Hard Two-Body Photodisintegration of ${}^3\mathrm{He}$

We have measured cross sections for the $\gamma {}^3\mathrm{He}\to pd$ reaction at photon energies of 0.4–1.4 GeV and a center-of-mass angle of 90°. We observe dimensional scaling above 0.7 GeV at this center-of-mass angle. This is the first observation of dimensional scaling in the photodisintegration of a nucleus heavier than the deuteron.

Figure 1

The $\beta$ distribution of particles detected in coincidence by the two high-resolution spectrometers in Hall A E03-101. The widths of the peaks result from the calibration and time resolution of the scintillators, from the momentum acceptance of the spectrometers ($\Delta p\approx \pm 3.5\%$), and the uncertainty of the path-length correction. The different scintillators in the two spectrometers lead to different widths of the distributions.

Figure 2

Event distributions measured by CLAS, of the missing mass of the proton $M{M}_p^2$ for ${\theta }_{p\mathrm{c}.\mathrm{m}.}=90°$, with (dashed red line) and without (blue solid line) the kinematic cuts, are shown. Events from the $pd$ final state are clearly identified in the peak. The background distribution (events rejected by the kinematic cuts) exhibits smooth behavior under the deuteron peak and reproduces the background shape outside of the peak.

Figure 3

The invariant cross section $d\sigma /dt$ multiplied by $s^{17}$ to remove the expected energy dependence. The DAPHNE data are taken from [28]. The RNA [18] calculation is normalized to our highest energy data point from JLab E03-101. The prediction of Laget is based on a diagrammatic hadronic model [41]. Constant $s^{17}$ scaling normalized to our highest energy data point is indicated by a solid red line. For a comment on the comparison between the CLAS and DAPHNE data, see [42].