Polarization Components in ${\pi }^0$ Photoproduction at Photon Energies up to 5.6 GeV

We present new data for the polarization observables of the final state proton in the ${}^{1}H(\overrightarrow{\gamma },\overrightarrow{p}){\pi }^0$ reaction. These data can be used to test predictions based on hadron helicity conservation and perturbative QCD. These data have both small statistical and systematic uncertainties and were obtained with beam energies between 1.8 and 5.6 GeV and for ${\pi }^0$ scattering angles larger than 75° in the center-of-mass frame. The data extend the polarization measurements database for neutral pion photoproduction up to $E_{\gamma }=5.6\mathrm{GeV}$. The results show a nonzero induced polarization above the resonance region. The polarization transfer components vary rapidly with the photon energy and ${\pi }^0$ scattering angle in the center-of-mass frame. This indicates that hadron helicity conservation does not hold and that the perturbative QCD limit is still not reached in the energy regime of this experiment.

Figure 1

The ${\pi }^0$ event selection at an incident photon energy $E_{\gamma }=3.951\mathrm{GeV}$. The distributions of the predicted energy deposition in the BigCal are plotted for the data (red solid line), the random background (green solid line) determined from time of flight spectra, the Monte Carlo simulation of ${\pi }^0$ events (light blue dotted line), and the MC simulation of $ep$ elastic events (blue dashed line). The black solid line is the sum of the MC simulation of ${\pi }^0$’s, $ep$ elastic events, and the measured random background. The simulated curves have been scaled to match the data. The two vertical lines are described in the text.

Figure 2

Top to bottom: Polarization transfer $C_x^{\mathrm{lab}}$, $C_z^{\mathrm{lab}}$, and induced polarization $P$ in the lab frame. Left to right: Different angles of ${\pi }^0$ in the c.m. frame. The “old data” could be found in the SAID database [23]. The three curves labeled Afanasev model [25], Farrar model [26], and SAID SP09 [27] are described in the text. Only the statistical uncertainties are shown.