Adjoint “quark” color fields in four-dimensional lattice gauge theory: Vacuum screening and penetration

The fields generated by “quarks” in the adjoint representation of SU(2) color are analyzed in the scaling region of the four-dimensional lattice theory. Evidence of vacuum screening of adjoint quarks is obtained from a comparison of quark-antiquark (QQ) flux-tubes for quarks in the adjoint (“isospin” j = 1) and fundamental (j = 12) representations. The component εj of the color-electric field strength in the direction parallel to the QQ axis is calculated. Near the quarks the ratio of fields εj=1εj=12 approaches the value 83, which is equal to the ratio of SU(2) Casimirs. In between the quarks, the ratio falls well below 83 at large R. εj also falls off rapidly as a function of distance x⊥ perpendicular to the QQ axis. However, the ratio εj=1εj=12 depends very weakly on x⊥. The flux-tubes in the two representations thus appear to have very similar cross-sections. This result could imply that the QCD vacuum is dual to a type I superconductor.