Structure in Backward Pion-Proton Elastic Scattering from 6 to 17 GeV/c

The angular distribution for elastic ${\pi }^{+}-p$ scattering has been measured over the angular range $135°<{\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}<180\mathrm{}°$ for pion laboratory momenta of 5.9, 9.9, and 13.7 GeV/c and also for ${\pi }^{-}p$ at 5.9 and 9.9 GeV/c. More limited angular distributions are presented for ${\pi }^{-}-p$ at 13.7 and 16.3 GeV/c and ${\pi }^{+}-p$ at 17.1 GeV/c. In both the ${\pi }^{-}-p$ and ${\pi }^{+}-p$ scattering the cross section rises by over 2 orders of magnitude in the backward direction; for ${\pi }^{+}-p$ only, there is a sharp dip at $u=-0.15\mathrm{}$ ${(\mathrm{G}\mathrm{e}\mathrm{V}/\mathit{c})}^2$.