Study of theρ′(1600)Mass Region Usingγp→π+π−pat 20 GeV

We have observed the ${\pi }^{+}{\pi }^{-}$ decay of the ${\rho }^{\prime }\mathrm{}\left(1600\right)\mathrm{}$ in the production reaction $\gamma p\to {\rho }^{\prime }p$ at 20 GeV. Using a calculation which takes into account the interference of the ${\rho }^{\prime }$ with the $\rho \mathrm{}\left(770\right)\mathrm{}$ and a Drell background, we find good evidence that this resonance is a radial excitation of the $\rho \mathrm{}\left(770\right)\mathrm{}$. The background interference strongly distorts the angular distributions predicted by a purely $s$-channel helicity-conserving production mechanism. We measure $m_0=(1.55\mathrm{}\pm 0.07)$ GeV/${\mathit{c}}^2$ and ${\Gamma }_0=\left({0.28}_{-0.08\mathrm{}}^{+0.03\mathrm{}}\right)$ GeV/${\mathit{c}}^2$.