Measurement of the pd→pdη cross section in complete kinematics

Abstract

The $pd\ensuremath{\rightarrow}pd\ensuremath{\eta}$ reaction has been studied in a kinematically complete experiment at five beam energies 927, 961, 996, 1032, and $1096\phantom{\rule{0.3em}{0ex}}\text{MeV}$. In contrast to our measurement of the $pd\ensuremath{\rightarrow}^{3}\mathrm{He}\ensuremath{\eta}$ at the same energies, all the c.m. angular distributions are consistent with isotropy. The energy dependence of the total cross section seems to follow a three-body phase space as modified by a proton-deuteron final-state interaction, and such an extrapolation is consistent with other near-threshold data. The distributions of the $pd$ and $p\ensuremath{\eta}$ invariant mass at fixed beam energy are both close to those of phase space. However, this is not the case for the $d\ensuremath{\eta}$ invariant mass, which shows significant structure in the first few MeV above threshold. This behavior is similar to that observed in the energy variation of the $pn\ensuremath{\rightarrow}d\ensuremath{\eta}$ total cross section and is the sign of a large $\ensuremath{\eta}$-deuteron scattering length that has been predicted in many theoretical models.