Implications of Precision Electroweak Data

الأصل · EN

There are two aspects to the 1995 summer update of the combined preliminary electroweak data from LEP and SLC. On the one hand, agreement between experiments and the Standard Model (SM) has improved for the line-shape and the asymmetry data. The τ widths and asymmetries are now consistent with e--μ--τ universality, and all the asymmetry data including the left-right asymmetry from SLC are consistent with the SM (16%CL). On the other hand, a discrepancy between experiments and SM predictions is sharpened for two observables, Rb and Rc, where Rq is the partial Z boson width ratio Γq/Γₕ. Rb is 3% larger (3.7σ) and Rc is 11% smaller (2.5σ) than the SM predictions. When combined, the SM is ruled out at the 99.99%CL for mₜ>170GeV. It is difficult to interpret the 11± 4% deficit of Rc, since if we allow only Γb and Γc to deviate from the SM then the precisely measured ratio Rₕ=Γₕ/Γℓ forces the QCD coupling to be αₛ≡ αₛ(mZ) MS=0.185± 0.041, which is uncomfortably large. The data can be consistent with the prefered αₛ (0.10<αₛ<0.13) only if the sum Γₕ=Σq Γq does not deviate significantly from the SM prediction. Possible experimental causes for the under-estimation of Rc are discussed. By assuming the SM value for Rc, the discrepancy in Rb decreases to 2% (3σ). The double tagging technique used for the Rb measurements is critically reviewed. A few theoretical models that can explain large Rb and small αₛ (= 0.104± 0.008) are discussed. If the QCD coupling αₛ is allowed to be fitted by the data, the standard S, T, U analysis for a new physics