Simultaneous determinations of τ polarisation and ντ helicity using the decay τ → a1ν

A method is presented to extract tau polarisation Pτ and tau neutrino helicity γVA simultaneously using τ leptons produced at the Z0 which decay into a1 and ντ. It provides better sensitivity than independent fits to Pτ or γVA separately and in fact can be shown to be optimal. Estimates of sensitivity from Monte Carlo studies and comments regarding the use of the technique in experiments are also presented. We also discuss how results of tau polarisation determinations using other decay modes may be included.