Scattering of 6-24 MeV Negative Pions by Hydrogen

The scattering of negative pions at laboratory energies between 6 and 24 MeV has been observed in a liquid hydrogen bubble chamber. The energy of each scattering was deduced from the ranges and angles of the scattered pion and recoil proton. Flux was measured by direct count of tracks of stopped pions. A maximum likelihood analysis was performed treating the strength of the small $P$-wave contribution as known. Based on 246 scattering events at a median energy of 13 MeV, the $S$-wave scattering length $a=\frac{(2\mathrm{}{\alpha }_1+{\alpha }_3)}{3\eta }$ is found to be 0.090±0.005 when the large Coulomb contribution is assumed to be the nonrelativistic amplitude for pure Coulomb scattering. Results are also presented with further Coulomb corrections. Values of ${\alpha }_1$ are computed from the combination of this experimental result with experiments on ${\pi }^{+}-p$ scattering.