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Analyzing powers and differential cross sections for pn --> [pi]⁻pp(¹ ₀) at 403 and 440 MeV Duncan, Fraser Andrew


There is considerable interest in the pn → π pp reaction which can proceed by a nonresonant channel from the isospin 0 pn initial state (an N Δ intermediate state cannot be formed). This thesis describes a measurement of analyzing powers and triple differential cross sections for a subset of this reaction, pn → π⁻ pp(¹S₀) by isolating the quasifree process in pd → π⁻ ppps. The experimental arrangement selects the relative S-wave component of the outgoing “diproton”. The experiment was done on TRIUMF beam line 1B using a LD₂ target; the pion was detected in a magnetic spectrometer, the two outgoing protons in a scintillator bar array. The spectator proton was undetected. Data were taken in August 1989 at 353, 403 and 440 MeV beam energies. Of these the 403 and 440 MeV data are analysed in this thesis and analyzing powers and triple differential cross sections as a function of pion scattering angle extracted at centre of mass kinetic energies, Tcm, of 55 and 70 MeV (corresponding to the 403 and 440 MeV beam energies respectively). Partial wave analysis of the data shows that, while the isospin 0 channel dominates the reaction, contributing approximately 75% of the cross section at the energies studied here, there are significant contributions from the s and d-wave pion, isospin 1 channels. Of particular importance is the contribution from the s-wave pion, isospin 1, channel whose interference with the isospin 0 channels produces the characteristic shapes of the cross sections and analyzing powers observed in the data. The d-wave pion, isospin 1 channels, are also required to fully explain the observed analyzing power distributions, and are essential for the Tcm = 70MeV data. Comparisons of the pion production data measured in this experiment with pion absorption measurements on ³He, where the absorption process is π⁻ pp(¹S₀) → pn, show a shift in the shape of the differential cross section which can be interpreted as due to differences in the wave functions for the free and bound diproton.

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