210 research outputs found
Solutions of the dispersion equation in the region of overlapping of zero-sound and particle-hole modes
In this paper the solutions of the zero-sound dispersion equation in the
random phase approximation (RPA) are considered. The calculation of the damped
zero-sound modes \omega_s(k) (complex frequency of excitation) in the nuclear
matter is presented. The method is based on the analytical structure of the
polarization operators \Pi(\omega,k). The solutions of two dispersion equations
with \Pi(\omega,k) and with Re(\Pi(\omega,k)) are compared. It is shown that in
the first case we obtain one-valued smooth solutions without "thumb-like"
forms. Considering the giant resonances in the nuclei as zero-sound excitations
we compare the experimental energy and escape width of the giant dipole
resonance (GDR) in the nucleus A with \omega_s(k) taken at a definite wave
vector k=k_A.Comment: 14 pages, 5 figures; revised versio
Resonance states below pion-nucleon threshold and their consequences for nuclear systems
Regular sequences of narrow peaks have been observed in the missing mass
spectra in the reactions pp --> p pi^+ X and pd --> ppX_1 below pion-production
threshold. They are interpreted in the literature as manifestations of
supernarrow light dibaryons, or nucleon resonances, or light pions forming
resonance states with the nucleon in its ground state. We discuss how existence
of such exotic states would affect properties of nuclear systems. We show that
the neutron star structure is drastically changed in all three cases. We find
that in the presence of dibaryons or nucleon resonances the maximal possible
mass of a neutron star would be smaller than the observational limit. Presence
of light pions does not contradict the observed neutron star masses. Light
pions allow for the existence of extended nuclear objects of arbitrary size,
bound by strong and electromagnetic forces.Comment: preprint ECT*-02-18, 6 pages, 3 figure
Elimination of spurious states in the quasiparticle time blocking approximation
The quasiparticle time blocking approximation (QTBA) is considered as a model
for the description of excitations in open-shell nuclei. The QTBA is an
extension of the quasiparticle random phase approximation that includes
quasiparticle-phonon coupling. In the present version of the QTBA, the pairing
correlations are included within the framework of the BCS approximation. Thus,
in this model, the spurious states appear, which are caused by the
breaking of the symmetry related to the particle-number conservation. In this
work, the method is described which solves the problem of the spurious
states in the QTBA with the help of the projection technique. The method is
illustrated by calculations of excitations in Sn nucleus.Comment: 12 pages, 3 figures - To appear in the proceedings of the 59-th
International Meeting on Nuclear Spectroscopy and Nuclear Structure, June
15-19, 2009, Cheboksary, Russi
Empirical Emission Functions for LPM Suppression of Photon Emission from Quark-Gluon Plasma
The LPM suppression of photon emission rates from the quark gluon plasma have
been studied at different physical conditions of the plasma given by
temperature and chemical potentials.The integral equation for the transverse
vector function (f(p_t)) consisting of multiple scattering effects is solved
for the parameter set {p,k,kappa,T}, for bremsstrahlung and AWS processes. The
peak positions of these distributions depend only on the dynamical variable
x=(T/kappa)|1/p-1/(p+k)|. Integration over these distributions multiplied by
x^2 factor also depends on this variable x,leading to a unique global emission
function g(x) for all parameters. Empirical fits to this dimensionless emission
function, g(x), are obtained. The photon emission rate calculations with LPM
suppression effects reduce to one dimensional integrals involving folding over
the empirical g(x) function with appropriate distribution functions and the
kinematic factors. Using this approach, the suppression factors for both
bremsstrahlung and AWS have been estimated for various chemical potentials and
compared with the variational method
Effects of resonant single-particle states on pairing correlations
Effects of resonant single-particle (s.p.) states on the pairing correlations
are investigated by an exact treatment of the pairing Hamiltonian on the Gamow
shell model basis. We introduce the s.p. states with complex energies into the
Richardson equations. The solution shows the property that the resonant s.p.
states with large widths are less occupied. The importance of many-body
correlations between bound and resonant prticle pairs is shown.Comment: 4 pages, 3 figures, to be published in Phys. Rev.
Effect of electron-phonon interaction on the shift and attenuation of optical phonons
Using the Boltzmann equation for electrons in metals, we show that the
optical phonons soften and have a dispersion due to screening in agreement with
the results reported recently [M. Reizer, Phys. Rev. B {\bf 61}, 40 (2000)].
Additional phonon damping and frequency shift arise when the electron--phonon
interaction is properly included.Comment: 4 pages, late
Characterization of neutrino signals with radiopulses in dense media through the LPM effect
We discuss the possibilities of detecting radio pulses from high energy
showers in ice, such as those produced by PeV and EeV neutrino interactions. It
is shown that the rich radiation pattern structure in the 100 MHz to few GHz
allows the separation of electromagnetic showers induced by photons or
electrons above 100 PeV from those induced by hadrons. This opens up the
possibility of measuring the energy fraction transmitted to the electron in a
charged current electron neutrino interaction with adequate sampling of the
angular distribution of the signal. The radio technique has the potential to
complement conventional high energy neutrino detectors with flavor information.Comment: 5 pages, 4 ps figures. Submitted to Phys. Rev. Let
Fractional Fokker-Planck Equation and Oscillatory Behavior of Cumulant Moments
The Fokker-Planck equation is considered, which is connected to the birth and
death process with immigration by the Poisson transform. The fractional
derivative in time variable is introduced into the Fokker-Planck equation. From
its solution (the probability density function), the generating function (GF)
for the corresponding probability distribution is derived. We consider the case
when the GF reduces to that of the negative binomial distribution (NBD), if the
fractional derivative is replaced to the ordinary one. Formulas of the
factorial moment and the moment are derived from the GF. The moment
derived from the GF of the NBD decreases monotonously as the rank j increases.
However, the moment derived in our approach oscillates, which is
contrasted with the case of the NBD. Calculated moments are compared with
those given from the data in collisions and in collisions.Comment: 10 pages, 8 figures, submitted to Phys. Rev.
Many Body Correlation Corrections to Superconducting Pairing in Two Dimensions.
We demonstrate that in the strong coupling limit (the superconducting gap
is as large as the chemical potential ), which is relevant to the
high- superconductivity, the correlation corrections to the gap and
critical temperature are about 10\% of the corresponding mean field
approximation values. For the weak coupling () the correlation
corrections are very large: of the order of 100\% of the corresponding mean
field values.Comment: LaTeX 12 page
Anomalous Normal-State Properties of High-T Superconductors -- Intrinsic Properties of Strongly Correlated Electron Systems?
A systematic study of optical and transport properties of the Hubbard model,
based on Metzner and Vollhardt's dynamical mean-field approximation, is
reviewed. This model shows interesting anomalous properties that are, in our
opinion, ubiquitous to single-band strongly correlated systems (for all spatial
dimensions greater than one), and also compare qualitatively with many
anomalous transport features of the high-T cuprates. This anomalous
behavior of the normal-state properties is traced to a ``collective single-band
Kondo effect,'' in which a quasiparticle resonance forms at the Fermi level as
the temperature is lowered, ultimately yielding a strongly renormalized Fermi
liquid at zero temperature.Comment: 27 pages, latex, 13 figures, Invited for publication in Advances in
Physic
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