In an experiment conducted at the Caltech 1.5 GeV electronsynchrotron, we measured the cross section for photoproduction ofeta mesons from deuterium. The measurement was performed by detectingand measuring the energies of both photons from the η → 2γ decaymode using two totally absorbing lead glass Cherenkov counters. Thesecounters were placed symmetrically about the photon beam line in orderto best detect eta mesons photoproduced along the beam line. Byvarying the placement of the Cherenkov counters and by varying the synchrotron end-point energy, we were able to obtain information onthe forward and backward cross section for photoproduction over anenergy range from about 725 MeV to about 1225 MeV.
Within the framework of the impulse approximation, we expressedour results as a sum of the differential cross section for etaphotoproduction from protons plus that from neutrons. The unfolding of the cross section was performed by finding the function that was thesmoothest (using a well-defined measure of smoothness) function thatfit the data to within a given degree of accuracy (as measured by X2).
The results show that for photon energy below 950 MeV, thecross section for photoproduction from neutrons is about equal in theforward and backward directions, and is about the same as the crosssection for photoproduction from protons. Above 950 MeV, a significantangular asymmetry is seen, and the neutron cross section shows signsof reaching a minimum, then rising. These results are interpreted in terms of contributing isobars.
