Showing papers by "Carlos A. Bertulani published in 2012"

Pygmy dipole resonance in 208Pb

Abstract: Scattering of protons of several hundred MeV is a promising new spectroscopic tool for the study of electric dipole strength in nuclei. A case study of 208 Pb shows that, at very forward angles, J π = 1 − states are strongly populated via Coulomb excitation. A separation from nuclear excitation of other modes is achieved by a multipole decomposition analysis of the experimental cross sections based on theoretical angular distributions calculated within the quasiparticle-phonon model. The B(E1) transition strength distribution is extracted for excitation energies up to 9 MeV; that is, in the region of the so-called pygmy dipole resonance (PDR). The Coulomb-nuclear interference shows sensitivity to the underlying structure of the E1 transitions, which allows for the first time an experimental extraction of the electromagnetic transition strength and the energy centroid of the PDR.

Constraining gluon shadowing using photoproduction in ultraperipheral pA and AA collisions

Abstract: Photoproduction of heavy quarks and exclusive production of vector mesons in ultraperipheral proton-nucleus and nucleus-nucleus collisions depend significantly on nuclear gluon distributions. In the present study we investigate quantitatively the extent of the applicability of these processes at the Large Hadron Collider (LHC) in constraining the shadowing component of nuclear gluon modifications.

Global investigation of odd-even mass differences and radii with isospin dependent pairing interactions

Abstract: Neutron and proton odd-even mass differences are systematically studied with Hartree-Fock (HF) + BCS calculations with Skyrme interactions and an isospin-dependent contact pairing interaction. The strength of pairing interactions is determined to reproduce empirical odd-even mass differences in a wide region of the mass table. By using the optimal parameter sets for proton and neutrons, we perform global (HF) + BCS calculations with Skyrme interactions and an isospin-dependent contact pairing interaction. The strength of pairing interactions is determined to reproduce empirical odd-even mass differences in a wide region of the mass table. By using the optimal parameter sets for proton and neutrons, we perform global HF + BCS calculations of nuclei and compare them with experimental data. The importance of the isospin dependence of the pairing interaction is singled out for odd-even mass differences in medium and heavy isotopes. Proton and neutron radii are studied systematically using the same model.

Big Bang nucleosynthesis with a non-Maxwellian distribution

Abstract: The abundances of light elements based on the big bang nucleosynthesis model are calculated using the Tsallis non-extensive statistics. The impact of the variation of the non-extensive parameter $q$ from the unity value is compared to observations and to the abundance yields from the standard big bang model. We find large differences between the reaction rates and the abundance of light elements calculated with the extensive and the non-extensive statistics. We found that the observations are consistent with a non-extensive parameter q = 1 + 0.05 - 0.12, indicating that a large deviation from the Boltzmann-Gibbs statistics (q = 1) is highly unlikely.

Nuclear effects in photoproduction of heavy quarks and vector mesons in ultraperipheral PbPb and p Pb collisions at energies available at the CERN Large Hadron Collider

Abstract: The comparison of photoproduction cross sections for $c\overline{c}$ and $b\overline{b}$ in PbPb and $p$Pb collisions can show sensitivity to nuclear shadowing effects. The photoproduction of vector mesons is even more sensitive to the underlying gluon distributions. In this study we present the cross sections and rapidity dependence of the photoproduction of heavy quarks and the exclusive production of vector mesons in ultraperipheral $p$Pb and PbPb collisions at the CERN Large Hadron Collider at $\sqrt{{s}_{{}_{NN}}}=5$ TeV and $\sqrt{{s}_{{}_{NN}}}=2.76$ TeV, respectively. The potentials of using these processes for constraining nuclear gluon shadowing are explored. It is found that the photoproduction of $J/\ensuremath{\psi}$ and $\ensuremath{\Upsilon}$ in PbPb collisions in particular exhibits very good sensitivity to gluon shadowing.

Tunneling, diffusion, and dissociation of Feshbach molecules in optical lattices

Abstract: The quantum dynamics of an ultracold diatomic molecule tunneling and diffusing in a one-dimensional optical lattice exhibits unusual features. While it is known that the process of quantum tunneling through potential barriers can break up a bound-state molecule into a pair of dissociated atoms, interference and reassociation produce intricate patterns in the time-evolving site-dependent probability distribution for finding atoms and bound-state molecules. We find that the bound-state molecule is unusually resilient against break up at ultralow binding energy Eb (Eb much smaller than the barrier height of the lattice potential). After an initial transient, the bound-state molecule spreads with a width that grows as the square root of time. Surprisingly, the width of the probability of finding dissociated atoms does not increase with time as a power law.

Extending the Kawai-Kerman-McVoy Statistical Theory of Nuclear Reactions to Intermediate Structure via Doorways

Abstract: Kawai, Kerman, and McVoy have shown that a statistical treatment of many open channels that are coupled by direct reactions leads to modifications of the Hauser- Feshbach expression for energy-averaged cross section (Ann. of Phys. 75, 156 (1973)). The energy averaging interval for this cross section is on the order of the width of sin- gle particle resonances, 1 MeV, revealing only a gross structure in the cross section. When the energy-averaging interval is decreased down to a width of a doorway state, 0:1 MeV, a so-called intermediate structure may be observed in cross sections. We extend the Kawai-Kerman-McVoy theory into the intermediate structure by leveraging a theory of doorway states developed by Feshbach, Kerman, and Lemmer (Ann. of Phys. 41, 230 (1967)). As a by-product of the extension, an alternative derivation of the central result of the Kawai-Kerman-McVoy theory is suggested. We quantify the e ect of the ap- proximations used in derivation by performing numerical computations for a large set of compound nuclear states.

Dark/Visible Parallel Universes and Big Bang Nucleosynthesis

Abstract: We develop a model for visible matter-dark matter interaction based on the exchange of a massive gray boson called herein the Mulato. Our model hinges on the assumption that all known particles in the visible matter have their counterparts in the dark matter. We postulate six families of particles five of which are dark. This leads to the unavoidable postulation of six parallel worlds, the visible one and five invisible worlds. A close study of big bang nucleosynthesis (BBN), baryon asymmetries, cosmic microwave background (CMB) bounds, galaxy dynamics, together with the Standard Model assumptions, help us to set a limit on the mass and width of the new gauge boson. Modification of the statistics underlying the kinetic energy distribution of particles during the BBN is also discussed. The changes in reaction rates during the BBN due to a departure from the Debye-Hueckel electron screening model is also investigated.

Neutrino and antineutrino cross sections in $^{12}$C

Abstract: We extend the formalism of weak interaction processes, obtaining new expressions for the transition rates, which greatly facilitate numerical calculations, both for neutrino-nucleus reactions and muon capture. We have done a thorough study of exclusive (ground state) properties of $^{12}$B and $^{12}$N within the projected quasiparticle random phase approximation (PQRPA). Good agreement with experimental data is achieved in this way. The inclusive neutrino/antineutrino ($ u/\tilde{ u}$) reactions $^{12}$C($ u,e^-)^{12}$N and $^{12}$C($\tilde{ u},e^+)^{12}$B are calculated within both the PQRPA, and the relativistic QRPA (RQRPA). It is found that the magnitudes of the resulting cross-sections: i) are close to the sum-rule limit at low energy, but significantly smaller than this limit at high energies both for $ u$ and $\tilde{ u}$, ii) they steadily increase when the size of the configuration space is augmented, and particulary for $ u/\tilde{ u}$ energies $> 200$ MeV, and iii) converge for sufficiently large configuration space and final state spin.

Dark/visible parallel universes and Big Bang nucleosynthesis

Abstract: We develop a model for visible matter-dark matter interaction based on the exchange of a massive gray boson called herein the Mulato. Our model hinges on the assumption that all known particles in the visible matter have their counterparts in the dark matter. We postulate six families of particles five of which are dark. This leads to the unavoidable postulation of six parallel worlds, the visible one and five invisible worlds. A close study of big bang nucleosynthesis (BBN), baryon asymmetries, cosmic microwave background (CMB) bounds, galaxy dynamics, together with the Standard Model assumptions, help us to set a limit on the mass and width of the new gauge boson. Modification of the statistics underlying the kinetic energy distribution of particles during the BBN is also discussed. The changes in reaction rates during the BBN due to a departure from the Debye-Hueckel electron screening model is also investigated.

Nuclear physics in the cosmos

Abstract: We observe photons and neutrinos from stars. Based on these observations, complemented by measurements of cosmic rays energies and composition, we have been able to constrain several models for the Big Bang and for stellar evolution. But that is not enough. We also need to help this effort with laboratory experiments. We are still far from being able to reproduce stellar environments in a terrestrial laboratory. But in many cases we can obtain accurate nuclear reaction rates needed for modeling primordial nucleosynthesis and hydrostatic burning in stars. The relevant reactions are difficult to measure directly in the laboratory at the small astrophysical energies. In recent years indirect reaction methods have been developed and applied to extract low-energy astrophysical S-factors. These methods require a combination of new experimental techniques and theoretical efforts, which are the subject of this review.

Medium Effects in Reactions with Rare Isotopes

Abstract: We discuss medium effects in knockout reactions with rare isotopes of weakly-bound nuclei at intermediate energies. We show that the poorly known corrections may lead to sizable modifications of knockout cross sections and momentum dsitributions.

Medium effects in direct reactions

Abstract: We discuss medium corrections of the nucleon-nucleon (NN) cross sections and their influence on direct reactions at intermediate energies $\gtrsim 50$ MeV/nucleon. The results obtained with free NN cross sections are compared with those obtained with a geometrical treatment of Pauli-blocking and with NN cross sections obtained with Dirac-Bruecker methods. We show that medium corrections may lead to sizable modifications for collisions at intermediate energies and that they are more pronounced in reactions involving weakly bound nuclei.