The spins of even-even nuclei are always 0^+ without any exception. This fact is believed to be a consequence of the strong attractive short range interaction. However, the predominance of 0^+ states has been recently recognized as the lowest states in shell model calculations with random two-body interactions. In this talk, we present our study of this problem taking a simple system such as four particles in a single j-shell. We have confirmed the dominance of 0^+ states as ground states and found an explanation of this phenomena.
[BA.002] Chiral Bands in Odd-Odd Triaxially Deformed Nuclei
K. Starosta (SUNY at Stony Brook)
In rotational bands built on high-j single-particle orbitals in odd-odd nuclei having triaxial shapes, the angular momenta of the valence proton, the valence neutron, and the collective rotation tend to align along the perpendicular axes of the triaxial core. This occurs when the Fermi level is low within the proton (neutron) subshell, but high within the neutron (proton) subshell resulting in their angular momenta oriented along the short and long axes, respectively. The core angular momentum is oriented along the intermediate axis because it has the largest moment of inertia according to the model of irrotational flow. These three mutually perpendicular vectors can be arranged to form two systems which differ by intrinsic chirality, a left- and a right-handed system; the two systems cannot be transformed into each other by rotation or space inversion, but are related by an operator, which involves time reversal. Chirality resulting from orthogonal coupling of angular momenta is unique to rotational bands in atomic nuclei since these are the only systems where a significant part of the total spin results from single-particle contributions. In relation to time reversal, chirality is a novel example of spontaneous symmetry breaking, on the same level as octupole deformation in relation to space inversion. The main experimental fingerprint of chirality in nuclear rotation is the doubling of states in rotational bands. \Delta I=1 doublet-band structures with remarkably similar experimental characteristics, recently observed for N=75 and N=73 isotones in the A\sim130 region, have been interpreted as chiral-band partners built on the \pih_11/2\nuh_11/2 configuration. Additional transition rate information is being investigated both experimentally and theoretically. The description of the chiral partner bands based on the microscopic Tilted Axis Cranking approach in the intrinsic, body-fixed reference frame and phenomenological core-particle coupling in the laboratory reference frame will be discussed.
[BA.003] Effective Interactions in the Shell Model Basis
Thomas Luu (Physics Dept. University of Washington)
The idea of utilizing the shell model as a rigorous effective theory is presented. In particular, the effective interactions problem is analyzed and an attempt to formulate a perturbative expansion of the effective interaction for the case of the deuteron is made. Within this expansion, two sources of non-perturbative behaviour are prevalent. The first source comes from the incorrect asymptotic behaviour of the single-particle basis functions that result from using the Harmonic Oscillator (H.O.) potential as a mean field. This behaviour is avoided by performing a kinetic energy operator resummation. The second source comes from the intrinsic hard core of the NN interaction. Renormalization Group tactics are used to 'soften' the core, thereby eliminating this non-perturbative behaviour. The validity of this expansion and its applications to higher A-body systems are also discussed.
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[BA.004] Clear evidence of three-nucleon force effects in Nd scattering
Hideyuki Sakai (University of Tokyo/RIKEN)
The first signature of the three-nucleon force (3NF) is the 3N binding energy. Modern nucleon-nucleon (NN) interactions underbind ^3H by 0.5-1 MeV depending on the choice of NN interactions. Although this gap between the theoretical and experimental binding energies can be filled with 3NF effects, this is not a clear signature of 3NF. Since the ^3H binding energy only puts a constraint on the overall strength, other signals for possible evidence of 3NF effects must be sought in 3N continuum studies. Among 3N scatterings, the nucleon-deuteron (Nd) scattering is very attractive since it offers a rich set of spin observables.
It has become possible to perform exact numerical calculations of the Faddeev equation for neutron-deuteron (nd) elastic scattering at intermediate energies using modern NN forces. The Bochum-Cracow group has pointed out(H. Wita\l)a et al., Phys. Rev. Lett. 81, 1183 (1998). that in such calculations significant 3NF effects appear in nd elastic scattering at energies near the minimum in the cross section. We have carried out \vecd+p elastic scattering at E_lab(d)=270, 200 and 140 MeV at the RIKEN Accelerator Research Facility and have obtained precise data(A part of data published: H. Sakai et al.) Phys. Rev. Lett. 84, 5288 (2000). for the cross sections, as well as the complete set of the deuteron vector and tensor analyzing powers A_y^d, A_xx, A_yy and A_xz covering a wide range of scattering angles \theta_c.m. = 10^\circ - 180^\circ. In addition, tensor-to-vector polarization (spin) transfer coefficients K^y'_xx and K^y'_yy were measured. These data are compared with exact Faddeev calculations by Bochum-Cracow group assuming nd scattering, in which modern data-equivalent NN forces, such as AV18, CD Bonn, Nijmegen I, II and 93, as well as 3NF, such as the Tucson-Melbourne(TM) and Urbana IX, are employed. Evidence of 3NF effects will be discussed.
The nd scattering at intermediate energies is very interesting not only because of 3NF, but also because of relativistic effects. Since there are no Coulomb force effects in nd scattering, a direct comparison with a Faddeev-type calculation is possible without reservation. We have measured the backward \vecn + d scattering together with \vecp + d scattering at E_lab(n/p)=250 MeV at the Research Center for Nuclear Physics (RCNP), Osaka University. We present also these new data, together with Faddeev calculations by Bochum-Cracow group, and discuss 3NF, as well as possible relativistic, effects.
This work is a collaboration of scientists from University of Tokyo, Saitama University, RIKEN, RCNP, Ruhr-University, and Jagellonian University.