Two-Body Photodisintegration of ${^3\rm{He}}$ between 0.4 and 1.5 GeV

The $\gamma {^3\rm{He}}\rightarrow pd$ reaction was measured with the CLAS detector at Jefferson Lab for photon energies between 0.4 and 1.5 GeV and proton CM angles $\theta^p_{CM}$ between 40$^\circ$ and 140$^\circ$. It is complementary to the three-body breakup of $^3$He with respect to studying three-body mechanisms. At all photon energies for our experiment, the differential cross sections exhibit a very strong forward-to-backward asymmetry --- approximately one order of magnitude. An interesting feature of the differential cross sections is that their slope does not depend on the photon energy and there is a change of slope at $\theta^p_{CM}=120^\circ$ seen at all photon energies. The invariant cross sections fall off with $s$ (where $s$ is the total CM energy) much faster than expected by the quark counting rules $[1]$. The latter predict that in the asymptotic regime $t\rightarrow\infty$ the invariant cross sections should scale as $s^{-17}$, whereas our data scale as $s^{-22}$. A comparison of our preliminary results with the cross sections predicted by Jean-Marc Laget's model $[2]$ shows that the differential cross sections for angles greater than 60$^\circ$ are sensitive to contributions from three-body mechanisms. The relative importance of the latter, with respect to one- and two-body mechanisms, is larger at 0.6 - 0.8 GeV than at higher energies. This has already been observed in our data for $\gamma {^3\rm{He}}\rightarrow ppn$ $[3]$ and seems to be a characteristic of the three-body mechanisms at medium energies. \vspace{6mm} \newline 1. S.J.~Brodsky and G.R.~Farrar, Phys. Rev. Lett. \textbf{31}, 1153 (1973) \\ 2. J-M.~Laget, Phys. Rev. C \textbf{38}, 2993 (1988)\\ 3. S.~Niccolai \textit{et al.}, Phys. Rev. C \textbf{70}, 064003 (2004)