Double-polarization observable G in neutral-pion photoproduction off the proton

Abstract

This paper reports on a measurement of the double-polarization observable G in π 0 photoproduction off the proton using the CBELSA/TAPS experiment at the ELSA accelerator in Bonn. The observable G is determined from reactions of linearly-polarized photons with longitudinally-polarized protons. The polarized photons are produced by bremsstrahlung off a properly oriented diamond radiator. A frozen spin butanol target provides the polarized protons. The data cover the photon energy range from 617 to 1325 MeV and a wide angular range. The experimental results for G are compared to predictions by the Bonn-Gatchina (BnGa), Jülich-Bonn (JüBo), MAID and SAID partial wave analyses. Implications of the new data for the pion photoproduction multipoles are discussed.

The impact of new polarization data from Bonn, Mainz and Jefferson Laboratory on $\gamma p \rightarrow \pi N$ multipoles

“…1. The recent new data include measurements of the beam asymmetry Σ [94,95], on T, P and H [96], G [38,97], and E [37,98] for the reaction γp → π 0 p. For the reaction γp → π + n, new data are available on the beam asymmetry Σ [95]. These data are (mostly) not used in the predictions.…”

“…The fits allowed for a renormalization of angular distributions, with a χ 2 penalty determined by the overall systematic error. In the new fits, this freedom was removed for the new polarized measurements [96,97,98]. A weighting factor of four in the χ 2 fit was applied to these data, while the set of previous measurements remained unweighted.…”

Spectroscopy of baryon resonances

“…The new fits, BnGa 2014-02 [48] (black solid line), SAID [97] (red dashed-dotted line), and JüBo [98] (blue dashed line), reproduce the data reasonably well [87]. For convenience, we show the data in Fig.…”

“…Interesting phenomena can be seen drawing the χ 2 /n df of each fitted order L max ≥ 1 against the center-of-mass energy W. Such plots are shown in Figs. 14, 15, and 16 for G [86,87], E [101], and T [102,103]. For specific truncation orders, bumps can be observed in the χ 2 -plots within certain energy regions.…”

Partial wave analysis

“…This is different in photoproduction of baryons. In photoproduction, a 7/2 + resonance needs an electric or magnetic E New photoproduction data on T , P, H [115,116] E [117], and G [81,118] for γp → π 0 p taken within the A.1 project at Bonn and data on E for γp → π + n [119] were included in the multi-channel BnGa-PWA and a large number of fits were performed. The fit required contributions from a Δ 7/2 + and a Δ 7/2 − resonance.…”