Osamu Morimatsu

Baryon-baryon spin-orbit interaction in a quark model

Abstract 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.

A study on the formation of Σ-hypernuclei by (K̄, π) reaction

Abstract The production of Σ-hypernuclear states is studied using the Green function method with both the Σ-escape process and the Λ-conversion process correctly taken into account. Some numerical results are given and discussed for the reaction 12 C(K − , π + ) with stopped K − . The bound state approximation in the calculation of the production rates is also examined.

The formation probabilities oF Σ-hypernuclei and the “unstable bound state”

Abstract A formalism for treating the processes forming unstable states such as Σ-hypemuclear states is given. The formation probabilities are shown to be expressed by the Green function for the Σ in the nucleus. The formalism is used for examining the effects of the “unstable bound state” which has been proposed as a candidate for explaining the observed sharp Σ-hypernuclear states.

Shear viscosity of a hadronic gas mixture

We discuss in detail the shear viscosity coefficient η and the viscosity to entropy density ratio η/s of a hadronic gas comprised of pions and nucleons. In particular, we study the effects of baryon chemical potential on η and η/s. We solve the relativistic quantum Boltzmann equations with binary collisions (ππ, πN, and NN) for a state slightly deviated from thermal equilibrium at temperature T and baryon chemical potential μ. The use of phenomenological amplitudes in the collision terms, which are constructed to reproduce experimental data, greatly helps to extend the validity region in the T-μ plane. The total viscosity coefficient η(T, μ) = η π + η N increases as a function of T and μ, indirectly reflecting energy dependences of binary cross sections. The increase in μ direction is due to enhancement of the nucleon contribution η N while the pion contribution η π diminishes with increasing μ. On the other hand, due to rapid growth of entropy density, the ratio η/s becomes a decreasing function of T and μ in a wide region of the T-μ plane. In the kinematical region we investigated T < 180 MeV, μ < 1 GeV, the smallest value of η/s is about 0.3. Thus, it never violates the conjectured lower bound η/s = 1/4π ∼ 0.1. The smallness of η/s in the hadronic phase and its continuity at T ≃ T c (at least for crossover at small μ) implies that the ratio will be small enough in the deconfined phase T ≥ T c . There is a nontrivial structure at low temperature and at around normal nuclear density. We examine its possible interpretation as the liquid-gas phase transition.

A Green's function method for hadrons in nuclei

Abstract A partial review is given of the Greens function method for calculating inclusive and semi-inclusive cross sections for quasi-free processes where various hadrons are produced in nuclear targets. The general properties of the Greens function related to the energy spectrum of the system is discussed. It is shown that the method provides us with a unique way of studying hadrons in nuclei in the form of the final state interaction between the produced hadron and the residual nucleus. The method is applied to the Σ-hyperon production by ( K − , π − ) reactions on nuclei in connection with the possibility of observing Σ-hypernuclear states, the production of hadronic atoms where the strong interaction interferes with the Coulomb interaction giving rise to interesting phenomena and the high energy electron scattering in the quasi-elastic region associated with the nuclear or color transparency.

Baryon-baryon interaction with a flip-flop model for quark confinement: The case of color SU(2)

Abstract The baryon-baryon interaction with a new type of confinement potential (a flip-flop model) free from the pathology of the color van der Waals force is studied in the framework of the quark-cluster model in the case of color SU(2). For moderate values of the parameter describing the confinement of the “hidden-color” part, the confinement potential turns out not to contribute to the scattering so much. The Galilei-invariant part of the spin-orbit interaction arising as a relativistic effect from the confinement potential is also studied in comparison with the one from the OGEP. The former does not contribute significantly between baryons even if its strength is determined so as to cancel the latter in the excited state of a single baryon, which suggests the possibility that the latter is an important source of the baryon-baryon spin-orbit interaction. Similar results are expected for the more realistic case of color SU(3).

Effects of the UA(1) breaking interaction on baryonic systems

Abstract The effects of the U A (1) breaking interaction on the baryon number one and two systems are estimated employing the six-quark flavour determinantal interaction as the effective interaction of quarks which reproduces the observed mass difference of η and η mesons. This is done by calculating the matrix elements of the U A (1) breaking hamiltonian with respect to unperturbed states of the MIT bag model and the nonrelativistic quark model. The determinantal interaction induces not only three-body but also two-body interactions of valence quarks. The two-body interaction is attractive, which gives rise to the NΔ mass difference with the magnitude less than one tenth of the observed one and attraction of two octet baryons at short distances whose magnitude is in the range 20∼80 MeV depending on the flavour channels and the choice of parameters. The three-body interaction is repulsive, which gives about 10∼20 MeV repulsion in the H-dibaryon channel and somewhat weaker repulsion in the flavour SU(3) octet and antidecuplet channels of two octet baryons at short distances. We also compare our results with those obtained by using the instanton-induced interaction.

A projected correlation function approach to the πNN coupling constant in QCD sum rules

Abstract We propose a new approach to construct QCD sum rules for the πNN coupling constant, g , starting from the vacuum-to-pion correlation function of the interpolating fields of two nucleons and taking its matrix element with respect to nucleon spinors. The new approach with the projected correlation function is advantageous because even in the chiral limit the dispersion integral can be parametrized with well-defined physical parameters. Another advantage of the new approach is that unwanted pole contribution is projected out. Calculating the Wilson coefficients of the operator product expansion of the correlation function up to O ( M B −4 ) and O ( m π ) where M B and m π are the Borel mass and the pion mass, respectively, we construct new QCD sum rules for the πNN coupling constant from the projected correlation function with consistently including O ( m π ) corrections. By numerically analyzing the obtained four sum rules we identified the most prominent one. After roughly estimating errors we obtained, g =10±3, as a result of this sum rule, which is in reasonable agreement with the empirical value. It is also found that the O ( m π ) correction is about 5%.

Microscopic identification of dissipative modes in relativistic field theories

We present an argument to support the existence of dissipative modes in relativistic field theories. In an O(N)

Shear viscosity of hadronic gas mixtures

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