OPTIMIZATION OF R(E+E-) AND FREEZING OF THE QCD COUPLANT AT LOW ENERGIES

The new result for the third-order QCD corrections to R(e+e-), unlike the old, incorrect result, is nicely compatible with the principle-of-minimal-sensitivity optimization method. Moreover, it leads to infrared fixed-point behavior: the optimized couplant alpha(s)/pi for R(e+e-) does not diverge at low energies, but ''freezes'' to a value 0.26 below about 300 MeV. This provides some direct theoretical evidence, purely from perturbation theory, for the ''freezing'' of the couplant-an idea that has long been a popular and successful phenomenological hypothesis. We use the ''smearing'' method of Poggio, Quinn, and Weinberg to compare the resulting theoretical prediction for R(e+e-) with experimental data down to the lowest energies, and find excellent agreement.