Looking beyond the Standard Model through Precision Electroweak Physics

The most important hint of physics beyond the Standard Model (SM) from the1995 precision electroweak data is that the most precisely measured quantities,the total, leptonic and hadronic decay widths of the $Z$ and the effective weakmixing angle, $\sin^2\theta_W$, measured at LEP and SLC, and the quark-leptonuniversality of the weak charged currents measured at low energies, all agreewith the predictions of the SM at a few $\times 10^{-3}$ level. By taking intoaccount the above constraints I examine implications of three possibledisagreements between experiments and the SM predictions. It is difficult tointerpret the 11\% (2.5-$\sigma$) deficit of the $Z$-partial-width ratio$R_c=\Gamma_c/\Gamma_h$, since it either implies an unacceptably large$\alpha_s$ or a subtle cancellation among hadronic $Z$ decay widths in order tokeep all the other successful predictions of the SM. The 2\% (3-$\sigma$)excess of the ratio $R_b=\Gamma_b/\Gamma_h$ may indicate the presence of a newrather strong interaction, such as the top-quark Yukawa coupling in thesupersymmetric (SUSY) SM or a new interaction responsible for the largetop-quark mass in the Technicolor scenario of dynamical electroweak symmetrybreaking. Another interpretation may be additional tree-level gaugeinteractions that couple only to the third generation of fermions. A commonconsequence of these attempts is a rather small $\alpha_s$, $\alpha_s(\mz)_{\msbar}=0.104\pm 0.08$. The 0.17\% (1-$\sigma$) deficit ...Comment: Talk presented at Yukawa International Seminar (YKIS)~'95, 23 pages, uuencoded compressed tar file of LaTeX file and 13 EPS files (uses ptptex.sty,wrapfig.sty,psfig.sty,axodraw.sty) PostScript version of complete paper available at ftp://ftp.kek.jp/kek/preprints/TH/TH-463/kekth463.ps.g