Tau polarimetry with inclusive decays.

Abstract

The spin asymmetry parameter ${\mathit{A}}_{\mathrm{\ensuremath{\tau}}}$ characterizing the angular distribution of the total hadron momentum in the decay of a polarized tau can be calculated rigorously using perturbative QCD and the operator product expansion. Perturbative QCD corrections to the free quark result ${\mathit{A}}_{\mathrm{\ensuremath{\tau}}}$=1/3 can be expressed as a power series in ${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$(${\mathit{M}}_{\mathrm{\ensuremath{\tau}}}$) and nonperturbative QCD corrections can be expanded systematically in powers of 1/${\mathit{M}}_{\mathrm{\ensuremath{\tau}}}^{2}$. The QCD prediction is ${\mathit{A}}_{\mathrm{\ensuremath{\tau}}}$=0.41\ifmmode\pm\else\textpm\fi{}0.02. A measurement in agreement with this prediction would provide strong support for the precise determination of ${\mathrm{\ensuremath{\alpha}}}_{3}$ from tau decay data.