121160071
Evidence for the microscopic formation of mixed-symmetry states from magnetic moment measurements
Using the transient field technique, the magnetic moments of the second excited 2{sup +} states in {sup 92,94}Zr have been measured for the first time. The large positive g factors, g(2{sub 2}{sup +};{sup 92}Zr)=+0.76(50) and g(2{sub 2}{sup +};{sup 94}Zr)=+0.88(27), which are in contrast to the known negative g factors of the 2{sub 1}{sup +} states, are found to be a consequence of weak proton-neutron coupling combined with the Z=40 subshell closure. From their large M1 transition strengths to the 2{sub 1}{sup +} states, in earlier works an assignment to the 2{sub 2}{sup +} states as proton-neutron symmetric and mixed-symmetry states has been made, which are now found to be polarized in their proton-neutron content. This fact allows to identify the underlying microscopic main configurations in the wave functions, which form the building blocks of symmetric and mixed-symmetry states in this region as valence nucleons are added and shell structure changes.
30767099
Microscopic restoration of proton-neutron mixed symmetry in weakly collective nuclei
Starting from the microscopic low-momentum nucleon-nucleon interaction V{sub lowk}, we present a systematic shell-model study of magnetic moments and magnetic dipole transition strengths of the basic low-energy one-quadrupole phonon excitations in nearly spherical nuclei. Studying in particular the even-even N=52 isotones from {sup 92}Zr to {sup 100}Cd, we find the predicted evolution of the predominantly proton-neutron nonsymmetric state reveals a restoration of collective proton-neutron mixed-symmetry structure near midshell. This provides a quantitative explanation for the existence of pronounced collective mixed-symmetry structures in weakly collective nuclei.
7797374
First direct double-β decay Q-value measurement of 82Se in support of understanding the nature of the neutrino.
In anticipation of results from current and future double-β decay studies, we report a measurement resulting in a (82)Se double-β decay Q value of 2997.9(3) keV, an order of magnitude more precise than the currently accepted value. We also present preliminary results of a calculation of the (82)Se neutrinoless double-β decay nuclear matrix element that corrects in part for the small size of the shell model single-particle space. The results of this work are important for designing next generation double-β decay experiments and for the theoretical interpretations of their observations.
118974414
Low momentum shell model effective interactions with all-order core polarizations
An all-order summation of core-polarization diagrams using the low-momentum nucleon-nucleon interaction V{sub low-k} is presented. The summation is based on the Kirson-Babu-Brown (KBB) induced interaction approach in which the vertex functions are obtained self-consistently by solving a set of nonlinear coupled equations. It is found that the solution of these equations is simplified using V{sub low-k}, which is energy independent, and using the Green's functions in the particle-particle and particle-hole channels. We have applied this approach to the sd-shell effective interactions and find that the results we calculated to all orders by using the KBB summation technique are remarkably similar to those of second-order perturbation theory, the average differences being less than 10%.
56294645
Three-body forces and shell structure in calcium isotopes
Understanding and predicting the formation of shell structure from nuclear forces is a central challenge for nuclear physics. While the magic numbers N = 2, 8, 20 are generally well understood, N = 28 is the first standard magic number that is not reproduced in microscopic theories with two-nucleon forces. In this paper, we show that three-nucleon forces give rise to repulsive interactions between two valence neutrons that are key to explain 48Ca as a magic nucleus, with a high 2+ excitation energy and a concentrated magnetic dipole transition strength. The repulsive three-nucleon mechanism improves the agreement with experimental binding energies. Communicated by Professor Jacek Dobaczewski
119225778
Tensor interaction constraints from beta decay recoil spin asymmetry of trapped atoms
We have measured the angular distribution of recoiling daughter nuclei emitted from the Gamow-Teller {beta} decay of spin-polarized {sup 80}Rb. The asymmetry of this distribution vanishes to lowest order in the standard model (SM) in pure Gamow-Teller decays, producing an observable very sensitive to new interactions. We measure the non-SM contribution to the asymmetry to be A{sub T}=0.015{+-}0.029 (stat) {+-}0.019 (syst), consistent with the SM prediction. We constrain higher-order SM corrections using the measured momentum dependence of the asymmetry, and their remaining uncertainty dominates the systematic error. Future progress in determining the weak magnetism term theoretically or experimentally would reduce the final errors. We describe the resulting constraints on fundamental four-Fermi tensor interactions.
119250162
Microscopic optical potential from chiral nuclear forces
The energy- and density-dependent single-particle potential for nucleons is constructed in a medium of infinite isospin-symmetric nuclear matter starting from realistic nuclear interactions derived within the framework of chiral effective field theory. The leading-order terms from both two- and three-nucleon forces give rise to real, energy-independent contributions to the nucleon self-energy. The Hartree-Fock contribution from the two-nucleon force is attractive and strongly momentum dependent, in contrast to the contribution from the three-nucleon force which provides a nearly constant repulsive mean field that grows approximately linearly with the nuclear density. Together, the leading-order perturbative contributions yield an attractive single-particle potential that is however too weak compared to phenomenology. Second-order contributions from two- and three-body forces then provide the additional attraction required to reach the phenomenological depth. The imaginary part of the optical potential, which is positive (negative) for momenta below (above) the Fermi momentum, arises at second-order and is nearly inversion-symmetric about the Fermi surface when two-nucleon interactions alone are present. The imaginary part is strongly absorptive and requires the inclusion of an effective mass correction as well as self-consistent single-particle energies to attain qualitative agreement with phenomenology.
118092886
Separation of Scales in the More Effective Field Theory and Moszkowski-Scott Methods
We compare the momentum-space decimation procedure used to construct the low momentum nucleon-nucleon interaction V(low-k) with the configuration-space separation method of Moszkowski and Scott. Each procedure defines a separation of scales in the nuclear many-body problem, and the extent to which these two scales coincide is studied. By studying the effects of the separation method on the relative S-state Kallio-Kolltveit potential, it is found that close agreement with V(low-k) is obtained as the configuration-space cutoff is lowered to ~ 1.0 fm.
17480164
A Hidden Local Field Theory Description of Dileptons in Relativistic Heavy Ion Collisions
Using the notion of “hadronic freedom” inferred from the vector manifestation of hidden local symmetry we argue that, contrary to the commonly held belief, the dileptons measured in relativistic heavy-ion collisions do not carry useful information directly relevant to the spontaneous breaking of chiral symmetry and hence to the mechanism for mass generation of light-quark hadrons. The dileptons emitted from those vector mesons whose masses are shifted by the vacuum change in temperature and/or density are strongly suppressed in the hadronic free region between the chiral restoration point and the “flash point” at which the vector mesons recover ∼ > 90% of their free-space on-shell masses and the full strong coupling strength. The spectral function deduced from the dilepton measurements reflects predominantly those vector mesons decaying at the “flash temperature” Tflash ∼ 120 MeV. Thus, � ≈ + � � ∗
58934407
Charged-current reactions in the supernova neutrino-sphere
We calculate neutrino absorption rates due to charged-current reactions $\nu_e+n \rightarrow e^- + p $ and $\bar{\nu}_e+p \rightarrow e^+ + n $ in the outer regions of a newly born neutron star called the neutrino-sphere. To improve on recent work which has shown that nuclear mean fields enhance the $\nu_e$ cross-section and suppress the $\bar{\nu}_e$ cross-section, we employ realistic nucleon-nucleon interactions that fit measured scattering phase shifts. Using these interactions we calculate the momentum-, density-, and temperature-dependent nucleon self-energies in the Hartree-Fock approximation. A potential derived from chiral effective field theory and a pseudo-potential constructed to reproduce nucleon-nucleon phase shifts at the mean-field level are used to study the equilibrium proton fraction and the charged-current rates are studied in detail. We compare our results to earlier calculations obtained using phenomenological mean-field models and to those obtained in the virial expansion valid at low density. We find that for typical ambient conditions in the neutrino-sphere, $T=5-10$ MeV and $\rho =10^{11}-10^{13}$ g/cm$^3$, the difference between the $\nu_{e}$ and $\bar{\nu}_e$ absorption rates is much larger than predicted earlier. Our results have important implications for heavy-element nucleosynthesis in supernovae and for supernova neutrino detection.
118658178
Quasiparticle interaction in nuclear matter with chiral three-nucleon forces☆
We derive the effective interaction between two quasiparticles in symmetric nuclear matter resulting from the leading-order chiral three-nucleon force. We restrict our study to the L=0,1 Landau parameters of the central quasiparticle interaction computed to first order. We find that the three-nucleon force provides substantial repulsion in the isotropic spin- and isospin-independent component F0 of the interaction. This repulsion acts to stabilize nuclear matter against isoscalar density oscillations, a feature which is absent in calculations employing low-momentum two-nucleon interactions only. We find a rather large uncertainty for the nuclear compression modulus K due to a sensitive dependence on the low-energy constant c3. The effective nucleon mass M⁎ on the Fermi surface, as well as the nuclear symmetry energy β, receive only small corrections from the leading-order chiral three-body force. Both the anomalous orbital g-factor δgl and the Landau–Migdal parameter gNN′ (characterizing the spin–isospin response of nuclear matter) decrease with the addition of three-nucleon correlations. In fact, δgl remains significantly smaller than its value extracted from experimental data, whereas gNN′ still compares well with empirical values. The inclusion of the three-nucleon force results in relatively small p-wave (L=1) components of the central quasiparticle interaction, thus suggesting an effective interaction of short range.