Publications (101)335.15 Total impact
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ABSTRACT: We study the effects of repulsive interactions on the critical density for the BoseEinstein transition in a homogeneous dilute gas of bosons. First, we point out that the simple mean field approximation produces no change in the critical density, or critical temperature, and discuss the inadequacies of various contradictory results in the literature. Then, both within the frameworks of Ursell operators and of Green's functions, we derive selfconsistent equations that include correlations in the system and predict the change of the critical density. We argue that the dominant contribution to this change can be obtained within classical field theory and show that the lowest order correction introduced by interactions is linear in the scattering length, a, with a positive coefficient. Finally, we calculate this coefficient within various approximations, and compare with various recent numerical estimates. 
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ABSTRACT: For a SU(N) YangMills theory, we present variational calculations using gaussian wave functionals combined with an approximate projection on gauge invariant states. The projection amounts to correcting the energy of the gaussian states by substracting the spurious energy associated with gauge rotations. Based on this improved energy functional, we perform variational calculations of the interaction energy in the presence of external electric and magnetic fields. We verify that the ultraviolet behaviour of our approximation scheme is consistent, as it should, with that expected from perturbation theory. In particular, we recover in this variational framework the standard oneloop beta function, with a transparent interpretation of the screening and antiscreening contributions. Comment: 40 pages, no figures 
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ABSTRACT: We show that the critical temperature of a uniform dilute Bose gas must increase linearly with the swave scattering length describing the repulsion between the particles. Because of infrared divergences, the magnitude of the shift cannot be obtained from perturbation theory, even in the weak coupling regime; rather, it is proportional to the size of the critical region in momentum space. By means of a selfconsistent calculation of the quasiparticle spectrum at low momenta at the transition, we find an estimate of the effect in reasonable agreement with numerical simulations.  [Show abstract] [Hide abstract]
ABSTRACT: We present microscopic calculations of neutrino propagation in hot neutron matter above nuclear density within the framework of the Random Phase Approximation . Calculations are performed for non degenerate neutrinos using various Skyrme effective interactions. We find that for densities just above nuclear density, spin zero sound is present at zero temperature for all Skyrme forces considered. However it disappears rapidly with increasing temperature due to a strong Landau damping. As a result the meanfree path is given, to a good approximation, by the mean field value. Because of the renormalization of the bare mass in the mean field, the medium is more transparent as compared to the free case. We find, in contrast, that at several times nuclear density, a new type of behavior sets in due to the vicinity of a magnetic instability. It produces a strong reduction of the mean free path. The corresponding transition density however occurs in a region where inputs from more realistic calculations are necessary for the construction of a reliable Skyrme type parametrization. Comment: 17 pages, 4 figures  [Show abstract] [Hide abstract]
ABSTRACT: Mean field theory for the time evolution of quantum meson fields is studied in terms of the functional Schroedinger picture with a timedependent Gaussian variational wave functional. We first show that the equations of motion for the variational wavefunctional can be rewritten in a compact form similar to the HartreeBogoliubov equations in quantum manybody theory and this result is used to recover the covariance of the theory. We then apply this method to the O(N) model and present analytic solutions of the mean field evolution equations for an Ncomponent scalar field. These solutions correspond to quantum rotations in isospin space and represent generalizations of the classical solutions obtained earlier by Anselm and Ryskin. As compared to classical solutions new effects arise because of the coupling between the average value of the field and its quantum fluctuations. We show how to generalize these solutions to the case of mean field dynamics at finite temperature. The relevance of these solutions for the observation of a coherent collective state or a disoriented chiral condensate in ultrarelativistic nuclear collisions is discussed. Comment: 31 pages, 2 Postscript figures, uses ptptex.sty  [Show abstract] [Hide abstract]
ABSTRACT: Analytic solutions of the mean field evolution equations for an Ncomponent scalar field with O(N) symmetry are presented. These solutions correspond to rotations in isospin space. They represent generalizations of the classical solutions obtained earlier by Anselm and Ryskin. As compared to classical solutions new effects arise because of the coupling between the average value of the field and quantum fluctuations.  [Show abstract] [Hide abstract]
ABSTRACT: Radii of sodium isotopes have been calculated by using the HartreeFockBCS model and the Generator Coordinate Method (GCM) with different forces. It is found that HartreeFockBCS results present a jump in both neutron and proton radii from 22Na to 23Na. However, configuration mixing calculations performed with the GCM result in a smooth increase of the neutron radius and an almost constant proton radius for the sodium isotopes. We analyze and discuss our results in the light of recent experimental data. 
Article: Variational calculations in gauge theories with approximate projection on gauge invariant states
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ABSTRACT: Variational calculations using Gaussian wave functionals combined with an approximate projection on gauge invariant states are presented. We find that the energy exhibits a minimum for a wave functional centered around a non vanishing background magnetic field. We show that divergences can be removed by a renormalization of the coupling constant. The resulting expectation value of the gluon condensate is found to be in qualitative agreement with phenomenological estimates.  [Show abstract] [Hide abstract]
ABSTRACT: We present a simple model to calculate energy distributions and multiplicities of the neutrons emitted in the interaction of a highenergy proton with a thick target. It is based on a dEdx formula proposed by Cugnon for the proton energy loss inside nuclear matter and on the assumption that the dissipated energy eventually becomes thermalized. Comparison with the results of existing codes and available data is made. An application to the determination of the sensitivity of multiplicities to various input parameters (e.g. the level density parameter) is described. The model is shown to provide further physical insight into the standard spallation codes.  [Show abstract] [Hide abstract]
ABSTRACT: We construct asymptotic solutions of the functional Schroedinger equation for a scalar field in the Gaussian approximation at large proper time. These solutions describe the late proper time stages of the expansion of a meson gas with boost invariant boundary conditions. The relevance of these solutions for the formation of a disoriented chiral condensate in ultra relativistic collisions is discussed. Comment: 9 pages, LATEX  [Show abstract] [Hide abstract]
ABSTRACT: The description of collective motion in nuclei at finite temperature using the framework of the random phase approximation is discussed. We focus on the special case of the isovector response function of hot nuclear matter using various effective Skyrme interactions.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the role of the effective nucleonnucleon interaction in the description of giant dipole resonances in hot nuclei. For this purpose we calculate the response function of hot nuclear matter to a small isovector external perturbation using various effective Skyrme interactions. We find that for Skyrme forces with an effective mass close to unity an undamped zero sound mode occurs at zero temperature. This mode gives rise in finite nuclei (calculated via the SteinwedelJenssen model) to a resonance whose energy agrees with the observed value. We find that zero sound disappears at a temperature of a few MeV, leaving only a broad peak in the dipole strength. For Skyrme forces with a small value of the effective mass (0.4 0.5), there is no zero sound at zero temperature but only a weak peak located too high in energy. The strength distribution in this case is nearly independent of temperature and shows small collective effects. The relevance of these results for the saturation of photon multiplicities observed in recent experiments is pointed out.  [Show abstract] [Hide abstract]
ABSTRACT: We calculate the isovector response function of hot nuclear matter using various effective Skyrme interactions. For Skyrme forces with a small effective mass the strength distribution is found to be nearly independent of temperature, and shows little collective effects. In contrast effective forces with an effective mass close to unity produce sizeable collective effects at zero temperature which disappear at temperatures of a few MeV. We discuss the relevance of these results for the saturation of the multiplicity of photons emitted by the giant dipole resonance in hot nuclei beyond $T$=3 MeV observed in recent experiments. 
Article: Self Similar Solutions of the Evolution Equation of a Scalar Field in an Expanding Geometry
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ABSTRACT: We consider the functional Schrodinger equation for a self interacting scalar field in an expanding geometry. By performing a time dependent scale transformation on the argument of the field we derive a functional Schrodinger equation whose hamiltonian is time independent but involves a timeodd term associated to a constraint on the expansion current. We study the mean field approximation to this equation and generalize in this case, for interacting fields, the solutions worked out by Bunch and Davies for free fields. Comment: 8 pages, Latex, IPNO/TH 9439  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the response function of hot nuclear matter to a small isovector external field using a simplified Skyrme interaction reproducing the value of the symmetry energy coefficient. We consider values of the momentum transfer corresponding to the dipole oscillation in heavy nuclei. We find that while at zero temperature the particle hole interaction is almost repulsive enough to have a sharp (zero sound type) collective oscillation, such is no longer the case at temperatures of a few MeV. As a result a broadening of the dipole resonance occurs, leading to its quasi disappearance by the time the temperature reaches 5 MeV. The sensitivity of the temperature evolution of the width when modifying the residual interaction strength is also examined.  [Show abstract] [Hide abstract]
ABSTRACT: We describe a nonperturbative calculation of the spectrum of SU(2) YangMills theory based on a Hamiltonian formulation. Our approach exploits gauge invariant variables similar to those used in nuclear physics to describe collective motion in nuclei. Comment: 18 pages Latex  [Show abstract] [Hide abstract]
ABSTRACT: Two phononstates of alkalimetal clusters (treated as jellium spheres) are calculated by using a method based on a perturbative construction of periodic orbits of the timedependent meanfield equations. Collective vibrations with various multipolarities in charged $Na^+_{21}$ are considered.
Publication Stats
6k  Citations  
335.15  Total Impact Points  
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Institutions

1998

Pierre and Marie Curie University  Paris 6
Lutetia Parisorum, ÎledeFrance, France


1989

Centro de Estudios y Experimentación de Obras Públicas
Madrid, Madrid, Spain


19751989

Institut de Physique Nucléaire de Lyon
Lyons, RhôneAlpes, France


19751986

Orsay Physics
Fuveau, ProvenceAlpesCôte d'Azur, France


1983

Université Libre de Bruxelles
 Theoretical Physics Unit
Bruxelles, Brussels Capital, Belgium


1977

University of California, Berkeley
Berkeley, California, United States


19701976

University of Oxford
Oxford, England, United Kingdom


19721973

Massachusetts Institute of Technology
Cambridge, Massachusetts, United States
