Peripheral Dipion Production by Pions of 12 and 18 GeV/c

A spark-chamber experiment on the peripheral production of 9245 pion pairs by 12- and 18-GeV/c incident pions is reported and analyzed in terms of a one-pion-exchange model in which the final state at the nucleon vertex contains generally one or more pions. The relevant dynamics and kinematics appropriate to this problem are reviewed, and the experimental and analysis techniques giving good resolution and detection-bias correction are discussed in some detail. From the results, fair agreement is found between the data and the one-pion-exchange calculation of the ${\rho }^0$ production cross sections and of the associated missing-mass spectra. The ${\rho }^0$ is found to be consistent with a single peak, and no evidence of peak splitting is observed. A search for a narrow $s$-wave dipion resonance is made with negative results. Normalizing to the ${\rho }^0$ meson, the $s$-wave ${\pi }^{+}{\pi }^{-}$ scattering cross section is computed from the abundant low-dipion-mass events, giving a cross section falling smoothly from 50 mb (300 MeV) to about 20 mb (600 MeV). No evidence of an $s$-wave resonance is found in this range of energies. Below 450 MeV, the pion-pion scattering asymmetry favors backward scattering (by 2½ standard deviations), which is consistent with a negative and falling $J=T=0\mathrm{}$ phase shift. The extrapolated forward-backward asymmetry and the $s$-wave cross section are both consistent with a $J=T=0\mathrm{}$ phase shift near|90°| at about 750 MeV.