S.B. Khadkikar

Hyperon-nucleon interaction in the quark cluster model☆

Abstract The lambda-nucleon and sigma-nucleon interaction is described in the nonrelativistic quark cluster model. The SU(3) flavor symmetry breaking due to the different quark masses is taken into account, i.e. different wavefunctions for the light (up, down) and heavy (strange) quarks are used in flavor and orbital space. The six-quark wavefunction is fully antisymmetrized. The model hamiltonian contains gluon exchange, pseudoscalar meson exchange and a phenomenological σ-meson exchange. The six-quark scattering problem is solved within the resonating group method. The experimental lambda-nucleon and sigma-nucleon cross sections are well reproduced.

Binding energy of the dihyperon in the quark cluster model

Abstract The dihyperon is described in the non-relativistic quark cluster model. The SU(3) flavor symmetry breaking due to the different quark masses is taken into account not only in the hamiltonian, but also in the quark wave functions. The interaction contains the one-gluon-exchange potential, pseudoscalar-meson exchange between quarks plus an additional phenomenological σ-meson exchange. The mass of the dihyperon is predicted to be (2211 ± 5) MeV, i.e. a binding energy of the dihyperon of (−20 ± 5) MeV below the ΛΛ threshold is found.

pp̄ Annihilation at rest into mesons in the quark rearrangement model and the quark annihilation modelt

Abstract pp annihilation at rest into mesons is calculated in the quark rearrangement model in which 3 quarks and 3 antiquarks rearrange into 3 mesons and in the quark annihilation model in which one or two quark-antiquark pairs are annihilated and a quark-antiquark pair is created to form two or three mesons. The constants of the quark-meson coupling are adjusted such that the meson-nucleon interactions agree with experiment. The radial dependences of the quark-meson couplings are all assumed to be the same. This is determined by the vector-meson-quark interaction derived from the charge form factor of the proton. In the case of the decay into three mesons both rearrangement and annihilation models explain with similar reliability the branching ratio of the decay products. In the case of the decay into two-meson annihilation gives a better agreement with experiment than rearrangement.

Surface and volume contributions to real and imaginary parts of the heavy-ion optical potential

Abstract Starting from the Feshbach expression for the optical potential, explicit formulae for the real and the imaginary parts of the optical potential between two heavy ions (HIs) are obtained. They are each composed of a volume and a surface term. The contributions to the volume term are calculated in two nuclear Fermi liquids which flow through each other starting from the realistic Reid soft core nucleon-nucleon (NN) interaction. Since the Fermi surface is formed by two spheres one obtains a complex Brueckner reaction matrix which is approximated by a complex, effective local interaction. It is used in a fully antisymmetrized double folding procedure to obtain the volume terms of the optical potential between the two HIs. The surface contributions are directly calculated in the collision of the two finite HIs. The collective surface vibrations (3 − octupole state and 2 + , 4 + ( T = 0) giant resonances for the 16 O− 16 O collision) are included as intermediate states. This yields especially an imaginary contribution at the surface which reduces the transparency found with the volume terms alone. The method is applied to 16 O− 16 O scattering at 83 and 332 MeV laboratory energy. The local approximations to the real and imaginary parts obtained in this way agree well with phenomenological fits. The surface terms improve the agreement of the differential cross section at 80 MeV where experimental data are available.

Folding computation of the 16O + 16O optical potential with a complex effective force

Abstract The 16O + 16O optical potential is obtained by using the folding method together with a previously defined complex effective nucleon-nucleon force closely related with the heavy-ion collision dynamics. This type of force allows the computation of both the real and the imaginary parts of the optical potential. Through the use of the folding method, finite-range effects are correctly incorporated. In that respect, the present results improve upon those obtained with the local density approximation. They also compare favourably with the phenomenologieal optical potentials.

Two-vacua RPA and the two-neutrino double-beta decay

Abstract A new method “two-vacua random-phase approximation” (TVRPA) for the description of the two-neutrino nuclear double-beta decay is developed. The nuclear wave functions of the intermediate odd-odd nucleus are constructed as linear combinations of proton-particle neutron-hole excitations from the ground state of the initial even-even ( N , Z ) nucleus and, at the same time, as neutron-particle proton-hole excitations from the ground state of the final ( N − 2, Z + 2) nucleus. The Ritz variational principle is used to obtain RPA-like equations. The ground state in the initial and final nucleus is approximated with HFB wave functions including proton-neutron pairing. A projection on numbers of protons and neutrons is performed additionally on the wave functions in both nuclei to restore the exact number of particles, broken by the quasi-particle transformation. The method is applied, as a preliminary test, to the 2ν double-beta decay in 20 48 Ca 28 . More realistic B (GT − ) and B (GT + ) Gamow-Teller strength distributions than QRPA and PQRPA are obtained. No dependence is found on the particle-particle strength parameter g pp , renormalizing Brueckner reaction matrix elements of the Bonn potential.

Two vacua particle number projected random phase approximation

Abstract A new Two Vacua Particle Number Projected Random Phase Approximation (TVRPA) method for nuclear structure calculations is presented. In the framework of this method excited states of the odd-odd nucleus (A,Z+1) are given as configuration mixing of the particle-hole excitations from the ground states of two even-even nuclei, (A,Z) and (A,Z+2). TVRPA has been applied for the study of the two neutrino mode of the nuclear double beta decay of 48 Ca.

The heavy-ion potential and its increasing transparency at intermediate energies

Abstract Starting from the Reid soft-core potential and the collision of two nuclear matters we solve the Bethe-Goldstone equation for the complex reaction matrix. The local density approximation including a finite range correction enables us to calculate the volume part of the optical potential between two heavy ions up to 100 MeV per nucleon. The surface contributions are calculated from the second-order Feshbach expression including low- and high-lying collective states. The data for 16 O- 16 O are nicely reproduced including the reduction of the reaction cross section above 20 to 30 MeV per nucleon. The maximum of the reaction cross section at 10–20 MeV per nucleon is essentially due to the surface contributions. The description of the 12 C- 12 C data is not so satisfactory due to the assumption of weak coupling and the neglect of some rotational excitations

Non-static contributions to the 0+ → 0+ ββ-decay in the presence of the Δ(32,32) resonance

Abstract Non-static contributions to the 0 ν 0 + → 0 + ββ-decay in the presence of the Δ( 3 2 , 3 2 ) resonance are considered in the constituent quark model. Even though it is then possible to have contributions of order P Δ in the ββ- decay width, they are negligible compared to the usual two-nucleon contribution.

The triaxial rotation vibration model in the XeBa region

Abstract We calculate the energy spectrum and B(E2) branching ratios for 130 Ba using a triaxial rotation vibration model. The results agree well with those obtained in the Interacting Boson Approximation (IBA) and with experiment.