Measurement of the ${}^{1}H(\mathrm{\gamma ⃗},\mathrm{p⃗}){\pi }^0$ Reaction Using a Novel Nucleon Spin Polarimeter

We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon. The measurement pioneers a novel polarimetry technique, which can be applied to many other nuclear and hadron physics experiments. The commissioning reaction of ${}^{1}H(\mathrm{\gamma ⃗},\mathrm{p⃗}){\pi }^0$ in the range $0.4<E_{\gamma }<1.4\mathrm{GeV}$ validates the technique and provides essential new data to constrain the excitation spectrum of the nucleon.

Figure 1

Illustration of the experimental setup. The liquid hydrogen target (yellow cylinder), situated at the center of the CB, is surrounded by the PID (blue) and the 2.25 cm thick graphite polarimeter. The 7.25 cm thick downstream cap covers the aperture to TAPS (not pictured). The PID was flush with the cap during production running but has been shifted here for visual clarity.

Figure 2

Top: the effective analyzing power, obtained by averaging the results from each scattering model, integrated over ${\theta }_{\mathrm{CM}}$, the pion angle defined in the center-of-mass frame of the photon and ${}^{1}H$ target. The shaded band indicates the systematic error, obtained from the difference between the two models. Bottom: comparison of ${\theta }_s$ distributions from experiment (circles) and simulation with (triangles) and without (squares) hadronic interactions. Nuclear scattered events lie in the shaded region.

Figure 3

$C_x^{*}$ excitation functions for $\mathrm{\gamma ⃗}p\to {\pi }^0\mathrm{p⃗}$ (black circles) for fixed pion polar angles ${\theta }_{\mathrm{cm}}$. Previous data came from JLab [2] (magenta triangles). The PWA solutions shown are SAID CM12 [36] (cyan long-dash line), SAID SN11 [37] (green short-dash line), BnGa2011-2 [38] (solid red line), and MAID07 [39] (violet dash-dotted line).