Abstract
We introduce optimal observables to measure the Pauli form factor F_2 of the tau -lepton in the pair-production process mathrm{e}^- mathrm{e}^+ rightarrow tau ^-tau ^+ from the intensity distribution of the decay products. The spin-density matrix for the production process is calculated in QED up to order alpha ^3 including virtual photon-loops and soft bremsstrahlung, as well the gamma Z^0 interference. We find that the decay channel (rho ^-nu _tau )times (rho ^+ {bar{nu }}_tau ) yields the best resolution for ReF_2(s) and ImF_2(s) due to its high branching fraction. We also study the bias that is introduced in the determination of F_2, if the production spin-density matrix is taken in tree-level (one-photon exchange) approximation.
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