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FIZIKA B 20 (2011) 1, 55-62
 
SPIN OBSERVABLES IN PION PHOTOPRODUCTION FROM A UNITARY AND CAUSAL EFFECTIVE FIELD THEORY
A. M. GASPARYANa,b and M. F. M. LUTZa
aGSI Helmholtzzentrum für Schwerionenforschung GmbH,
Planckstrasse 1, 64291 Darmstadt, Germany
bSSC RF ITEP, Bolshaya Cheremushkinskaya 25, 117218 Moscow, Russia
Received 29 December 2010; Revised manuscript received 24 November 2011
Accepted 17 December 2011 Online 4 January 2012
Pion photoproduction is analyzed with the chiral Lagrangian.
Partial-wave amplitudes are obtained by an analytic extrapolation
of sub-threshold reaction amplitudes computed in chiral perturbation
theory, where the constraints set by electromagnetic-gauge invariance,
causality and unitarity are used to stabilize the extrapolation.
The experimental data set is reproduced up to energies
√s ≅ 1300 MeV in terms of the parameters relevant
at order Q3. We present and discuss predictions for various
spin observables.
PACS numbers: 13.60.Le, 11.30.Rd
UDC 539.126, 539.122
Keywords: pion photoproduction, chiral perturbation theory, stabilization of
extrapolation, spin observables
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