Forward elastic scattering of 40-GeV/c. pi. /sup -/ mesons by protons

Apparatus consisting of a recoil-nucleus detector and a proportional-chamber spectrometer has been used to measure the ratio of the real and imaginary parts of the amplitude for forward scattering of 40-GeV/c ..pi../sup -/ mesons by protons. The recoil detector consisted of an ionization chamber filled with hydrogen which served simultaneously as a gas target and a recoil-nucleus detector. The detector measured the energy of the recoil nucleus, the polar angle of recoil, and the position of the point of interaction inside the chamber working volume. The energy resolution in the region of Coulomb-nuclear interference was 60 keV (FWHM). The proportional-chamber spectrometer measured the scattering angle and the momentum of the scattered particles. The experimental value obtained for the ratio of the real and imaginary parts of the forward scattering amplitude is rho=-0.037 +- 0.017 and is in good agreement with calculations with the dispersion relations.