Debye Screening in the QCD plasma

Various definitions for the QCD Debye mass and its evaluation are reviewed in a non-perturabtive framework for the study of screening of general static sources. While it is possible to perturbatively integrate over scales $\sim T$ and thus construct a 3d effective theory, the softer scales $\sim gT$ and $\sim g^2T$ are strongly coupled for temperatures \lsim 10^7 GeV and require lattice simulations. Within the effective theory, a lattice treatment of screening at finite quark densities \mu \lsim 4/T is also possible.Comment: 12 pages, 4 figures. Invited talk at Strong and Electroweak Matter, Marseille, France, June 13-17, 200

Constraining the QCD phase diagram by tricritical lines at imaginary chemical potential

We present unambiguous evidence from lattice simulations of QCD with three degenerate quark species for two tricritical points in the (T,m) phase diagram at fixed imaginary \mu/T=i\pi/3 mod 2\pi/3, one in the light and one in the heavy mass regime. These represent the boundaries of the chiral and deconfinement critical lines continued to imaginary chemical potential, respectively. It is demonstrated that the shape of the deconfinement critical line for real chemical potentials is dictated by tricritical scaling and implies the weakening of the deconfinement transition with real chemical potential. The generalization to non-degenerate and light quark masses is discussed.Comment: 4 pages, 5 figure

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 $r_b \approx 1.25{\rm fm}, 2.3 r_0$, or in units of the lightest glueball, $r_b m_{0++} \approx 9.7$. We furthermore resolve the first excitation of the flux-tube and observe its breaking as well. The result for $r_b$ is in remarkable quantitative agreement with the three-dimensional one.Comment: 15 pages, 3 figures; increased statistics; gauge-fixing clarified; typos correcte

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

QCD phase diagram for small densities from simulations at imaginary mu

We present results on the QCD phase diagram for small densities without reweighting. Our simulations are performed with an imaginary chemical potential mu_I for which the fermion determinant is positive. On an 8^3x4 lattice with 2 flavors of staggered quarks, we map out the pseudo-critical temperature T_c(mu_I). For mu_I/T < pi/3, this is an analytic function whose Taylor expansion converges rapidly, with truncation errors smaller than statistical ones. The result is analytically continued to give the location of the pseudo-critical line for real mu_B<500 MeV.Comment: Lattice2002(nonzerot), 3 pp, 5 figure