205 research outputs found
A study of center vortices in SU(2) and SU(3) gauge theories
We show how center vortices and Abelian monopoles both appear as local gauge
ambiguities in the Laplacian Center gauge. Numerical results, for SU(2) and
SU(3), support the view that the string tension obtained in the
center-projected theory matches the full string tension when the continuum
limit is taken.Comment: 9 pages, 4 figures; talk presented at the International Workshop on
Non-Perturbative Methods and Lattice QCD, Guangzhou, May 200
The phase diagram of N_f=3 QCD for small baryon densities
We demonstrate how to locate the critical endpoint of the QCD phase
transition by means of simulations at imaginary \mu. For the three flavor
theory, we present numerical results for the pseudo-critical line as a function
of chemical potential and bare quark mass, as well as the bare quark mass
dependence of the endpoint.Comment: 3 pages, 5 eps-figs, Lattice2003(nonzero
QCD phase diagram at small densities from simulations with imaginary mu
We review our results for the QCD phase diagram at baryonic chemical
potential mu_B \leq pi T. Our simulations are performed with an imaginary
chemical potential mu_I for which the fermion determinant is positive. For 2
flavors of staggered quarks, we map out the phase diagram and identify the
pseudo-critical temperature T_c(mu_I). For mu_I/T \leq pi/3, this is an
analytic function, whose Taylor expansion is found to converge rapidly, with
truncation errors far smaller than statistical ones. The truncated series may
then be continued to real mu, yielding the corresponding phase diagram for mu_B
\lsim 500 MeV. This approach provides control over systematics and avoids
reweighting. We outline our strategy to find the (2+1)-flavor critical point.Comment: 5 pages, 4 figures. To appear in the proceedings of Strong and
Electroweak Matter (SEWM 2002), Heidelberg, Germany, 2-5 Oct 200
Laplacian Center Vortices
I present a unified picture of center vortices and Abelian monopoles. Both
appear as local gauge ambiguities in the Laplacian Center Gauge. This gauge is
constructed for a general SU(N) theory. Numerical evidence is presented, for
SU(2) and SU(3), that the projected theory confines with a string tension
similar to the non-Abelian one.Comment: 9 pages, 5 figures; talk presented at "Confinement 2000", Osaka,
March 200
The QCD Phase Diagram for Three Degenerate Flavors and Small Baryon Density
We present results for the phase diagram of three flavor QCD for \mu_B ~ 500
MeV. Our simulations are performed with imaginary chemical potential \mu_I for
which the fermion determinant is positive. Physical observables are then fitted
by truncated Taylor series and continued to real chemical potential. We map out
the location of the critical line T_c(\mu_B) with an accuracy up to terms of
order (\mu_B/T)^6. We also give first results on a determination of the
critical endpoint of the transition and its quark mass dependence. Our results
for the endpoint differ significantly from those obtained by other methods, and
we discuss possible reasons for this.Comment: 20 pages, 10 figures. Eqn (20) corrected, increased statistics, more
accurate results. Version to appear in Nucl.Phys.
Towards a controlled study of the QCD critical point
The phase diagram of QCD, as a function of temperature T and quark chemical
potential mu, may contain a critical point (mu_E,T_E) whose non-perturbative
nature makes it a natural object of lattice studies. However, the sign problem
prevents the application of standard Monte Carlo techniques at non-zero baryon
density. We have been pursuing an approach free of the sign problem, where the
chemical potential is taken as imaginary and the results are Taylor-expanded in
mu/T about mu=0, then analytically continued to real mu.
Within this approach we have determined the sensitivity of the critical
chemical potential mu_E to the quark mass, d(\mu_E)^2/dm_q|_{\mu_E=0}. Our
study indicates that the critical point moves to {\em smaller} chemical
potential as the quark mass {\em increases}. This finding, contrary to common
wisdom, implies that the deconfinement crossover, which takes place in QCD at
mu=0 when the temperature is raised, will remain a crossover in the mu-region
where our Taylor expansion can be trusted. If this result, obtained on a coarse
lattice, is confirmed by simulations on finer lattices now in progress, then we
predict that no {\em chiral} critical point will be found for mu_B \lesssim 500
MeV, unless the phase diagram contains additional transitions.Comment: 4 pages, 6 figures, proceedings of Quark Matter 2008, Jaipur (India),
Feb. 2008, to appear in J. Phys.
Adjoint string breaking in 4d SU(2) Yang-Mills theory
We compute the static potential of adjoint sources in SU(2) Yang-Mills theory
in four dimensions by numerical Monte Carlo simulations. Following a recent
calculation in 2+1 dimensions, we employ a variational approach involving
string and gluelump operators and obtain clear evidence for string breaking and
the saturation of the potential at large distances. For the string breaking
scale we find , or in units of the lightest
glueball, . We furthermore resolve the first
excitation of the flux-tube and observe its breaking as well. The result for
is in remarkable quantitative agreement with the three-dimensional one.Comment: 15 pages, 3 figures; increased statistics; gauge-fixing clarified;
typos correcte
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