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Phys Rev Lett. 2014 Jan 10;112(1):012003. Epub 2014 Jan 10.

First measurement of the helicity asymmetry for γp → pπ0 in the resonance region.

Author information

1
Helmholtz-Institut für Strahlen-und Kernphysik, Universität Bonn, 53115 Bonn, Germany.
2
Helmholtz-Institut für Strahlen-und Kernphysik, Universität Bonn, 53115 Bonn, Germany and Petersburg Nuclear Physics Institute, Gatchina, 188300 Russia.
3
Physikalisches Institut, Universität Bonn, 53115 Bonn, Germany.
4
Institut für Experimentalphysik I, Ruhr-Universität Bochum, 44780 Bochum, Germany.
5
Helmholtz-Institut für Strahlen-und Kernphysik, Universität Bonn, 53115 Bonn, Germany and II. Physikalisches Institut, Universität Gießen, 35392 Gießen, Germany.
6
Department Physik, Universität Basel, 4056 Basel, Switzerland.
7
Department of Physics, Florida State University, Tallahassee, Florida 32306, USA.
8
II. Physikalisches Institut, Universität Gießen, 35392 Gießen, Germany.
9
Petersburg Nuclear Physics Institute, Gatchina, 188300 Russia.

Abstract

The first measurement of the helicity dependence of the photoproduction cross section of single neutral pions off protons is reported for photon energies from 600 to 2300 MeV, covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, and BnGa partial wave analyses. Strikingly large differences between data and predictions are observed, which are traced to differences in the helicity amplitudes of well-known and established resonances. Precise values for the helicity amplitudes of several resonances are reported.

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