[en] Results are presented concerning shape coexistence and neutron-proton correlations at low and high spins in the N=Z nucleus ⁵⁶Ni obtained within the complex version of the Excited Vampir variational approach. The coexistence of small and moderately deformed oblate and prolate configurations, strongly mixed in some cases, may explain the irregularities observed for the yrast line, as well as the presence of other nonyrast states connected with them by E2 or Ml transitions. The first deformed band is well reproduced and predictions on the structure of next deformed bands are presented. These are the first microscopic calculations for ⁵⁶Ni including in the model space the 0g_9/2 and 1d_5/2 spherical orbitals besides the full pf-shell. Contaminations due to the center of momentum motion are discussed and a rough method to eliminate them at least approximately is presented

[en] The ep→e'N'π'γ' and γp→N'γ'γ' reactions in the Δ resonance region are investigated in the framework of the relativistic field-theoretical spectral decomposition method for the γ*N→γ'π'N' transition amplitude. This approach allows us to express the amplitude through three-dimensional diagrams with on-mass shell intermediate states. The Δ resonance is constructed as intermediate cluster state vertical bar ψ_Δ > with real bare mass M_Δ∼1232 MeV and with a complex propagator of Breit-Wigner shape for the P₃₃ πN amplitude. Thus, the usual off-mass shell ambiguities in spin (3(2)) effective Lagrangians do not affect the present description. Using a unified treatment of nucleons, mesons, and Δ's as bound systems of quarks according to the Haag-Nishijima-Zimmermann field-theoretical approach, it is shown that quark-gluon degrees of freedom only change the rule for constructing the form factors in three-dimensional diagrams for obtaining the γ^*N → γ'π'N' transition amplitude. However, the structure of these three-dimensional diagrams remains the same as in formulations with only meson and nucleon degrees of freedom. A simple redefinition of the γ*N→γ'π'N' amplitude which insures gauge invariance in every order of perturbation theory with an arbitrary choice of phenomenological vertices is considered. We calculate the leading diagram of this amplitude in the Δ resonance region. It is demonstrated that the differential cross section of the ep→e'N'π'γ' and γp→N'π'γ' reactions is sensitive to the value of the magnetic moment of the Δ⁺ resonance. Thus, the measurement of the differential cross section for ep→e'N'π'γ' and γp→N'π'γ' reactions can be used for determining the magnetic dipole moment of Δ⁺

[en] The inclusive process of the neutrinoless muon-to-electron conversion in nuclei is investigated by using the renormalized quasiparticle random-phase approximation (RQRPA). This approach is an improvement of the ordinary QRPA which restores the Pauli principle. It is a more reliable method for extremely small transition matrix elements as for the (μ^-,e^-) conversion. Our calculations refer to a set of nuclei throughout the periodic table but we specifically investigate those isotopes for which experiments are done or planned. We study in particular ⁴⁸Ti which is at present used as a target at PSI in the new ongoing μ-e conversion experiment with the SINDRUM II spectrometer

[en] Mass spectra of heavy meson ground states D, D_s, B and B_s are calculated based on a relativistic quark potential model with mean field approach. Radiative and hadronic pion decays and Isgur-Wise functions of semileptonic decays of these heavy states are investigated. In our approach, both light and heavy quark wave functions are treated independently by solving Dirac equation self-consistently. Linear confinement and one-gluon exchange interaction are taken into account. The calculations show that the mass splitting between the triplet ³S₁ and singlet ¹S₀ heavy meson states affects the Isgur-Wise functions. Our results also illustrate the success of heavy quark effective theory in the mass spectra of the heavy mesons. Our calculations for the radiative and strong decays of the heavy mesons are in agreement with the data

No abstract available

[en] The pair structure and the average pairing gaps of realistic wave functions obtained within the complex Excited Vampir variational approach are investigated in order to evaluate the neutron-proton pairing correlations at low and high spins in medium mass N≅Z nuclei. The number of isovector J^π=0⁺ pairs is calculated for the lowest few 0⁺ states in two chains of nuclei in the A≅70 mass region. The results indicate the dominant role played by the isovector neutron-proton pairing correlations in the structure of odd-odd N=Z nuclei and the reduction of their importance with increasing neutron excess in even-even nuclei. The evolution of particular isovector and isoscalar pairs with increasing angular momentum is analyzed for the odd-odd N=Z nucleus ⁷⁴Rb and the even-even N=Z nucleus ⁷²Kr. It turns out that in the nucleus ⁷⁴Rb the neutron-proton correlations play an essential role for the alignment of the yrast positive-parity even-spin band

[en] Results are presented on shape coexistence, shape transition and neutron-proton correlations at low and high spin states in the nuclei ⁷²Kr and ⁷⁴Kr. They have been obtained within the complex version of the Excited Vampir variational approach. Oblate-prolate coexistence and mixing at low spin is found to be responsible for the irregular sequence of states identified at low excitation energy. At high spins the coexistence of states with different prolate deformations results in complex multiple band structures. The present investigation indicates furthermore that particular neutron-proton matrix elements of the effective interaction play an essential role for the shape mixing

[en] By using the four-dimensional relativistic harmonic oscillator model, the Q²-evolution of the helicity amplitudes of the low-lying resonances P₃₃(1232), S₁₁(1535), and D₁₃(1520) are estimated. The results show that the Lorentz contraction causes remarkable effects on those helicity amplitudes

[en] We investigate nuclear matter properties in the relativistic Brueckner approach. The in-medium on-shell T-matrix is represented covariantly by five Lorentz invariant amplitudes from which we deduce directly the nucleon self-energy. We discuss the ambiguities of this approach and the failure of previously used covariant representations in reproducing the nucleon self-energies on the Hartree-Fock level. To enforce correct Hartree-Fock results we develop a subtraction scheme which treats the bare nucleon-nucleon potential exactly in accordance with the different types of meson exchanges. For the remaining ladder kernel, which contains the higher order correlations, we employ two different covariant representations in order to study the uncertainty inherent in the approach. The nuclear matter bulk properties are only slightly sensitive to the explicit representation used for the kernel. However, we obtain new Coester lines for the various Bonn potentials which are shifted towards the empirical region of saturation. In addition the nuclear equation-of-state turns out to be significantly softer in the new approach

No abstract available