Discrepancy Between the Vector-Dominance Model and Pion Production by Polarized Photons

The helicity density matrix for the process ${\pi }^{-}p\to {\pi }^{+}{\pi }^{-}n$ has been studied as a function of $M_{\pi \pi }$. The ratio $\frac{{{\rho }_{1-1}}^{\mathrm{hel}}}{{{\rho }_{11}}^{\mathrm{hel}}}$ shows no variation with $M_{\pi \pi }$ and it thus seems unlikely that the large discrepancy between the vector-dominance prediction and the observed asymmetry of single pions produced by linearly polarized photons could be the result of density-matrix distortion caused by background interference.