A Study of the hadronic resonance structure in the decay tau ---> 3 pi neutrino(tau)

The hadronic structure of the decay of the $\tau$ lepton to three charged particles, $\tau\rightarrow 3\pi\nu_\tau$, is studied using data collected by the DELPHI detector at LEP between 1992 and 1995. The invariant mass of the $3\pi$ system, $m_{3\pi}$, is fitted using the models of KIhn and Santamaria, Isgur Morningstar and Reader, and Feindt. The $3\pi$ mass spectrum and the Dalitz projections are compared with each model. Below \mbox{$m_{3\pi}^2 = 2.3~ {\GeV}^2$}, all are in good qualitative agreement. Above \mbox{$m_{3\pi}^2 = 2.3~ {\GeV}^2$}, anomalous behaviour is observed, consistent with the existence of a hitherto unseen decay mode of the $\tau$ through a radial excitation of the $a_1$ meson.