### Session C8: Nuclear Theory I

 Saturday, April 22, 2006 1:30PM - 1:42PM C8.00001: Pygmy-Resonance as a Manifestation of Strong Coupling to Continuum Alexander Volya , Vladimir Zelevinsky The collective strength of giant resonances is built on coherent interaction of many simple particle-hole type excitations with specific quantum numbers. For isovector modes, and for the giant dipole resonance in particular, the collective strength forms a main peak shifted up in energy. The remaining unshifted low-energy strength becomes more pronounced in neutron-rich nuclei giving rise to the pygmy-branch of the giant resonance. Microscopically it can be presented as a vibration of the neutron skin against the core. Standard RPA calculations do not reveal an important role played by the continuum coupling. The particle-hole excitations, especially in loosely bound nuclei, are quasistationary states in the continuum. In such cases, the interaction through decay channels competes with a conventional shell model interaction and, for sufficiently large decay widths and overlapping resonances, leads to the collectivization of the widths (the phenomenon ofsuper-radiation"). The interplay of two collectivities was originally considered in [1], where the enhancement of the pygmy-branch was predicted on the base of schematic models. We present the first results of microscopic calculations in the framework of the recently advanced version of the continuum shell model [2]. \\ 1. V.V. Sokolov and V.G. Zelevinsky, Fizika (Zagreb) {\bf 22}, 303 (1990).\\ 2. A. Volya and V. Zelevinsky, Phys. Rev. Lett. {\bf 94} (2005) 052501. Saturday, April 22, 2006 1:42PM - 1:54PM C8.00002: Structure of Light Nuclei Using Chiral Three-Nucleon Interaction. V.G. Gueorguiev , P. Navratil , J.P. Vary , W.E. Ormand Modern high-precision nucleon-nucleon (NN) interactions adjusted to reproduce the phase shifts in the two nucleon system need to be augmented by three-nucleon interactions (TNI) in order to reproduce properties of multi nucleon systems. Chiral perturbation theory (${\chi}PT$) at next-to-next-to-leading order (N$^2$LO) predicts three types of TNI terms. A two-pion exchange term, which is frequently used, has parameters determined from the two-nucleon ${\chi}PT$-potential and/or pion-nucleon scattering data. The other two terms, the one-pion exchange with NN-contact term and the NNN-contact term, are accompanied by undetermined low-energy constants. We include all three TNI terms in {\it ab initio} no-core shell model (NCSM) calculations. The two undetermined TNI constants are fit to binding energies of $s$-shell nuclei. The effect on the nuclear structure of adding N$^2$LO TNI is then studied in NCSM calculations for p-shell nuclei, such as $^{10,11}$B and $^{12,13}$C that are particularly sensitive to the presence of and the form of the TNI. Saturday, April 22, 2006 1:54PM - 2:06PM C8.00003: Structure of $p$-shell Nuclei with Interactions Derived from Chiral Effective-Field Theory W. E. Ormand , V. G. Gueorguiev , P. Navratil , J. P. Vary Traditional realistic nucleon-nucleon (NN) interactions based on precision fits to 2-body data have not produced high-quality descriptions of light nuclei. Effective-Field theories (EFT) based on chiral-perturbation theory provide a natural scheme to derive inter-nucleon interactions and predict a three-nucleon interaction at next-to-next-to-leading order (N2LO). A key feature of these EFT potentials is a set of parameters; some of which are determined by the EFT NN couplings, while others are chosen to reproduce the binding energies of A=3 and 4 nuclei. We have developed the tools to utilize EFT-based potentials, including the NNN terms, in the ab initio no-core shell model (NCSM). We have also improved our shell-model codes to increase the scope of our calculations with three-nucleon interactions to the point where model spaces up to $6 \hbar\Omega$ are accessible for all $p$-shell nuclei. We will show results of large-basis NCSM calculations for light p-shell nuclei, especially masses $A =10, 11, 12, 13$ and highlight the impact the N2LO TNI and its parameters on their structure. Support from LDRD contract No. 04-ERD-058 and DOE grant SCW0498 is acknowledged. Saturday, April 22, 2006 2:06PM - 2:18PM C8.00004: Possible Collective Effects of Residual Three-Body Forces Vladimir Zelevinsky , Tony Sumaryada , Alexander Volya The {\sl ab-initio} calculations for light nuclei show the influence of three-body forces. Here we discuss possible effects of residual three-body forces added to the effective two-body interaction. The well known monopole corrections determined by the single-particle occupation numbers can be interpreted as a result of the mean-field averaging over the third particle. A classification of three-body effective interactions according to angular momentum of participating pairs shows a possibility of less trivial collective effects. The monopole renormalization of pairing can be considered exactly. We give an example of predictions that works for the xenon isotopes and allows one to estimate the typical strength of the effect. We also discuss cubic interactions of collective phonons which may be enhanced due to the three-body contribution. The octupole-quadrupole-octupole vertex is especially interesting leading to strong phonon coupling in agreement with the recent experiments for xenon isotopes [W.F. Mueller {\sl et al.}, in press]. This can also be important for the enhancement of the nuclear Schiff moment in nuclei with soft octupole and quadrupole modes. Saturday, April 22, 2006 2:18PM - 2:30PM C8.00005: Benchmark result on the $^4$He photoabsorbtion cross section Sofia Quaglioni , Ionel Stetcu , Bruce R. Barrett , Sonia Bacca , Petr Navr\'atil , Winfried Leidemann , Giuseppina Orlandini , Nir Barnea , Calvin W. Johnson We present the results of a benchmark calculation of the $^4$He photoabsorbtion cross section via the Lorentz integral transform approach, using both the no-core shell model (NCSM) and the effective interaction hyperspherical harmonic (EIHH) expansion methods. The present study, which is fully microscopic in the treatment of the dynamics in both the initial and final states, uses a semirealistic nucleon-nucleon potential, with the goal to test the reliability of the NCSM for the description of such processes. We find that the results of both EIHH and NCSM agree, which opens the the way for the use of two- and three-body forces obtained from EFT, and for applications to heavier nuclei by means of the NCSM. S.Q., I.S., and B.R.B acknowledge partial support by NFS grants PHY0070858 and PHY0244389. The work was performed in part under the auspices of the U. S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. P.N. received support from LDRD contract 04-ERD-058. W.L. and G.O. acknowledge support by the grant COFIN03 of the Italian Ministry of University and Research. N.B. acknowledges support by the Israel Science Foundation (Grant No. 202/02). C.W.J. acknowledges USDOE grant No.DE-FG02-03ER41272. Saturday, April 22, 2006 2:30PM - 2:42PM C8.00006: A cluster model of $^{6}$He Jeremy Armstrong , Alexander Sakharuk , Vladimir Zelevinsky Small nuclei provide an ideal testing ground of few-body theories. $^{6}$He is particularly interesting in that it shows an extended particle distribution similar to a halo nucleus, is loosely bound, and is a Borromean system. We apply the Brink Formalism in secondary quantization to study the structure of $^{6}$He. This formalism allows for the proper treatment of Fermi statistics and correct projection into eigenstates of angular momentum. The alpha plus dineutron configuration and cigar'' (neutron, alpha, neutron chain) configuration were studied to obtain binding energies, charge radii, matter radii, and B(E2) for $^{6}$He. We discuss the relative weight of both configurations as well as the size of the interference term between them in the overall $^{6}$He wavefunction. Saturday, April 22, 2006 2:42PM - 2:54PM C8.00007: Precision extraction of $a_{nn}$ from $\pi^-d\to nn\gamma$ using $\chi$PT Anders G{\aa}rdestig , Daniel Phillips We present an updated chiral perturbation theory calculation of the $\pi^-d\to nn\gamma$ reaction. By using other observables to constrain the unknown short-distance physics, we can drastically reduce the theoretical error in the extracted neutron-neutron scattering length. Further consequences of the results will be discussed. Saturday, April 22, 2006 2:54PM - 3:06PM C8.00008: Pionic Color Transparency Arnold Larson , Gerald Miller , Mark Strikman A search for pionic color transparency effects in nuclear reactions is currently under experimental investigation. However, it is unclear if the transparency effect is greater than the effect of the momentum dependence of the nuclear pion content. We review the use of a semi-classical formula to compute pion transparency and discuss the use of pionic response in determining the relative importance of these two effects. Saturday, April 22, 2006 3:06PM - 3:18PM C8.00009: Pionic Contributions in a Quark Model of Nucleons and Nuclei T. Goldman , Richard R. Silbar A successful relativistic quark model of small nuclei[1] did not include long-range effects of pion exchanges between quarks. We add these contributions to themodel with a short-range cutoff (to avoid double counting). We report on the changes induced in model parameters and binding energies. [1]T. Goldman, K.R. Maltman, G.J. Stephenson, Jr., and K.E. Schmidt, \textit{Nucl. Phys}. \textbf{A481}, 621 (1988).