5,428 research outputs found
Evidence for a conformal phase in SU(N) gauge theories
We discuss the existence of a conformal phase in SU(N) gauge theories in four
dimensions. In this lattice study we explore the model in the bare parameter
space, varying the lattice coupling and bare mass. Simulations are carried out
with three colors and twelve flavors of dynamical staggered fermions in the
fundamental representation. The analysis of the chiral order parameter and the
mass spectrum of the theory indicates the restoration of chiral symmetry at
zero temperature and the presence of a Coulomb-like phase, depicting a scenario
compatible with the existence of an infrared stable fixed point at nonzero
coupling. Our analysis supports the conclusion that the onset of the conformal
window for QCD-like theories is smaller than Nf=12, before the loss of
asymptotic freedom at sixteen and a half flavors. We discuss open questions and
future directions.Comment: 11 pages, 11 figures; extended analysis, conclusions unchanged.
(version to appear in PRD
QCD in One Dimension at Nonzero Chemical Potential
Using an integration formula recently derived by Conrey, Farmer and
Zirnbauer, we calculate the expectation value of the phase factor of the
fermion determinant for the staggered lattice QCD action in one dimension. We
show that the chemical potential can be absorbed into the quark masses; the
theory is in the same chiral symmetry class as QCD in three dimensions at zero
chemical potential. In the limit of a large number of colors and fixed number
of lattice points, chiral symmetry is broken spontaneously, and our results are
in agreement with expressions based on a chiral Lagrangian. In this limit, the
eigenvalues of the Dirac operator are correlated according to random matrix
theory for QCD in three dimensions. The discontinuity of the chiral condensate
is due to an alternative to the Banks-Casher formula recently discovered for
QCD in four dimensions at nonzero chemical potential. The effect of temperature
on the average phase factor is discussed in a schematic random matrix model.Comment: Latex, 23 pages and 5 figures; Added two references and corrected
several typo
A simple model for the microscopic effective pairing interaction
The microscopic effective pairing interaction in the -channel is
investigated for two different values of the chemical potential starting
from the separable form of the Paris NN-potential. It is shown that, within a
high accuracy, this effective interaction can be approximated by the off-shell
free T-matrix taken at the negative energy .Comment: LaTeX, 8 pages, 6 ps-figure
One,Two,Zero: Scales of Strong Interactions
We discuss our results on QCD with a number of fundamental fermions ranging
from zero to sixteen. These theories exhibit a wide array of fascinating
phenomena which have been under close scrutiny, especially in recent years,
first and foremost is the approach to conformality. To keep this review
focused, we have chosen scale generation, or lack thereof as a guiding theme,
however the discussion will be set in the general framework of the analysis of
the phases and phase transitions of strong interactions at zero and nonzero
temperature.Comment: 15 pages, prepared for IJMPA Special Issue 'Recent Nonperturbative
Developments in QCD-like Theories
On the particle spectrum and the conformal window
We study the SU(3) gauge theory with twelve flavours of fermions in the
fundamental representation as a prototype of non-Abelian gauge theories inside
the conformal window. Guided by the pattern of underlying symmetries, chiral
and conformal, we analyze the two-point functions theoretically and on the
lattice, and determine the finite size scaling and the infinite volume fermion
mass dependence of the would-be hadron masses. We show that the spectrum in the
Coulomb phase of the system can be described in the context of a universal
scaling analysis and we provide the nonperturbative determination of the
fermion mass anomalous dimension gamma*=0.235(46) at the infrared fixed point.
We comment on the agreement with the four-loop perturbative prediction for this
quantity and we provide a unified description of all existing lattice results
for the spectrum of this system, them being in the Coulomb phase or the
asymptotically free phase. Our results corroborate the view that the fixed
point we are studying is not associated to a physical singularity along the
bare coupling line and estimates of physical observables can be attempted on
either side of the fixed point. Finally, we observe the restoration of the U(1)
axial symmetry in the two-point functions.Comment: 40 pages, 22 figure
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