Effects of physics beyond the standard model on electroweak observables ares studied using the Peskin-Takeuchi isospin-conserving, S, and -breaking, T, parametrization of ‘‘new’’ quantum loop corrections. Experimental constraints on S and T are presented. Atomic parity-violating experiments are shown to be particularly sensitive to S with existing data giving S=-2.7±2.0±1.1. That constraint has important implications for generic technicolor models which predict S≃0.1${\mathit{N}}_{\mathit{T}}$${\mathit{N}}_{\mathit{D}}$ (${\mathit{N}}_{\mathit{T}}$ is the number of technicolors, ${\mathit{N}}_{\mathit{D}}$ is the number of technidoublets).

• Received 30 August 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.2963