Spin Polarization in $\gamma d\to \vec{n}p$ at Low Energies with a Pionless Effective Field Theory

Abstract

With the use of pionless effective field theory including dibaryon fields, we study the $\gamma d\to \vec{n}p$ reaction for the laboratory photon energy $E_{\gamma }^{lab}$ ranging from threshold to 30 MeV. Our main goal is to calculate the neutron polarization $P_{y^{\prime }}$ defined as $P_{y^{\prime }}=({\sigma }_{+}-{\sigma }_{-})/({\sigma }_{+}+{\sigma }_{-})$, where ${\sigma }_{+}$ and ${\sigma }_{-}$ are the differential cross sections for the spin-up and spin-down neutrons, respectively, along the axis perpendicular to the reaction plane. We also calculate the total cross section as well as the differential cross section $\sigma (\theta )$, where $\theta$ is the colatitude angle. Although the results for the total and differential cross sections are found to agree reasonably well with the data, the results for $P_{y^{\prime }}$ show significant discrepancy with the experiment. We comment on this discrepancy.

Comments: 15 pages, 7 figures, discussion about di-baryon formalism detailed, minor corrections, accepted for publication in Phys. Rev. C