Photoproduction of NeutralρMesons

An experimental study of the photoproduction of charged pion pairs is reported. The pions were detected with two mirror-image magnetic spectrometers, each of which consisted of a half-quadrupole magnet and a number of scintillation counters. The dipion mass spectrum was searched over by fixing the position of the spectrometer arms, fixing the maximum photon energy, and varying the current in each of the half-quadrupole magnets. Measurements were made for symmetrical pairs with opening angles of 18.2° and 32.1° for photon energies of 5.5 and 3.0 BeV and for hydrogen and carbon targets; some measurements were also made with aluminum and copper targets. Measurements were made of asymmetric pairs and symmetrical pairs whose resultant momentum made an angle of 6° with respect to the photon beam. In the course of the experiment, the invariant mass of the dipion system ranged from 0.300 to 1.50 BeV. The only significant structure seen was at the mass of the ${\rho }^0$ meson. The measurements showed that the $\rho$ photoproduction from hydrogen is due to a diffractionlike mechanism. The data from complex nuclei showed that the production at 0° was coherent and that the $\rho \text{'}\mathrm{s}$ were completely polarized. No evidence was found for a forward-backward asymmetry in the decay of the photoproduced $\rho \text{'}\mathrm{s}$.