τ decay and the structure of the a1

The decay of the tau into three pions and a neutrino is analysed using different methods to account for the resonance structure, which is usually ascribed to the a1. One scenario is based on the recently developed techniques to generate axial-vector resonances dynamically, whereas in a second calculation the a1 is introduced as an explicit resonance. After explaining the model in detail, the influence of different assumptions on the results are investigated. In the molecule scenario the spectral function is described reasonably well by adjusting only one free parameter. This result can be systematically improved by adding higher order corrections to the iterated Weinberg-Tomozawa interaction. Treating the a1 as an explicit resonance on the other hand does not improve the results. (orig.)