Thea1inτdecay

Abstract

The decay τ→${\nu }_{\tau }$πππ provides a potentially powerful means of observing the axial-vector isovector state expected in the region of 1.2 GeV. Extraction of the properties of this resonance is, however, complicated by its broad width. We examine the problems of studying such a resonance, especially the model dependence of its deduced mass and width. Within a clearly defined and well-tested model we find $m_{a_1=1220\mathrm{}\pm 15}$ MeV and ${\Gamma }_{a_1=420\mathrm{}\pm 40}$ MeV.