Probing the direct step of relativistic heavy ion fragmentation: (/sup 12/C,/sup 11/B + p) at 2.1 GeV/nucleon with C and CH2 targets

We have measured the momentum distributions and the excitation energy of the p,/sup 11/B pair in the (/sup 12/C,/sup 11/B + p) reaction at 2.1 GeV/nucleon with C and CH2 targets. The cross section separates into three regions: (1) nucleon-nucleon quasi-elastic scattering, (2) nucleon-nucleon inelastic scattering, and (3) a low excitation energy and momentum transfer peak. The cross sections, by region, are as follows: (1) 8.8 +- 2.5 mb, (2) 10.1 +- 2.2 mb, and (3) 0.81 +- 0.45 mb for H(/sup 12/C,/sup 11/B + p)X and (1) 11.1 +- 2.4 mb, (2) 24.1 +- 3.7 mb, and (3) 4.50 +- 0.67 mb for C(/sup 12/C,/sup 11/B + p)X. The shapes of the first two regions can be fit with a nucleon-nucleon cascade model including π production. However, the cascade model prediction for the quasi-elastic component is too large by a factor of 3. The low momentum transfer peak is consistent with two mechanisms, (1) an excitation and decay via proton emission of the carbon projectile and (2) a projectile proton scattering diffractively off the C target. Finally, the Fermi momentum determined from the transverse momentum distribution is 160 +- 11 MeV/c compared to 190 +- 11 MeV/c from the longitudinal momentum distribution