45 research outputs found
A Gribov equation for the photon Green's function
We present a derivation of the Gribov equation for the gluon/photon Green's
function D(q). Our derivation is based on the second derivative of the
gauge-invariant quantity Tr ln D(q), which we interpret as the gauge-boson
`self-loop'. By considering the higher-order corrections to this quantity, we
are able to obtain a Gribov equation which sums the logarithmically enhanced
corrections. By solving this equation, we obtain the non-perturbative running
coupling in both QCD and QED. In the case of QCD, alpha_S has a singularity in
the space-like region corresponding to super-criticality, which is argued to be
resolved in Gribov's light-quark confinement scenario. For the QED coupling in
the UV limit, we obtain a \propto Q^2 behaviour for space-like Q^2=-q^2. This
implies the decoupling of the photon and an NJLVL-type effective theory in the
UV limit.Comment: 12 pages, 5 figures; version to be published in Eur. Phys. J.
Scaling functions for O(4) in three dimensions
Monte Carlo simulation using a cluster algorithm is used to compute the
scaling part of the free energy for a three dimensional O(4) spin model. The
results are relevant for analysis of lattice studies of high temperature QCD.Comment: 12 pages, 6 figures, uses epsf.st
The first dozen years of the history of ITEP Theoretical Physics Laboratory
The theoretical investigations at ITEP in the years 1945-1958 are reviewed.
There are exposed the most important theoretical results, obtained in the
following branches of physics: 1) the theory of nuclear reactors on thermal
neutrons; 2) the hydrogen bomb project ("Tube" in USSR and "Classical Super" in
USA); 3) radiation theory; ~4) low temperature physics; 5) quantum
electrodynamics and quantum field theories; 6) parity violation in weak
interactions, the theory of -decay and other weak processes; 7) strong
interaction and nuclear physics. To the review are added the English
translations of few papers, originally published in Russian, but unknown (or
almost unknown) to Western readers.Comment: 55 pages, 5 fig
Logarithmic Corrections in the 2D XY Model
Using two sets of high-precision Monte Carlo data for the two-dimensional XY
model in the Villain formulation on square lattices, the scaling
behavior of the susceptibility and correlation length at the
Kosterlitz-Thouless phase transition is analyzed with emphasis on
multiplicative logarithmic corrections in the finite-size
scaling region and in the high-temperature phase near
criticality, respectively. By analyzing the susceptibility at criticality on
lattices of size up to we obtain , in agreement with
recent work of Kenna and Irving on the the finite-size scaling of Lee-Yang
zeros in the cosine formulation of the XY model. By studying susceptibilities
and correlation lengths up to in the high-temperature phase,
however, we arrive at quite a different estimate of , which is
in good agreement with recent analyses of thermodynamic Monte Carlo data and
high-temperature series expansions of the cosine formulation.Comment: 13 pages, LaTeX + 8 postscript figures. See also
http://www.cond-mat.physik.uni-mainz.de/~janke/doc/home_janke.htm
Magnetic Oscillations in Dense Cold Quark Matter with Four-Fermion Interactions
The phase structures of Nambu-Jona-Lasinio models with one or two flavours
have been investigated at non-zero values of and , where is an
external magnetic field and is the chemical potential. In the phase
portraits of both models there arise infinitely many massless chirally
symmetric phases, as well as massive ones with spontaneously broken chiral
invariance, reflecting the existence of infinitely many Landau levels. Phase
transitions of first and second orders and a lot of tricritical points have
been shown to exist in phase diagrams. In the massless case, such a phase
structure leads unavoidably to the standard van Alphen-de Haas magnetic
oscillations of some thermodynamical quantities, including magnetization,
pressure and particle density. In the massive case we have found an oscillating
behaviour not only for thermodynamical quantities, but also for a dynamical
quantity as the quark mass. Besides, in this case we have non-standard, i.e.
non-periodic, magnetic oscillations, since the frequency of oscillations is an
-dependent quantity.Comment: latex, 29 pages, 8 figure
Damping of spin waves and singularity of the longitudinal modes in the dipolar critical regime of the Heisenberg-ferromagnet EuS
By inelastic scattering of polarized neutrons near the (200)-Bragg
reflection, the susceptibilities and linewidths of the spin waves and the
longitudinal spin fluctuations were determined separately. By aligning the
momentum transfers q perpendicular to both \delta S_sw and the spontaneous
magnetization M_s, we explored the statics and dynamics of these modes with
transverse polarizations with respect to q. In the dipolar critical regime,
where the inverse correlation length kappa_z(T) and q are smaller than the
dipolar wavenumber q_d, we observe:(i) the static susceptibility of \delta
S_sw^T(q) displays the Goldstone divergence while for \delta S_z^T(q) the
Ornstein-Zernicke shape fits the data with a possible indication of a
thermal(mass-)renormalization at the smallest q-values, i.e. we find
indications for the predicted 1/q divergence of the longitudinal
susceptibility; (ii) the spin wave dispersion as predicted by the
Holstein-Primakoff theory revealing q_d=0.23(1)\AA^{-1}in good agreement with
previous work in the paramagnetic and ferromagnetic regime of EuS; (iii) within
experimental error, the (Lorentzian) linewidths of both modes turn out to be
identical with respect to the q^2-variation, the temperature independence and
the absolute magnitude. Due to the linear dispersion of the spin waves they
remain underdamped for q<q_d. These central results differ significantly from
the well known exchange dominated critical dynamics, but are quantitatively
explained in terms of dynamical scaling and existing data for T>=T_C. The
available mode-mode coupling theory, which takes the dipolar interactions fully
into account, describes the gross features of the linewidths but not all
details of the T- and q-dependencies. PACS: 68.35.Rh, 75.40.GbComment: 10 pages, 7 figure
Critical Dynamics of Magnets
We review our current understanding of the critical dynamics of magnets above
and below the transition temperature with focus on the effects due to the
dipole--dipole interaction present in all real magnets. Significant progress in
our understanding of real ferromagnets in the vicinity of the critical point
has been made in the last decade through improved experimental techniques and
theoretical advances in taking into account realistic spin-spin interactions.
We start our review with a discussion of the theoretical results for the
critical dynamics based on recent renormalization group, mode coupling and spin
wave theories. A detailed comparison is made of the theory with experimental
results obtained by different measuring techniques, such as neutron scattering,
hyperfine interaction, muon--spin--resonance, electron--spin--resonance, and
magnetic relaxation, in various materials. Furthermore we discuss the effects
of dipolar interaction on the critical dynamics of three--dimensional isotropic
antiferromagnets and uniaxial ferromagnets. Special attention is also paid to a
discussion of the consequences of dipolar anisotropies on the existence of
magnetic order and the spin--wave spectrum in two--dimensional ferromagnets and
antiferromagnets. We close our review with a formulation of critical dynamics
in terms of nonlinear Langevin equations.Comment: Review article (154 pages, figures included
On the analysis of the pi -> e nu gamma experimental data
The most general amplitude for the radiative pion decay pi -> e nu gamma
including terms beyond V-A theory is considered. The experimental constraints
on the decay amplitude components are discussed. A model independent
presentation of the results of high statistics and high resolution experiments
is suggested.Comment: 5 pages, 2 figure
Towards a renormalizable standard model without fundamental Higgs scalar
We investigate the possibility of constructing a renormalizable standard
model with purely fermionic matter content. The Higgs scalar is replaced by
point-like fermionic self-interactions with couplings growing large at the
Fermi scale. An analysis of the UV behavior in the point-like approximation
reveals a variety of non-Gaussian fixed points for the fermion couplings. If
real, such fixed points would imply nonperturbative renormalizability and evade
triviality of the Higgs sector. For point-like fermionic self-interactions and
weak gauge couplings, one encounters a hierarchy problem similar to the one for
a fundamental Higgs scalar.Comment: 18 pages, 4 figure
Constraints on possible phase transitions above the nuclear saturation density
We compare different models for hadronic and quark phases of cold baryon-rich
matter in an attempt to find a deconfinement phase transition between them. For
the hadronic phase we consider Walecka-type mean-field models which describe
well the nuclear saturation properties. We also use the variational chain model
which takes into account correlation effects. For the quark phase we consider
the MIT bag model, the Nambu-Jona-Lasinio and the massive quasiparticle models.
By comparing pressure as a function of baryon chemical potential we find that
crossings of hadronic and quark branches are possible only in some exceptional
cases while for most realistic parameter sets these branches do not cross at
all. Moreover, the chiral phase transition, often discussed within the
framework of QCD motivated models, lies in the region where the quark phases
are unstable with respect to the hadronic phase. We discuss possible physical
consequences of these findings.Comment: 28 pages, 18 PostScript figures, submitted to Phys. Rev.