Measurement of the Analyzing Power in $\overrightarrow{p}d\to (pp)n$ with a Fast Forward ${}^{1}S_0$ Proton Pair

A measurement of the analyzing power $A_y$ of the $\overrightarrow{p}d\to (pp)+n$ reaction was carried out at the ANKE spectrometer at COSY at beam energies of 0.5 and 0.8 GeV by detection of a fast forward proton pair of small excitation energy $E_{pp}<3\mathrm{M}\mathrm{e}\mathrm{V}$. The $S$-wave dominance in the fast diproton is experimentally demonstrated in this reaction. While at $T_p=0.8\mathrm{G}\mathrm{e}\mathrm{V}$ the measured analyzing power almost vanishes, it rises to nearly unity at $T_p=0.5\mathrm{G}\mathrm{e}\mathrm{V}$ for neutrons emitted at ${\theta }_n^{\mathrm{c}.\mathrm{m}.}=167°$. The results are compared with a model taking into account one-nucleon exchange, single scattering, and $\Delta (1232)$ excitation in the intermediate state. The model describes fairly well the unpolarized cross section obtained earlier and the analyzing power at 0.8 GeV; it fails to reproduce $A_y$ at 0.5 GeV.

Figure 1

Top view of the ANKE spectrometer with the forward detector (FD) and the silicon-detector telescope (SDT, see inset). Diprotons from the breakup reaction stem from the region indicated in the distribution of the polar angle projection ${\theta }_{xz}$ vs particle momentum (lower right). Protons from $pd$-elastic scattering are distributed along the kinematical locus.

Figure 2

Proton pairs are identified from the correlation of the TOF differences $\Delta t_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{s}}$ and $\Delta t(\overrightarrow{p_1},\overrightarrow{p_2})$.

Figure 3

Acceptance corrected distribution of events as function of $\cos {\theta }_k^{\mathrm{c}.\mathrm{m}.}$ for 0.5 GeV (left panel) and 0.8 GeV (right).

Figure 4

Angular dependence of the analyzing power $A_y$ as function of the neutron polar angle ${\theta }_n^{\mathrm{c}.\mathrm{m}.}$ for $T_p=0.5$ ($•$) and 0.8 GeV ($\circ$). The lines show predictions for $A_y$ at 0.5 GeV (solid) and 0.8 GeV (dashed) from the $\mathrm{O}\mathrm{N}\mathrm{E}+\mathrm{S}\mathrm{S}+\Delta$ model [9, 14], with the CD-Bonn potential.