Large-Angle ${\pi }^{-}+p$ Elastic Scattering at 3.63 $\frac{\mathrm{GeV}}{c}$

The differential cross section for elastic scattering of $3.63-\frac{\mathrm{GeV}}{c} {\pi }^{-}$ mesons on protons was studied with a hydrogen bubble chamber, the emphasis being on large-angle scattering. From 90 to 180° in the barycentric system, the cross section is roughly flat with an average value of 2.7±1.0 μb/sr. Near and at 180°, there may be a slight peak of magnitude 10±6 μb/sr. But if such a peak exists, it is only one-third to one-fourth the size of the 180° peak found in 4.0 $\frac{\mathrm{GeV}}{c} {\pi }^{+}+p$ elastic scattering. In addition to comparison with other ${\pi }^{-}+p$ and ${\pi }^{+}+p$ large-angle elastic-scattering measurements, this measurement is compared with large-angle $p+p$ elastic scattering. In the forward hemisphere a small peak or a plateau exists at $cos {\theta }^{*}=+0.60\mathrm{}$. This appears to be a second diffraction maximum such as has been found in lower-energy $\pi +p$ elastic scattering. A survey of indications of such a second diffraction maximum in other $\pi +p$ measurements shows that it always occurs in the vicinity of $-t=1.2\mathrm{} {\left(\frac{\mathrm{GeV}}{c}\right)}^2$, where $t$ is the square of the four-momentum transfer. As the incident momentum increases, the relative size of this second maximum decreases.