LIGHT PAIR CORRECTIONS TO THE Z-LINESHAPE PARAMETERS

We study the effect of the light fermion pair correction in the Z lineshape parameters. We observe corrections which are modest with respect to the present experimental accuracy but will become relevant in the near future: Mz changes by about 1.3 MeV, GAMMA(z) by more than 2 MeV and the peak cross section by around 0.25%. This estimate has been obtained for a realistic experimental strategy. A new third order formula for the total cross section with multiple photon and lepton pair contributions (featuring exponentiation) is presented. It provides the total cross section near the Z resonance with precision DELTA-delta/delta less-than-or-equal-to 3.7 x 10(-4) for /square-root s-M(z)/less-than-or-equal-to 7.5 GeV. This precision includes DELTA-delta/delta less-than-or-equal-to 3.5 x 10(-4) from pure bremsstrahlung and DELTA/delta less-than-or-equal-to 0.2 x 10(-4) from lepton pairs. We also discuss the light quark pair contribution. We estimate (conservatively) its uncertainty to be DELTA-delta/delta less-than-or-equal-to 1.8 x 10(-4).