Proton production in neutrino-neon collisions in the 15 ft. bubble chamber at the tevatron

Proton production was studied in v-20 Ne interactions in the 15 ft. Bubble Chamber at Fermilab, and the results were used to test models of the intranuclear cascade. The Bubble Chamber was filled with a heavy Ne-H2 mixture, and was exposed to the wide-band Quadrupole Triplet neutrino beam produced by 800 GeV protons from the Tevatron. It is shown that open-quotes grayclose quotes protons, which are in the momentum range 0.3-1.2 GeV/c, are the relevant ones for investigating nuclear effects. Neutrino events are measured to have a net charge excess of 0.39 ± 0.02 over what would be expected if there were no nuclear effects. It is found that the fully corrected excess of gray protons is 0.34 ± 0.02, which accounts for most or all of the charge excess. The mean net charge of events increases linearly with the number of gray protons, with a slope of 0.73 ± 0.03 as expected if the charge excess consists of gray protons. The fractional distribution of gray protons agrees with the model of Andersson, Otterlund and Stenlund, and is not consistent with a Poisson distribution. It is shown for the first time that gray proton production decreases with the total hadronic effective mass, corroborating the model that it is mostly low-energy particles that undergo rescattering. A significant 19% of the gray protons are in the backward hemisphere with respect to the neutrino direction, which is kinematically forbidden in the absence of nuclear effects. Backward proton multiplicity is observed to increase linearly with forward multiplicity. It is shown for the first time that neutrino interactions are consistent with models that explain backward protons as originating from pion absorption, specifically the mechanisms: π +open-quotes bclose quotes → N + N and π + open-quotes bclose quotes → Δ + N with Δ → π + N, where open-quotes bclose quotes is a two-nucleon cluster within the nucleus