The Nuclear Transparency of Pion - photoproduction from 4 He at 12 GeV June 20 , 2007

We propose to perform a coincidence γn → πp differential cross section measurements at the quasi-free kinematics from deuterium and He in order to extract nuclear transparency over a momentum transferred square (|t|) range of 3.0 9.0 GeV. A rise in the nuclear transparency as a function of |t| is predicted to be a signature of the onset of Color Transparency. Recent experiments have reported hints of Color Transparency like effects at relatively low momentum transfer squared in pion photoand electro-production from nuclei. Unambiguous observation of Color Transparency would uniquely point to the role of color in exclusive processes at high momentum transfers. In addition, the occurrence of such effects is a necessary (but not sufficient) condition for the approach to the factorization regime in meson electroproduction experiments, necessary for accessing the Generalized Parton Distributions. In conjunction, by straddling the Charm threshold, this experiment will exploring the rather mysterious energy dependence that the BNL A(p,2p) nuclear transparency experiments have indicated. The proposed experiment will be carried out in the upgraded Hall-C, using a 2050 μA electron beam impinging on a 6% copper radiator, a liquid deuterium target and liquid helium target. The proposed experiment seeks to measure the nuclear transparency of the the γn → πp process from He at the center-of-mass angle of 90, up to the highest |t| values that can easily be reached at a 12-GeV JLab, using the HMS and SHMS spectrometers. We request a total of 350 hours of beam time with a maximum beam current of 50 μA. This measurement requires five different photon energies between 3.3 and 11.0 GeV.