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Published: 1988

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One of the central issues in nuclear physics is the identification of clear signatures for quarks in nuclei. A guiding principle in this search is to perform experiments with high energy electromagnetic probes of the simplest nucleus, the deuteron, a system which is particularly amenable to theoretical interpretation. It has long been known that the quark counting rules seem to apply for electron elastic scattering from the pion and nucleon. However, the rapid decline in the cross section for electron scattering from the deuteron, a six-quark system, renders an experiment at high momentum transfer unfeasible. Clearly, the dimensional-scaling region is not reached for electron-deuteron elastic scattering as indicated. This led Brodsky and Chertok to analyze these data in terms of the reduced nuclear amplitudes and produce, in effect, scaling at a lower momentum transfer. Presently, this is the only known case where the reduced nuclear amplitude analysis seems to apply. In the present work we abandon elastic electron scattering in favor of an exclusive photoreaction with the deuteron. The simplest process involving a nucleus is the .gamma.d .-->. pn reaction. Here, n = 13 and an s−11 dependence is expected where the quark counting rules are valid. Naively, one would expect that the energy region where the quark counting rules are valid to be most naturally described in terms of a quark or parton basis rather than a nucleon basis, and thereby provide a signature quark effects in nuclei. Hitherto no s−11 dependence has been observed for the data below 1 GeV. As a test of the energy-dependence for the .gamma.d .-->. pn reaction, we have performed the first measurements for this process above 1 GeV.