Identification of thed+p→He3+ηReaction Very Near Threshold: Cross Section and Deuteron Tensor Analyzing Power

From the interactions of a deuteron beam in a liquid-hydrogen target at energies near the $d+p\to {}_{}{}^3{}_{}{}^{}\mathrm{He}+\eta$ threshold, $\eta$ events were selected on the basis of the momentum-analyzed ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ with a background of less than 2%. This opens the way to a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-tagged $\eta$ beam at Saturne, with a possible flux of ${10}^4$-${10}^5$ $\eta \text{'}\mathrm{s}$ per pulse. The present data allow an evaluation of the cross section down to less than 0.5 MeV above threshold. The deuteron tensor analyzing power $t_{20}$ has also been measured: The small negative value found at threshold (-0.15 ± 0.05) seems incompatible with models based on one-pion exchange.