I know understand! The vector current is even under a G-parity transformation. I recall that G-parity is a multiplicative quantum number used for hadrons classification. It is defined as a rotation about the 2-axis, followed by charge conjugation, i.e. C \mathrm{e}^{i\pi I_{2}} where I_{2}...
http://www.physics.gla.ac.uk/~cmcneile/papers/rhodecay.ps
http://arxiv.org/pdf/hep-lat/0301020v4.pdf
These are two examples of the articles I mentioned
I'm studying the \tau \rightarrow \rho \; \nu_{\tau} decay. I'm asked to calculate the decay width, using a parameterization of the matrix element of the hadronic current. I actually find a matrix element of the form:
\left<\rho |\; \bar{u} \gamma^{\mu} \left( 1-\gamma^{5} \right ) d \; |...