The Proton Electric Pygmy Dipole Resonance

The evolution of the low-lying E1 strength in proton-rich nuclei is analyzed in the framework of the self-consistent relativistic Hartree-Bogoliubov (RHB) model and the relativistic quasiparticle random-phase approximation (RQRPA). Model calculations are performed for a series of N=20 isotones and Z=18 isotopes. For nuclei close to the proton drip-line, the occurrence of pronounced dipole peaks is predicted in the low-energy region below 10 MeV excitation energy. From the analysis of the proton and neutron transition densities and the structure of the RQRPA amplitudes, it is shown that these states correspond to the proton pygmy dipole resonance.Comment: 7 pages, 4 figures, to be published in Phys. Rev. Let

Nuclear Structure and Response based on Correlated Realistic NN Interactions

Starting from the Argonne V18 nucleon-nucleon (NN) interaction and using the Unitary Correlation Operator Method, a correlated interaction v_UCOM has been constructed, which is suitable for calculations within restricted Hilbert spaces. In this work we employ the v_UCOM in Hartree-Fock, perturbation-theory and RPA calculations and we study the ground-state properties of various closed-shell nuclei, as well as some excited states. The present calculations provide also important feedback for the optimization of the v_UCOM and valuable information on its properties. The above scheme offers the prospect of ab initio calculations in nuclei, regardless of their mass number. It can be used in conjunction with other realistic NN interactions as well, and with various many-body methods (Second RPA, QRPA, Shell Model, etc.).Comment: 3 pages, incl. 2 figures; Proc. Int. Conf. on Frontiers in Nuclear Structure, Astrophysics and Reactions (FINUSTAR), Kos, Greece, Sept.200

Relativistic description of exotic collective excitation phenomena in atomic nuclei

The low-lying dipole and quadrupole states in neutron rich nuclei, are studied within the fully self-consistent relativistic quasiparticle random-phase approximation (RQRPA), formulated in the canonical basis of the Relativistic Hartree-Bogoliubov model (RHB), which is extended to include the density dependent interactions. In heavier nuclei, the low-lying E1 excited state is identified as a pygmy dipole resonance (PDR), i.e. as a collective mode of excess neutrons oscillating against a proton-neutron core. Isotopic dependence of the PDR is characterized by a crossing between the PDR and one-neutron separation energies. Already at moderate proton-neutron asymmetry the PDR peak is calculated above the neutron emission threshold, indicating important implications for the observation of the PDR in (gamma,gamma') scattering, and on the theoretical predictions of the radiative neutron capture rates in neutron-rich nuclei. In addition, a novel method is suggested for determining the neutron skin of nuclei, based on measurement of excitation energies of the Gamow-Teller resonance relative to the isobaric analog state.Comment: 8 pages, 3 figures, invited talk at the international workshop "Blueprints for the nucleus: From First Principles to Collective Motion", May 17-22. 2004, Istanbul, Turkey; to appear in Int. J. Mod. Phys.

Toroidal dipole resonances in the relativistic random phase approximation

The isoscalar toroidal dipole strength distributions in spherical nuclei are calculated in the framework of a fully consistent relativistic random phase approximation. It is suggested that the recently observed "low-lying component of the isoscalar dipole mode" might in fact correspond to the toroidal giant dipole resonance. Although predicted by several theoretical models, the existence of toroidal resonances has not yet been confirmed in experiment. The strong mixing between the toroidal resonance and the dipole compression mode might help to explain the large discrepancy between theory and experiment on the position of isoscalar giant dipole resonances.Comment: 10 pages, 3 figures; Phys.Rev.C, in prin

Reconstruction of Gravitational Lensing Using WMAP 7-Year Data

Gravitational lensing by large scale structure introduces non-Gaussianity into the Cosmic Microwave Background and imprints a new observable, which can be used as a cosmological probe. We apply a four-point estimator to the Wilkinson Microwave Anisotropy Probe (WMAP) 7-year coadded temperature maps alone to reconstruct the gravitational lensing signal. The Gaussian bias is simulated and subtracted, and the higher order bias is investigated. We measure a gravitational lensing signal with a statistical amplitude of $\mathcal {C}$ = $1.27\pm 0.98$ using all the correlations of the W- and V-band Differencing Assemblies (DAs). We therefore conclude that WMAP 7-year data alone, can not detect lensing.Comment: 10 pages, 10 figure

A New Method for Selecting Exclusive Semileptonic Charmless B-Decays at e+e−e^+ e^- Colliders at the Υ(4S)\Upsilon(4S)

We introduce a new method for selecting exclusive semileptonic charmless B-decays in the presence of a large background. The method can be applied to charged and neutral B-mesons decaying into any exclusive neutral or charged hadronic final state. The method is designed for high luminosity \eplemi colliders operating at the $\Upsilon(4S)$. It employs an improved partial reconstruction technique for \Dstar-mesons and a novel 0-C event fit to both B-meson's decay products resulting in the kinematics of all particles (including neutrinos) in the event. The charged lepton energies are accessible from 1.0 \GeV to the kinematic limit.Comment: 10 pages with 7 figures in subdirectory fi

Low-energy dipole excitations towards the proton drip-line: doubly magic 48Ni

The properties of the low-energy dipole response are investigated for the proton-rich doubly magic nucleus $^{48}$Ni, in a comparative study of two microscopic models: fully self-consistent Relativistic Random-Phase Approximation(RRPA) based on the novel density-dependent meson-exchange interactions, and Continuum Random-Phase Approximation(CRPA) using Skyrme-type interactions with the continuum properly included. Both models predict the existence of the low-energy soft mode, i.e. the proton pygmy dipole resonance (PDR), for which the transition densities and RPA amplitudes indicate the dynamics of loosely bound protons vibrating against the rest of the nucleons. The CRPA analysis indicates that the escape width for the proton PDR is rather large, as a result of the coupling to the continuum.Comment: 12 pages, 3 figures, accepted for publication in Phys. Lett.

Inclusive charged-current neutrino-nucleus reactions calculated with the relativistic quasiparticle random phase approximation

Inclusive neutrino-nucleus cross sections are calculated using a consistent relativistic mean-field theoretical framework. The weak lepton-hadron interaction is expressed in the standard current-current form, the nuclear ground state is described with the relativistic Hartree-Bogoliubov model, and the relevant transitions to excited nuclear states are calculated in the relativistic quasiparticle random phase approximation. Illustrative test calculations are performed for charged-current neutrino reactions on $^{12}$C, $^{16}$O, $^{56}$Fe, and $^{208}$Pb, and results compared with previous studies and available data. Using the experimental neutrino fluxes, the averaged cross sections are evaluated for nuclei of interest for neutrino detectors. We analyze the total neutrino-nucleus cross sections, and the evolution of the contribution of the different multipole excitations as a function of neutrino energy. The cross sections for reactions of supernova neutrinos on $^{16}$O and $^{208}$Pb target nuclei are analyzed as functions of the temperature and chemical potential.Comment: 28 pages, 8 figures, 2 tables, submitted to Phys. Rev.