Polarization components in $\pi^{0}$ photoproduction at photon energies up to 5.6 GeV

TL;DR

This study provides new polarization data for neutral pion photoproduction at photon energies up to 5.6 GeV, challenging existing theoretical predictions and indicating that perturbative QCD limits are not yet reached.

Contribution

It offers high-precision polarization measurements extending the data range up to 5.6 GeV, testing hadron helicity conservation and perturbative QCD predictions.

Findings

Non-zero induced polarization observed above resonance region

Rapid variation of polarization transfer components with energy and angle

HHC does not hold; pQCD limit not reached in this energy regime

Abstract

We present new data for the polarization observables of the final state proton in the $^{1}H(\vec{\gamma},\vec{p})\pi^{0}$ reaction. These data can be used to test predictions based on hadron helicity conservation (HHC) and perturbative QCD (pQCD). 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$^{\circ}$ in center-of-mass (c.m.) frame. The data extend the polarization measurements data base for neutral pion photoproduction up to $E_{\gamma}=5.6 GeV$. The results show non-zero induced polarization above the resonance region. The polarization transfer components vary rapidly with the photon energy and $\pi^{0}$ scattering angle in c.m. frame. This indicates that HHC does not hold and that the pQCD limit is still not reached in the energy regime of this experiment.