1,006 research outputs found
Can universality of the QCD evolution be checked in W boson decays into hadrons?
Hadron multiplicity from boson is calculated in pQCD. The agreement of
our theoretical predictions with the LEP data says in favor of universality of
the QCD evolution in hard processes.Comment: 14 pages, 7 figure
On passage through resonances in volume-preserving systems
Resonance processes are common phenomena in multiscale (slow-fast) systems.
In the present paper we consider capture into resonance and scattering on
resonance in 3-D volume-preserving slow-fast systems. We propose a general
theory of those processes and apply it to a class of viscous Taylor-Couette
flows between two counter-rotating cylinders. We describe the phenomena during
a single passage through resonance and show that multiple passages lead to the
chaotic advection and mixing. We calculate the width of the mixing domain and
estimate a characteristic time of mixing. We show that the resulting mixing can
be described using a diffusion equation with a diffusion coefficient depending
on the averaged effect of the passages through resonances.Comment: 23 pages and 9 Figure
RPA Green's Functions of the Anisotropic Heisenberg Model
We solve in random-phase approximation the anisotropic Heisenberg model,
including nearest and next-nearest neighbour interactions by calculating all
Green's functions and pair correlation functions in a cumulant decoupling
scheme. The general exposition is pedagogic in tone and is intended to be
accessible to any graduate student or physicist who is not an expert in the
field.Comment: 26 pages, 4 figure
Thermal Bogoliubov transformation in nuclear structure theory
Thermal Bogoliubov transformation is an essential ingredient of the thermo
field dynamics -- the real time formalism in quantum field and many-body
theories at finite temperatures developed by H. Umezawa and coworkers. The
approach to study properties of hot nuclei which is based on the extension of
the well-known Quasiparticle-Phonon Model to finite temperatures employing the
TFD formalism is presented. A distinctive feature of the QPM-TFD combination is
a possibility to go beyond the standard approximations like the thermal
Hartree-Fock or the thermal RPA ones.Comment: 8 pages, Proceedings of the International Bogolyubov Conference
"Problems of Theoretical and Mathematical Physics", August 23 -- 27, 2009,
Dubna, Russi
The effective action and equations of motion of curved local and global vortices: Role of the field excitations
The effective actions for both local and global curved vortices are derived,
based on the derivative expansion of the corresponding field theoretic actions
of the nonrelativistic Abelian Higgs and Goldstone models. The role of
excitations of the modulus and the phase of the scalar field and of the gauge
field (the Bogolyubov-Anderson mode) emitted and reabsorbed by vortices is
elucidated. In case of the local (gauge) magnetic vortex, they are necessary
for cancellation of the long distance divergence when using the transverse form
of the electric gauge field strength of the background field. In case of global
vortex taking them into account results in the Greiter-Wilczek-Witten form of
the effective action for the Goldstone mode. The expressions for transverse
Magnus-like force and the vortex effective mass for both local and global
vortices are found. The equations of motion of both type of vortices including
the terms due to the field excitations are obtained and solved in cases of
large and small contour displacements.Comment: 16 pages, no figures; accepted for publication in Int. Journ. Mod.
Phys.
Effects of Frustrated Surface in Heisenberg Thin Films
We study by extensive Monte Carlo (MC) simulations and analytical Green
function (GF) method effects of frustrated surfaces on the properties of thin
films made of stacked triangular layers of atoms bearing Heisenberg spins with
an Ising-like interaction anisotropy. We suppose that the in-plane surface
interaction can be antiferromagnetic or ferromagnetic while all other
interactions are ferromagnetic. We show that the ground-state spin
configuration is non linear when is lower than a critical value .
The film surfaces are then frustrated. In the frustrated case, there are two
phase transitions related to disorderings of surface and interior layers. There
is a good agreement between MC and GF results. In addition, we show from MC
histogram calculation that the value of the ratio of critical exponents
of the observed transitions is deviated from the values of two and
three Ising universality classes. The origin of this deviation is discussed
with general physical arguments.Comment: 9 pages, 16 figure
Localization of Bogoliubov quasiparticles in interacting Bose gases with correlated disorder
We study the Anderson localization of Bogoliubov quasiparticles (elementary
many-body excitations) in a weakly interacting Bose gas of chemical potential
subjected to a disordered potential . We introduce a general mapping
(valid for weak inhomogeneous potentials in any dimension) of the Bogoliubov-de
Gennes equations onto a single-particle Schr\"odinger-like equation with an
effective potential. For disordered potentials, the Schr\"odinger-like equation
accounts for the scattering and localization properties of the Bogoliubov
quasiparticles. We derive analytically the localization lengths for correlated
disordered potentials in the one-dimensional geometry. Our approach relies on a
perturbative expansion in , which we develop up to third order, and we
discuss the impact of the various perturbation orders. Our predictions are
shown to be in very good agreement with direct numerical calculations. We
identify different localization regimes: For low energy, the effective
disordered potential exhibits a strong screening by the quasicondensate density
background, and localization is suppressed. For high-energy excitations, the
effective disordered potential reduces to the bare disordered potential, and
the localization properties of quasiparticles are the same as for free
particles. The maximum of localization is found at intermediate energy when the
quasicondensate healing length is of the order of the disorder correlation
length. Possible extensions of our work to higher dimensions are also
discussed.Comment: Published versio
Bare vs effective pairing forces. A microscopic finite-range interaction for HFB calculations in coordinate space
We propose a microscopic effective interaction to treat pairing correlations
in the channel. It is introduced by recasting the gap equation
written in terms of the bare force into a fully equivalent pairing problem.
Within this approach, the proposed interaction reproduces the pairing
properties provided by the realistic force very accurately. Written in
the canonical basis of the actual Bogolyubov transformation, the force takes
the form of an off-shell in-medium two-body matrix in the superfluid phase
multiplied by a BCS occupation number . This interaction is finite
ranged, non local, total-momentum dependent and density dependent. The factor
emerging from the recast of the gap equation provides a natural
cut-off and makes zero-range approximations of the effective vertex meaningful.
Performing such an approximation, the roles of the range and of the density
dependence of the interaction can be disentangled. The isoscalar and isovector
density-dependences derived ab-initio provide the pairing force with a strong
predictive power when extrapolated toward the drip-lines. Although finite
ranged and non local, the proposed interaction makes HFB calculations of finite
nuclei in coordinate space tractable. Through the two-basis method, its
computational cost is of the same order as for a zero-range force.Comment: 43 pages, 13 figures. Published versio
Lessons from All Logs Summation in Yukawa Theories
Some features of old results in the total summation of all logarithmic
contributions of all diagrams in Yukawa theory are presented. We discuss some
lessons from this picture for the description of Pomeron, odderon, etc. in QCD.Comment: 6 pages, 2 figure
Observation of Leggett's collective mode in a multi-band MgB2 superconductor
We report observation of Leggett's collective mode in a multi-band MgB2
superconductor with T_c=39K arising from the fluctuations in the relative phase
between two superconducting condensates. The novel mode is observed by Raman
spectroscopy at 9.4 meV in the fully symmetric scattering channel. The observed
mode frequency is consistent with theoretical considerations based on the first
principle computations.Comment: Accepted for PR
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