Yigal Shamir

Zero of the discrete beta function in SU(3) lattice gauge theory with color sextet fermions

We have carried out a Schrodinger functional calculation for the SU(3) lattice gauge theory with two flavors of Wilson fermions in the sextet representation of the gauge group. We find that the discrete beta function, which governs the change in the running coupling under a discrete change of spatial scale, changes sign when the Schrodinger functional renormalized coupling is in the neighborhood of g{sup 2}=2.0. The simplest explanation is that the theory has an infrared-attractive fixed point, but more complicated possibilities are allowed by the data. While we compare rescalings by factors of 2 and 4/3, we work at a single lattice spacing.

Infrared fixed point in SU(2) gauge theory with adjoint fermions

We apply Schroedinger-functional techniques to the SU(2) lattice gauge theory with N{sub f}=2 flavors of fermions in the adjoint representation. Our use of hypercubic smearing enables us to work at stronger couplings than did previous studies, before encountering a critical point and a bulk phase boundary. Measurement of the running coupling constant gives evidence of an infrared fixed point g{sub *} where 1/g{sub *}{sup 2}=0.20(4)(3). At the fixed point, we find a mass anomalous dimension {gamma}{sub m}(g{sub *})=0.31(6).

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

Running coupling and mass anomalous dimension of SU(3) gauge theory with two flavors of symmetric-representation fermions

{N}_{f}=2

Observations on staggered fermions at nonzero lattice spacing

flavors of symmetric-representation fermions. The fermions are discretized with the tadpole-improved clover action. Our simulations are done on lattices of length

Mass anomalous dimension in sextet QCD

L=6

Renormalization-group analysis of the validity of staggered-fermion QCD with the fourth-root recipe

, 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 on "Chiral anomalies and rooted staggered fermions"

We have measured the running coupling constant of SU(3) gauge theory coupled to N{sub f}=2 flavors of symmetric representation fermions, using the Schroedinger functional scheme. Our lattice action is defined with hypercubic smeared links which, along with the larger lattice sizes, bring us closer to the continuum limit than in our previous study. We observe that the coupling runs more slowly than predicted by asymptotic freedom, but we are unable to observe fixed point behavior before encountering a first order transition to a strong coupling phase. This indicates that the infrared fixed point found with the thin-link action is a lattice artifact. The slow running of the gauge coupling permits an accurate determination of the mass anomalous dimension for this theory, which we observe to be small, {gamma}{sub m} < or approx. 0.6, over the range of couplings we can reach. We also study the bulk and finite-temperature phase transitions in the strong coupling region.

Breakdown of staggered fermions at nonzero chemical potential

We show that the use of the fourth-root trick in lattice QCD with staggered fermions corresponds to a nonlocal theory at nonzero lattice spacing, but argue that the nonlocal behavior is likely to go away in the continuum limit. We give examples of this nonlocal behavior in the free theory, and for the case of a fixed topologically nontrivial background gauge field. In both special cases, the nonlocal behavior indeed disappears in the continuum limit. Our results invalidate a recent claim that at nonzero lattice spacing an additive mass renormalization is needed because of taste-symmetry breaking.

Localization properties of lattice fermions with plaquette and improved gauge actions

We extend our previous study of the SU(3) gauge theory with N_f=2 flavors of fermions in the sextet representation of color. Our tool is the Schroedinger functional method. By changing the lattice action, we push the bulk transition of the lattice theory to stronger couplings and thus reveal the beta function and the mass anomalous dimension gamma_m over a wider range of coupling, out to g^2 ~ 11. Our results are consistent with an infrared fixed point, but walking is not ruled out. Our main result is that gamma_m never exceeds 0.45, making the model unsuitable for walking technicolor. We use a novel method of extrapolation to the large-volume/continuum limit, tailored to near-conformal theories.