Article

# Polarization Components in π 0 Photoproduction at Photon Energies up to 5.6 GeV

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(Impact Factor: 7.51). 09/2011; 108(22). DOI: 10.1103/PhysRevLett.108.222004
Source: arXiv

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.

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##### Article: Measurement of the $^{1}H$($\vec{\gamma}$, $\vec{p}$)$\pi^{0}$ reaction using a novel nucleon spin polarimeter
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ABSTRACT: We report the first large-acceptance measurement of polarization transfer from a polarized photon beam to a recoiling nucleon, pioneering a novel polarimetry technique with wide application to future nuclear and hadronic physics experiments. The commissioning measurement of polarization transfer in the $^{1}H$($\vec{\gamma}$,$\vec{p}$)$\pi^{0}$ reaction in the range $0.4<E_{\gamma}<1.4$ GeV is highly selective regarding the basic parameterizations used in partial wave analyses to extract the nucleon excitation spectrum. The new data strongly favor the recently proposed Chew-Mandelstam formalism.
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##### Article: Photocouplings at the Pole from Pion Photoproduction
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ABSTRACT: The reactions $\gamma p\to\pi^0 p$ and $\gamma p\to\pi^+ n$ are analyzed in a semi-phenomenological approach up to $E\sim2.3$ GeV. Fits to differential cross section and single and double polarization observables are performed. A good overall reproduction of the available photoproduction data is achieved. The J\"ulich2012 dynamical coupled-channel model -which describes elastic $\pi N$ scattering and the world data base of the reactions $\pi N\to\eta N$, $K\Lambda$, and $K\Sigma$ at the same time - is employed as the hadronic interaction in the final state. The framework guarantees analyticity and, thus, allows for a reliable extraction of resonance parameters in terms of poles and residues. In particular, the photocouplings at the pole can be extracted and are presented.
European Physical Journal A 01/2014; 50(6). DOI:10.1140/epja/i2014-14101-3 · 2.74 Impact Factor