460 research outputs found
New analytic running coupling in QCD: higher loop levels
The properties of the new analytic running coupling are investigated at the
higher loop levels. The expression for this invariant charge, independent of
the normalization point, is obtained by invoking the asymptotic freedom
condition. It is shown that at any loop level the relevant function has
the universal behaviors at small and large values of the invariant charge. Due
to this feature the new analytic running coupling possesses the universal
asymptotics both in the ultraviolet and infrared regions irrespective of the
loop level. The consistency of the model considered with the general definition
of the QCD invariant charge is shown.Comment: LaTeX 2.09, 12 pages with 5 EPS figures, uses mpla1.sty; enlarged
version is accepted for publication in Mod. Phys. Lett.
Casimir energy of a compact cylinder under the condition
The Casimir energy of an infinite compact cylinder placed in a uniform
unbounded medium is investigated under the continuity condition for the light
velocity when crossing the interface. As a characteristic parameter in the
problem the ratio is used, where and
are, respectively, the permittivity and permeability of the material
making up the cylinder and and are those for the
surrounding medium. It is shown that the expansion of the Casimir energy in
powers of this parameter begins with the term proportional to . The
explicit formulas permitting us to find numerically the Casimir energy for any
fixed value of are obtained. Unlike a compact ball with the same
properties of the materials, the Casimir forces in the problem under
consideration are attractive. The implication of the calculated Casimir energy
in the flux tube model of confinement is briefly discussed.Comment: REVTeX, 12 pages, 1 figure in a separate fig1.eps file, 1 table;
minor corrections in English and misprints; version to be published in Phys.
Rev. D1
Two-Photon Excitation of Low-Lying Electronic Quadrupole States in Atomic Clusters
A simple scheme of population and detection of low-lying electronic
quadrupole modes in free small deformed metal clusters is proposed. The scheme
is analyzed in terms of the TDLDA (time-dependent local density approximation)
calculations. As test case, the deformed cluster is considered.
Long-living quadrupole oscillations are generated via resonant two-photon
(two-dipole) excitation and then detected through the appearance of satellites
in the photoelectron spectra generated by a probe pulse. Femtosecond pump and
probe pulses with intensities and
pulse duration fs are found to be optimal. The modes of
interest are dominated by a single electron-hole pair and so their energies,
being combined with the photoelectron data for hole states, allow to gather new
information about mean-field spectra of valence electrons in the HOMO-LUMO
region. Besides, the scheme allows to estimate the lifetime of electron-hole
pairs and hence the relaxation time of electronic energy into ionic heat.Comment: 4 pages, 4 figure
Shock Wave Structure in a Strongly Nonlinear Granular Lattice with Viscous Dissipation
The shock wave structure in a one-dimensional lattice (e.g. granular chain)
with a power law dependence of force on displacement between particles with
viscous dissipation is considered and compared to the corresponding long wave
approximation. A dissipative term depending on the relative velocity between
neighboring particles is included in the discrete model to investigate its
influence on the shape of steady shock profiles. The critical viscosity
coefficient is obtained from the long-wave approximation for arbitrary values
of the exponent n and denotes the transition from an oscillatory to a monotonic
shock profile in stronly nonlinear systems. The expression for the critical
viscosity coefficient converges to the known equation for the critical
viscosity in the weakly nonlinear case. Values of viscosity based on this
expression are comparable to the values obtained in the numerical analysis of a
discrete particle lattice with a Herzian contact interaction corresponding to n
= 3/2. An initial disturbance in a discrete system approaches a stationary
shock profile after traveling a short distance that is comparable to the width
of the leading pulse of a stationary shock front. The shock front width is
minimized when the viscosity is equal to its critical value.Comment: 20 pages, 6 figure
STIRAP transport of Bose-Einstein condensate in triple-well trap
The irreversible transport of multi-component Bose-Einstein condensate (BEC)
is investigated within the Stimulated Adiabatic Raman Passage (STIRAP) scheme.
A general formalism for a single BEC in M-well trap is derived and analogy
between multi-photon and tunneling processes is demonstrated. STIRAP transport
of BEC in a cyclic triple-well trap is explored for various values of detuning
and interaction between BEC atoms. It is shown that STIRAP provides a complete
population transfer at zero detuning and interaction and persists at their
modest values. The detuning is found not to be obligatory. The possibility of
non-adiabatic transport with intuitive order of couplings is demonstrated.
Evolution of the condensate phases and generation of dynamical and geometric
phases are inspected. It is shown that STIRAP allows to generate the
unconventional geometrical phase which is now of a keen interest in quantum
computing.Comment: 9 pages, 6 figures. To be published in Laser Physics (v. 19, n.4,
2009
Mode-by-mode summation for the zero point electromagnetic energy of an infinite cylinder
Using the mode-by-mode summation technique the zero point energy of the
electromagnetic field is calculated for the boundary conditions given on the
surface of an infinite solid cylinder. It is assumed that the dielectric and
magnetic characteristics of the material which makes up the cylinder
and of that which makes up the surroundings obey the relation . With this
assumption all the divergences cancel. The divergences are regulated by making
use of zeta function techniques. Numerical calculations are carried out for a
dilute dielectric cylinder and for a perfectly conducting cylindrical shell.
The Casimir energy in the first case vanishes, and in the second is in complete
agreement with that obtained by DeRaad and Milton who employed a Green's
function technique with an ultraviolet regulator.Comment: REVTeX, 16 pages, no figures and tables; transcription error in
previous version corrected, giving a zero Casimir energy for a tenuous
cylinde
Low-energy M1 states in deformed nuclei: spin-scissors or spin-flip?
The low-energy states in deformed Dy and spherical Ni are
explored in the framework of fully self-consistent Quasiparticle Random-Phase
Approximation (QRPA) with various Skyrme forces. The main attention is paid to
orbital and spin excitations. The obtained results are compared with the
prediction of the low-energy {\it spin-scissors} resonance suggested
within Wigner Function Moments (WFM) approach. A possible relation of this
resonance to low-energy spin-flip excitations is analyzed. In connection with
recent WFM studies, we consider evolution of the low-energy spin-flip states in
Dy with deformation (from the equilibrium value to the spherical
limit). The effect of tensor forces is briefly discussed. It is shown that two
groups of states observed at 2.4-4 MeV in Dy are rather explained
by fragmentation of the orbital strength than by the occurrence of the
collective spin-scissors resonance. In general, our calculations do not confirm
the existence of this resonance.Comment: 7 pages, 7 figures, submitted to Physics of Atomic Nuclei. arXiv
admin note: text overlap with arXiv:2102.13580. As compared with the previous
version, Ref. [1] was removed, Ref [33] was replaced, description of Fig. 3
was modifie
Extended analytic QCD model with perturbative QCD behavior at high momenta
In contrast to perturbative QCD, the analytic QCD models have running
coupling whose analytic properties correctly mirror those of spacelike
observables. The discontinuity (spectral) function of such running coupling is
expected to agree with the perturbative case at large timelike momenta;
however, at low timelike momenta it is not known. In the latter regime, we
parametrize the unknown behavior of the spectral function as a sum of (two)
delta functions; while the onset of the perturbative behavior of the spectral
function is set to be 1.0-1.5 GeV. This is in close analogy with the "minimal
hadronic ansatz" used in the literature for modeling spectral functions of
correlators. For the running coupling itself, we impose the condition that it
basically merges with the perturbative coupling at high spacelike momenta. In
addition, we require that the well-measured nonstrange semihadronic (V+A) tau
decay ratio value be reproduced by the model. We thus obtain a QCD framework
which is basically indistinguishable from perturbative QCD at high momenta (Q >
1 GeV), and at low momenta it respects the basic analyticity properties of
spacelike observables as dictated by the general principles of the local
quantum field theories.Comment: 15 pages, 6 figures; in v2 Sec.IV is extended after Eq.(48) and
refs.[51-52] added; v2 published in Phys.Rev.D85,114043(2012
Microscopic analysis of dipole electric and magnetic strengths in Gd
The dipole electric () and magnetic () strengths in strongly deformed
Gd are investigated within a fully self-consistent Quasiparticle Random
Phase Approximation (QRPA) with Skyrme forces SV-bas, SLy6 and SG2. We inspect,
on the same theoretical footing, low-lying dipole states and the isovector
giant dipole resonance in channel and orbital scissors resonance as well
as spin-flip giant resonance in channel. Besides, toroidal mode and
low-energy spin-flip excitations are considered. The calculations show a
good agreement with available experimental data, except for the recent NRF
measurements of M. Tamkas et al for strength at 4-6 MeV, where, in
contradiction with our calculations and previous data, almost no
strength was observed.Comment: 8 pages, 4 figure
- …