Determining $\alpha_s$ from Measurements at $Z$: How Nature Prompts us about New Physics

Abstract

The value of $\alpha_s (M_Z)$ emerging from the so called global fits based mainly on the data at the $Z$ peak (and assuming the standard model) is three standard deviations higher than the one stemming from the low-energy phenomenology. The corresponding value of $\Lambda_{\rm QCD}$ is very large, $\sim$ 500 MeV, and is incompatible with crucial features of QCD. If persists, the discrepancy should be interpreted as due to contributions to the $Z$-quark-antiquark vertices which go beyond the standard model.