Cross section and complete set of proton spin observables in $\overrightarrow{p}d$ elastic scattering at 250 MeV

The angular distributions of the cross section, the proton analyzing power, and all proton polarization transfer coefficients of $\overrightarrow{p}d$ elastic scattering were measured at 250 MeV. The range of center-of-mass angles was $10°–165°$ for the cross section and the analyzing power, and about $10°–95°$ for the polarization transfer coefficients. These are the first measurements of a complete set of proton polarization observables for $\overrightarrow{p}d$ elastic scattering at intermediate energies. The present data are compared with theoretical predictions based on exact solutions of the three-nucleon Faddeev equations and modern realistic nucleon-nucleon potentials combined with three-nucleon forces (3NF), namely, the Tucson-Melbourne (TM) $2\pi$-exchange model, a modification thereof ${(\mathrm{T}\mathrm{M}}^{\prime }$) closer to chiral symmetry, and the Urbana IX model. Large effects of the three-nucleon forces are predicted. The inclusion of the three-nucleon forces gives a good description of the cross section at angles below the minimum. However, appreciable discrepancies between the data and predictions remain at backward angles. For the spin observables the predictions of the TM 3NF model deviate strongly from the other two 3NF models, which are close together, except for $K_y^{y^{\prime }}.$ In the case of the analyzing power all 3NF models fail to describe the data at the upper half of the angular range. In the restricted measured angular range the polarization transfer coefficients are fairly well described by the ${\mathrm{TM}}^{\prime }$ and Urbana IX 3NF models, whereas the TM 3NF model mostly fails. The transfer coefficient $K_y^{y^{\prime }}$ is best described by the Urbana IX but the theoretical description is still insufficient to reproduce the experimental data. These results call for a better understanding of the spin structure of the three-nucleon force and very likely for a full relativistic treatment of the three-nucleon continuum.