ MeV). It is shown that our model provides a description of $J/ψ\rightarrowϕππ/ϕKK$ data comparable in quality with those of alternative models. Furthermore, the formalism developed for the $ππ$ system is consistently extended to the $πη$ interaction leading to a description of the $a_0(980)$ as a dynamically generated threshold effect (which is therefore neither a conventional $q\overline{q}$ state nor a $K\overline{K}$ bound state). Exploring the corresponding pole position the $a_0(980)$ is found to be rather broad ($m_{a_0}=991$ MeV, $Γ_{a_0}=202$ MeV). The experimentally observed smaller width results from the influence of the nearby $K\overline{K}$ threshold on this pole.
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