NN tensor and LS potentials in a quark model with quark-antiquark excitations

A more detailed study is made of the NN interaction in a quark model in which the (qq̄) excitations inherent in the quark-gluon interaction are explicitly incorporated into the model space. A unified treatment for all types of exchange terms of the (3q)-(3q) to (3q)-(3q)(qq̄) coupling kernels is used to calculate the space parts of the full coupling kernels and their Wigner transforms in complete analytic form. The present investigation focuses on the noncentral parts of the NN interaction. The tensor force gains almost its full strength from coupling kernels of Nπ and Nρ type. If the Nπ contribution is adjusted to fit the experimental pion-nucleon coupling constant the predicted strength of the full tensor force is in reasonable agreement with that of conventional OBEP's over the range in which the tensor force can act. The LS force gains contributions from both the pure (3q)-(3q) and the coupling kernels, but the dominant contributions (about 60–65% of the triplet-odd LS potential in the 0.7–1.0 fm range), come from the coupling kernels and particularly from the Nω and Nρ components. The triplet-odd LS potential derived from the full quark-exchange kernel is in remarkably good agreement with the OBEP LS potential over the significant range. Both the tensor and LS potentials are approximated surprisingly well over their full range by the simple (qq̄) exchange terms of our model. The ³P RGM phase shifts are calculated to show that both tensor and LS forces of our quark model are in good agreement with the experimental facts.     

RGM calculation of the NN spin-orbit interaction in a quark model

A quark-cluster model supplemented by an effective-meson-exchange potential (EMEP) is applied to the NN ³P scattering using the resonating group method (RGM). The repulsive central and the attractive spin-orbit NN interactions are obtained from the quark-exchange interaction including the symmetric spin-orbit term in the one-gluon-exchange potential. When the long- and medium-range central and tensor forces between nucleons are introduced as the EMEP, the results obtained seem to explain a considerable part of the empirical spin-orbit effect in the ³P scattering phase shifts.     

The quark-antiquark pair production mechanism in a quark jet

We assume that $\text{q}\text{q}$ pairs in a quark jet are generated similarly to e⁺e^? pairs in a constant uniform electric field. In this way we obtain relative yields and a lower bound for the average transverse momenta (ATM) of all quark flavors. In particular, the ATM of strange quarks would be larger than those of nonstrange ones. From an analysis of the mass dependence of the widths of the parent-trajectory hadrons an estimation of the radius of the colour electric field tube is obtained.     

NN potential with a six-quark core from the constituent quark model

A mesonic nucleon-nucleon potential from an effective quark interchange mechanism for non-overlapping nucleons is obtained from the constituent quark model. This is supplemented at short range by a phenomenological, non-mesonic potential describing the transition to a six-quark core with a discrete energy spectrum. NN phase shifts and low energy parameters are calculated and compared with data and phase shift analyses. Core parameters are extracted from the fits.     

K-p scattering and hyperon excitations in a chiral quark model

A chiral quark-model approach is employed to study theˉ KN scattering at low energies.The processes of Kˉp →Σ 0 π 0,Λπ 0 andˉ K 0 n at P K 800 MeV/c (i.e.the center mass energy W 1.7 GeV) are investigated.The analysis shows that the Λ(1405)S 01 dominates the processes Kˉp →Σ 0 π 0,ˉ K 0 n in the energy region considered here.Around P K 400 MeV/c,the Λ(1520)D 03 is responsible for a strong resonant peak in the cross section of Kˉp →Σ 0 π 0 andˉ K 0 n.To reproduce the data,an unexpectedly large coupling for Λ(1520)D 03 to KN is needed.In contrast,the coupling for Λ(1670)S 01 to KN appears to be weak,which could be due to configuration mixings between Λ(1405)S 01 and Λ(1670)S 01 .By analyzing Kˉ p →Λπ 0,evidences for two low mass S-wave states,Σ(1480)S 11 and Σ(1560)S 11,seem to be available.With these two states,the reaction Kˉp →ˉ K 0 n can also be described well.However,it is difficult to understand the low masses of Σ(1480)S 11 and Σ(1560)S 11 .The s-channel amplitudes for Kˉp →Λπ 0 are also larger than the naive quark model expectations.The non-resonant background contributions,i.e.t-channel and/or u-channel,also play important roles in the explanation of the angular distributions due to amplitude interferences.     

K－p scattering and hyperon excitations in a chiral quark model

A chiral quark-model approach is employed to study theˉ KN scattering at low energies.The processes of Kˉp → Σ 0 π 0,Λπ 0 andˉ K 0 n at P K 800 MeV/c （i.e.the center mass energy W 1.7 GeV） are investigated.The analysis shows that the Λ（1405）S 01 dominates the processes Kˉp → Σ 0 π 0,ˉ K 0 n in the energy region considered here.Around P K 400 MeV/c,the Λ（1520）D 03 is responsible for a strong resonant peak in the cross section of Kˉp → Σ 0 π 0 andˉ K 0 n.To reproduce the data,an unexpectedly large coupling for Λ（1520）D 03 to KN is needed.In contrast,the coupling for Λ（1670）S 01 to KN appears to be weak,which could be due to configuration mixings between Λ（1405）S 01 and Λ（1670）S 01 .By analyzing Kˉ p → Λπ 0,evidences for two low mass S-wave states,Σ（1480）S 11 and Σ（1560）S 11,seem to be available.With these two states,the reaction Kˉp →ˉ K 0 n can also be described well.However,it is difficult to understand the low masses of Σ（1480）S 11 and Σ（1560）S 11 .The s-channel amplitudes for Kˉp → Λπ 0 are also larger than the naive quark model expectations.The non-resonant background contributions,i.e.t-channel and/or u-channel,also play important roles in the explanation of the angular distributions due to amplitude interferences.     

Three-nucleon interaction in a quark cluster model

Taking into account the quark structure of the nucleons, a new three-nucleon potential is derived. It contains two parts: the pairwise interactions and a three-nucleon force. The pairwise interactions are described by the NN potentials of the quark-compound-bag model in the off-shell generalized form. The three-body force has the form of a separable potential which has a pole singularity on the energy axis. The residue at the pole is determined by the three-nucleon vertex given explicitly through all possible intermediate states formed by the 6q and 9q bags. The solution of the three-nucleon problem is obtained in an analytical form by means of the known solution of the problem with ordinary pairwise interaction. It is shown that: (i) the 9q bag is an additional source of an attractive interaction between three nucleons; (ii) the pole singularity of the three-nucleon force exhibits a resonance structure of the three-nucleon scattering amplitude; (iii) the total and partial widths of the quasi-discrete level are defined by the three-nucleon vertex. Some consequences of the new potential are discussed.     

Baryon-baryon spin-orbit interaction in a quark model

The baryon-baryon spin-orbit interactions are studied within the framework of a nonrelativistic quark-cluster model. The origin of the spin-orbit interactions is taken to be the Galilei-invariant part of the spin-orbit term in the one-gluon-exchange potential between quarks. It gives, for example, the NN spin-orbit interaction which is qualitatively similar to the empirical ones. The baryon-nucleus spin-orbit interactions are also considered along this line. The N- and Σ-nucleus spin-orbit interactions are of comparable strength, while the Λ-nucleus spin-orbit interaction is weak. The main origin of the difference between the Λ -nucleus and Σ-nucleus spin-orbit interactions is the presence of the comparatively strong antisymmetric LS (ALS) terms for both NΛ and NΣ interactions but with opposite signs. Other sources of the spin-orbit interactions are briefly discussed in connection with the problem of the spin-orbit effect in the excited baryon spectra.     

The potential of quark-antiquark interaction

A phenomenological potential of quark-antiquark interaction (1) is considered within the quasiclassical framework. It is shown that the Van Royen-Weisskopf relation has a different origin for pseudoscalar and vector mesons. For pseudoscalar mesons, it follows from the SU(4) symmetry of strong interactions, while for vector mesons, the Van Royen-Weisskopf relation is a consequence of a certain dynamical mechanism of quark-antiquark interaction. On the basis of a nonrelativistic quark model it is shown that the parameter takes the same value =0.21 (GeV)² for different quark-antiquark systems. Taking this value of , the energy spectra of quark-antiquark (c¯c)-, (b¯b)-, and (t¯t)-systems are constructed, and the partial widths of the decays V⁰e⁺e^– of these levels, are determined. A condition for the occurrence of the first negative-energy level of a quark-antiquark (q₁¯q₂)-system is found. A comparison with the experiment is made.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 104–108, April, 1984.In conclusion, I want to thank Yu. P. Nikitin and I. L. Rozental' for useful discussions and critical comments.     

Weak hyperon nucleon interaction in a quark model

The nonmesonic decay moded of Λ in nuclear matter, ΛN → NN, and production of Λ in proton-neutron scattering, pn → Λn, are studied theoretically. The weak transition of baryon is described by the one meson exchange mechanism and the direct quark-quark interaction mechanism.     

Nucleon-nucleon interaction in a “non-symmetric” quark model

We extend the work of Warke and Shanker on the nucleon-nucleon interaction to include colored nucleon clusters. By treating a two-nucleon state as a mixture of a color singlet-singlet component and a color octet-octet component, it is shown that, with a few percent admixture of the octet-octet component, a Coulomb-type quark-quark potential can generate a nucleon-nucleon potential comparable to the phenomenological central-force potentials. It also leads to a color van der Waals potential which is too strong in comparison with experiment.     

The light-baryon spectrum in a relativistic quark model with instanton-induced quark forces

This is the second of a series of three papers treating light-baryon resonances up to 3 GeV within a relativistically covariant quark model based on the three-fermion Bethe-Salpeter equation with instantaneous two- and three-body forces. In this paper we apply the covariant Salpeter framework (which we developed in the first paper, U. L?ring, K. Kretzschmar, B.Ch. Metsch, H.R. Petry, Eur. Phys. J. A 10, 309 (2001)) to specific quark model calculations. Quark confinement is realized by a linearly rising three-body string potential with appropriate spinorial structures in Dirac space. To describe the hyperfine structure of the baryon spectrum we adopt 't Hooft's residual interaction based on QCD-instanton effects and demonstrate that the alternative one-gluon exchange is disfavored on phenomenological grounds. Our fully relativistic framework allows to investigate the effects of the full Dirac structures of residual and confinement forces on the structure of the mass spectrum. In the present paper we present a detailed analysis of the complete non-strange-baryon spectrum and show that several prominent features of the nucleon spectrum such as, e.g., the Roper resonance and approximate “parity doublets” can be uniformly explained due to a specific interplay of relativistic effects, the confinement potential and 't Hooft's force. The results for the spectrum of strange baryons will be discussed in a subsequent paper, see U. L?ring, B.Ch. Metsch, H.R. Petry, this issue, p. 447. Received: 27 March 2001 / Accepted: 17 April 2001     

The three-nucleon problems with weak NN interaction

We consider the three-nucleon system in which each two-nucleon pair interacts both strongly and weakly. The strong part is assumed to be separable but an addition of small non-separable term is possible. The weak two-nucleon interaction (which is absolutely non-separable) is assumed to be given in terms of its partial wave components in momentum space. These elements have been calculated by Lassey and McKellar starting from recent models of non-relativistic (strangeness conserving non-leotonic) NN weak interaction. An expression for the weak part of the elastic nucleon-deuteron scattering amplitude is derived within the framework of two potential scattering theory. The weak three-body driving terms are explicitly evaluated. They reflect weak form factors responsible for the weak dissociation of two nucleon isobars (including parity impurity of the deuteron wave function) as well as weak NN scattering. The expressions appearing below are numerically tractable and can be used to elaborate on previous calculations.