Inclusive Pion Single Charge Exchange in Helium -4 in the Delta-Resonance Region.

The doubly-differential cross sections, d ^{2}sigma/dOmega dE, for the inclusive ^4He( pi^{-},pi ^{0})X reaction have been measured with outgoing pi^0^ectra measured at angles of 30^circ, 50^circ, 80^ circ, 105^circ, and 130^circ for an incident beam energy of 160 MeV. The angular distribution, d sigma/dOmega, and total cross section, sigma, are presented as well. The data were taken during an experiment performed at the Clinton P. Anderson Meson Physics Facility at the Los Alamos National Laboratory (LAMPF) using a liquid He target and the LAMPF pi^0^ectrometer. This measurement is of pion single-charge-exchange (SCX), which, along with double-charge-exchange (DCX) and inelastic scattering (where the pion does not change charge), is one of three possible reactions leading to a pion in the final state. In ^4He, measurements of the other two reactions have been performed in the Delta-resonance region, where scattering is affected by the strong Delta(1232) resonance in the pi-N system. The data presented in this thesis allow for a direct comparison of the reaction mechanisms for each process. Inelastic scattering in ^4He is dominated by scattering from a single nucleon or quasifree scattering, while double-scattering dominates DCX (two like nucleons are required by charge conservation for this reaction to occur). Pion absorption is a competing reaction, as higher-order scattering processes leading to a pion in the final state are made improbable due to the strong possiblity the pion will be absorbed at each interaction. SCX, however, cannot proceed through the intermediate ^5S_2 Delta -N state, which is the primary two-body absorption channel, due to isospin, parity, and angular momentum arguments. The data show that SCX in ^4He is dominated by quasifree scattering. The angular distribution supports this as it resembles SCX from the free proton at backward scattering angles. At forward angles, the angular distribution is suppressed by Pauli blocking. A comparison between the doubly-differential cross sections for SCX and inelastic scattering indicates that (1) the magnitudes follow those predicted by simple isospin arguments, (2) the scattering processes behind both are very similar. Calculations based on the plane wave impulse approximation and distorted wave impulse approximation (PWIA and DWIA) have been made of the cross section for the SCX reaction. The more realistic DWIA calculation agrees reasonably well with the data. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253 -1690.).