Measurement of the left-right asymmetry on Z Boson production by electron-positron collisions

We present the first measurement of the left-right cross-section asymmetry (ALn) in Z boson production observed at the SLAC Linear Collider. The left-right asymmetry provides a direct measurement of the e- -Z coupling and thus of the effective weak mixing angle; in addition, ALR is sensitive to the unknown top-quark and Higgs-boson masses. In 1992 the SLD detector recorded 10224 Z events produced by the collision of longitudinally polarized electrons with an unpolarized positron beam at a center-of-mass energy of 91.55 GeV. The average electron beam polarization during the run was (22.4 ± 0.6)%. We measure ALR to be 0.101 ± 0.044 (stat.) ± 0.004 (syst.), which determines the effective weak mixing angle to be sin 2 oet = 0.2377 ± 0.0056 (stat.) ± 0.0005 (syst.). This measurement of ALn is in agreement with comparable measurements and is consistent with Standard-Model predictions for allowed top and Higgs masses.