Precise measurement of the e+ e- ---> mu+ mu- reaction at s**(1/2) = 57.77-GeV

The reaction e (cid:49) e (cid:50) ! (cid:109) (cid:49) (cid:109) (cid:50) has been measured at (cid:65) s (cid:53) 57.77 GeV, based on 289.6 (cid:54) 2.6 pb (cid:50) 1 data collected with the VENUS detector at TRISTAN. The production cross section is measured in bins of the production angle within an angular acceptance of (cid:117) cos (cid:117) (cid:117) (cid:60) 0.75, according to a model-independent definition. The result is consistent with the prediction of the standard electroweak theory. Although a trend in measurements at lower energies that the total cross section tends to be smaller than the prediction remains, the discrepancy is not significant. The model-independent result is converted to the differential cross section in the effective-Born scheme by unfolding photon-radiation effects. This result can be extrapolated to quantities for the full solid angle as (cid:115) totEB (cid:53) 30.05 (cid:54) 0.59 pb and A FBEB (cid:53)(cid:50) 0.350 (cid:54) 0.017, by imposing an ordinary assumption on the production-angle dependence. The converted results are used to set constraints on extensions of the standard theory. S-matrix parametrization, and possible contributions from contact interactions and heavy neutral-scalar exchanges are examined. (cid:64) S0556-2821 (cid:126) 98 (cid:33) 04309-4 (cid:35)