40 research outputs found
Non trivial critical exponents for finite temperature chiral transitions at fixed total fermion number
We analyze the finite temperature chiral restoration transition of the
-dimensional Gross-Neveu model for the case of a large number of
flavors and fixed total fermion number. This leads to the study of the model
with a nonzero imaginary chemical potential. In this formulation of the theory,
we have obtained that, in the transition region, the model is described by a
chiral conformal field theory where the concepts of dimensional reduction and
universality do apply due to a transmutation of statistics which makes fermions
act as if they were bosons, having zero energy. This result should be generic
for theories with dynamical symmetry breaking, such as Quantum Chromodynamics.Comment: 14 pages Latex, no figures, final version to be published in Phys.
Lett.
Dimensional reduction of the chiral-continous Gross-Neveu model
We study the finite-temperature phase transition of the generalized
Gross-Neveu model with continous chiral symmetry in euclidean
dimensions. The critical exponents are computed to the leading order in the
expansion at both zero and finite temperatures. A dimensionally reduced
theory is obtained after the introduction of thermal counterterms necessary to
cancel thermal divergences that arise in the limit of high temperature.
Although at zero temperature we have an infinitely and continously degenerate
vacuum state, we show that at finite temperature this degeneracy is discrete
and, depending on the values of the bare parameters, we may have either total
or partial restoration of symmetry. Finally we determine the universality class
of the reduced theory by a simple analysis of the infrared structure of
thermodynamic quantities computed using the reduced action as starting point.Comment: Latex, 25 pages, 4 eps fig., uses epsf.sty and epsf.te
Nonperturbative bound on high multiplicity cross sections in phi^4_3 from lattice simulation
We have looked for evidence of large cross sections at large multiplicities
in weakly coupled scalar field theory in three dimensions. We use spectral
function sum rules to derive bounds on total cross sections where the sum can
be expresed in terms of a quantity which can be measured by Monte Carlo
simulation in Euclidean space. We find that high multiplicity cross sections
remain small for energies and multiplicities for which large effects had been
suggested.Comment: 23 pages, revtex, seven eps figures revised version: typos corrected,
some rewriting of discusion, same resul
Further results for the two-loop Lcc vertex in the Landau gauge
In the previous paper hep-th/0604112 we calculated the first of the five
planar two-loop diagrams for the Lcc vertex of the general non-Abelian
Yang-Mills theory, the vertex which allows us in principle to obtain all other
vertices via the Slavnov-Taylor identity. The integrand of this first diagram
has a simple Lorentz structure. In this letter we present the result for the
second diagram, whose integrand has a complicated Lorentz structure. The
calculation is performed in the D-dimensional Euclidean position space. We
initially perform one of the two integrations in the position space and then
reduce the Lorentz structure to D-dimensional scalar single integrals. Some of
the latter are then calculated by the uniqueness method, others by the
Gegenbauer polynomial technique. The result is independent of the ultraviolet
and the infrared scale. It is expressed in terms of the squares of spacetime
intervals between points of the effective fields in the position space -- it
includes simple powers of these intervals, as well as logarithms and
polylogarithms thereof, with some of the latter appearing within the Davydychev
integral J(1,1,1). Concerning the rest of diagrams, we present the result for
the contributions correponding to third, fourth and fifth diagrams without
giving the details of calculation. The full result for the Lcc correlator of
the effective action at the planar two-loop level is written explicitly for
maximally supersymmetric Yang-Mills theory.Comment: 29 pages, 1 figure, minor changes; three references added, one new
paragraph in Introduction added, Note Added is extended; to appear in JHE
QED and String Theory
We analyze the D9-D9bar system in type IIB string theory using Dp-brane
probes. It is shown that the world-volume theory of the probe Dp-brane contains
two-dimensional and four-dimensional QED in the cases with p=1 and p=3,
respectively, and some applications of the realization of these well-studied
quantum field theories are discussed. In particular, the two-dimensional QED
(the Schwinger model) is known to be a solvable theory and we can apply the
powerful field theoretical techniques, such as bosonization, to study the
D-brane dynamics. The tachyon field created by the D9-D9bar strings appears as
the fermion mass term in the Schwinger model and the tachyon condensation is
analyzed by using the bosonized description. In the T-dualized picture, we
obtain the potential between a D0-brane and a D8-D8bar pair using the Schwinger
model and we observe that it consists of the energy carried by fundamental
strings created by the Hanany-Witten effect and the vacuum energy due to a
cylinder diagram. The D0-brane is treated quantum mechanically as a particle
trapped in the potential, which turns out to be a system of a harmonic
oscillator.
As another application, we obtain a matrix theory description of QED using
Taylor's T-duality prescription, which is actually applicable to a wide variety
of field theories including the realistic QCD. We show that the lattice gauge
theory is naturally obtained by regularizing the matrix size to be finite.Comment: 33 pages, Latex, 4 figures, a reference adde
Symmetry Nonrestoration in a Gross-Neveu Model with Random Chemical Potential
We study the symmetry behavior of the Gross-Neveu model in three and two
dimensions with random chemical potential. This is equivalent to a four-fermion
model with charge conjugation symmetry as well as Z_2 chiral symmetry. At high
temperature the Z_2 chiral symmetry is always restored. In three dimensions the
initially broken charge conjugation symmetry is not restored at high
temperature, irrespective of the value of the disorder strength. In two
dimensions and at zero temperature the charge conjugation symmetry undergoes a
quantum phase transition from a symmetric state (for weak disorder) to a broken
state (for strong disorder) as the disorder strength is varied. For any given
value of disorder strength, the high-temperature behavior of the charge
conjugation symmetry is the same as its zero-temperature behavior. Therefore,
in two dimensions and for strong disorder strength the charge conjugation
symmetry is not restored at high temperature.Comment: 16 pages, 3 figure
Scaling critical behavior of superconductors at zero magnetic field
We consider the scaling behavior in the critical domain of superconductors at
zero external magnetic field. The first part of the paper is concerned with the
Ginzburg-Landau model in the zero magnetic field Meissner phase. We discuss the
scaling behavior of the superfluid density and we give an alternative proof of
Josephson's relation for a charged superfluid. This proof is obtained as a
consequence of an exact renormalization group equation for the photon mass. We
obtain Josephson's relation directly in the form , that
is, we do not need to assume that the hyperscaling relation holds. Next, we
give an interpretation of a recent experiment performed in thin films of
. We argue that the measured mean field like
behavior of the penetration depth exponent is possibly associated with a
non-trivial critical behavior and we predict the exponents and
for the correlation lenght and specific heat, respectively. In the
second part of the paper we discuss the scaling behavior in the continuum dual
Ginzburg-Landau model. After reviewing lattice duality in the Ginzburg-Landau
model, we discuss the continuum dual version by considering a family of
scalings characterized by a parameter introduced such that
, where is the bare mass of the magnetic
induction field. We discuss the difficulties in identifying the renormalized
magnetic induction mass with the photon mass. We show that the only way to have
a critical regime with is having , that
is, with having the scaling behavior of the renormalized photon mass.Comment: RevTex, 15 pages, no figures; the subsection III-C has been removed
due to a mistak
Expanding non homogeneous configurations of the model
A time dependent variational approach is considered to derive the equations
of movement for the model. The temporal evolution of the model
is performed numerically in the frame of the Gaussian approximation in a
lattice of 1+1 dimensions given non homogeneous initial conditions (like
bubbles) for the classical and quantum parts of the field which expands. A
schematic model for the initial conditions is presented considering the model
at finite fermionic density. The non zero fermionic density may lead either to
the restoration of the symmetry or to an even more asymmetric phase. Both kinds
of situations are considered as initial conditions and the eventual differences
in early time dynamics are discussed. In the early time evolution there is
strong energy exchange between the classical and quantum parts of the field as
the initial configuration expands. The contribution of the quantum fluctuations
is discussed especially in the strong coupling constant limit. The continuum
limit is analyzed.Comment: 23 pages (latex) plus thirteen figures in eps file
Diferentes fontes de fibra em dietas a base de palma forrageira na alimentação de ovinos
Asymptotic safety of simple Yukawa systems
We study the triviality and hierarchy problem of a Z_2-invariant Yukawa
system with massless fermions and a real scalar field, serving as a toy model
for the standard-model Higgs sector. Using the functional RG, we look for UV
stable fixed points which could render the system asymptotically safe. Whether
a balancing of fermionic and bosonic contributions in the RG flow induces such
a fixed point depends on the algebraic structure and the degrees of freedom of
the system. Within the region of parameter space which can be controlled by a
nonperturbative next-to-leading order derivative expansion of the effective
action, we find no non-Gaussian fixed point in the case of one or more fermion
flavors. The fermion-boson balancing can still be demonstrated within a model
system with a small fractional flavor number in the symmetry-broken regime. The
UV behavior of this small-N_f system is controlled by a conformal Higgs
expectation value. The system has only two physical parameters, implying that
the Higgs mass can be predicted. It also naturally explains the heavy mass of
the top quark, since there are no RG trajectories connecting the UV fixed point
with light top masses.Comment: 14 pages, 3 figures, v2: references added, typos corrected, minor
numerical correction