Rho-meson production by pions at 15 GeV/c: A study of vector dominance and of rho-omega interference

We present the results of a wire spark chamber experiment studying the reaction n-p -+ 7r’+a-n at 15 GeV/c. The differential cross section,density matrix elements, and dipion mass distributions are given in the rho region for momentum transfers (t) less than 0.30 (GeV/$. The density-matrix elements and differential cross section exhibit structure in the forward direction (-t < m2,). Using the vector dominance model, a comparison is made with polarized and unpolarized single-pion photoproduction data which also exhibit structure in the forward direction. Qualitative agreement with the vector dominance model is found and, in particular, the &ransverse rho differential cross section displays a pronounced forward peak, analogous to that observed in single-pion photoproduction. Quantitative agreement is found for the unnatural-parity-exchange cross section and for the asymmetry at small momentum transfers. Elsewhere there are disagreements in detail which may not be explained by changes in normalization or the value of the rho-photon coupling constant. The data show a significant (> 4 standard deviations) rhoomega interference effect in the dipion mass distribution for 0. I< ItI< 0.3 (GeV/c)2. The overall phase between the amplitudes in the observed data is dominated by transverse production and is shown to be -1.40 * 0.45 (rad) which differs from the phase found at lower energies by -N -r/2. The lower limit on the branching ratio I’(w -+ 2s)/I’(w + 37r) is 0. ‘7% at the 95% confidence level.