93 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
Experiences with OpenMP in tmLQCD
An overview is given of the lessons learned from the introduction of
multi-threading using OpenMP in tmLQCD. In particular, programming style,
performance measurements, cache misses, scaling, thread distribution for hybrid
codes, race conditions, the overlapping of communication and computation and
the measurement and reduction of certain overheads are discussed. Performance
measurements and sampling profiles are given for different implementations of
the hopping matrix computational kernel.Comment: presented at the 31st International Symposium on Lattice Field Theory
(Lattice 2013), 29 July - 3 August 2013, Mainz, German
Fixed point merger in the SU(N) gauge beta functions
We discuss qualitative behavior of the SU(N) gauge beta functions in QCD with
many massless flavors. Non-perturbative beta functions can be obtained by
extracting the renormalized trajectories in the exact renormalization group
framework. We examine the renormalization group equations for the general
four-fermi couplings as well as the gauge coupling obtained in a simple
approximation scheme. It is shown that the gauge beta function possesses not
only an IR but also a UV fixed point in the conformal window. These fixed
points merge with each other and disappear at the edge of the conformal window.
The scaling dimensions of the quark mass operator at the fixed points are also
shown.Comment: 33 pages, 18 figure
Holography of the Conformal Window
Inspired by the model of Jarvinen and Kiritsis, we present a simple
holographic model for the on set of chiral symmetry breaking at the edge of the
conformal window in QCD in the Veneziano limit. Our most naive model enforces
the QCD two loop running coupling on a D3/D7 holographic brane system. The mass
of the holographic field, describing the chiral condensate in the model, is
driven below the BF bound when the running is sufficiently strong, triggering
chiral symmetry breaking for N_f/N_c<2.9. This model though contains too great
a remnant of supersymmetry and does not correctly encode the perturbative
anomalous dimensions of QCD. In a second model we impose the QCD anomalous
dimension result and find chiral symmetry breaking sets in for N_f/N_c=4 at a
BKT-type phase transition. In this case the transition is triggered when the
anomalous dimension of the mass operator \gamma_m=1.Comment: 10 pages, 2 figures, v2: minor corrections, improved Figure
Phase structure of SU(3) gauge theory with two flavors of symmetric-representation fermions
We have performed numerical simulations of SU(3) gauge theory coupled to Nf=2
flavors of symmetric representation fermions. The fermions are discretized with
the tadpole-improved clover action. Our simulations are done on lattices of
length L=6, 8, and 12. In all simulation volumes we observe a crossover from a
strongly coupled confined phase to a weak coupling deconfined phase.
Degeneracies in screening masses, plus the behavior of the pseudoscalar decay
constant, indicate that the deconfined phase is also a phase in which chiral
symmetry is restored. The movement of the confinement transition as the volume
is changed is consistent with avoidance of the basin of attraction of an
infrared fixed point of the massless theory.Comment: 12 pages, 11 figure
Infrared Evolution and Phase Structure of a Gauge Theory Containing Different Fermion Representations
We study the evolution of an asymptotically free vectorial SU() gauge
theory from the ultraviolet to the infrared and the resultant phase structure
in the general case in which the theory contains fermions transforming
according to several different representations of the gauge group. We discuss
the sequential fermion condensation and dynamical mass generation that occur,
and comment on the effect of bare fermion mass terms.Comment: 13 pages, late
Chiral symmetry of QCD with twelve light flavors
We study QCD with twelve light flavors at intermediate values of the bare
lattice coupling. We contrast the results for the order parameter with
different theoretical models motivated by the physics of the Goldstone phase
and of the symmetric phase, and we perform a model independent analysis of the
meson spectrum inspired by universal properties of chiral symmetry. Our
analysis favors chiral symmetry restoration.Comment: 7 pages, 3 figures. To appear in the proceedings of the XXVIII
International Symposium on Lattice Field Theory, Lattice2010, June 14-19,
2010, Villasimius, Ital
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