Isospin Violation in τ → 3πντ

Isospin violating signals in the τ − → (3π) − ν τ decay mode are discussed. For the τ − → π − π − π + ν τ decay mode, isospin violation arises from the vector current contribution in the τ − → ωπ − ν τ decay with the subsequent isospin violating ω decay into π + π −. We demonstrate that such effects may be observed in presently available data through the measurement of the interference effects of these vector current contributions with the dominating axial vector current, i.e. through a measurement of the structure functions W F , W G , W H and W I. In the case of the τ − → π 0 π 0 π − ν τ decay mode, a vector current contribution is generated by ηπ 0 mixing in the decay chain τ − → ηρ − ν τ → π 0 π 0 π − ν τ. We find that this effect is rather small, the magnitude of the associated interference terms being too low for present statistics.