A1−ρ−πSystem andπ→lνγDecay

Abstract

It is pointed out that an accurate experimental determination of the ratio $r$ of the structure-dependent axial-vector to the vector contribution in $\pi \to l\nu \gamma$ decay can settle the question of subtraction in the dispersion relation for the relevant amplitude which determines the axial-vector contribution in this decay, as well as in the electromagnetic form factor of the pion. Further, it is shown that the value of $r$ is sensitive to the value of the parameter $\gamma$ which enters into the $A_1-\rho -\pi$ system for theoretical determination of the $A_1\to \rho \pi$ and $\rho \to 2\pi$ widths.