Measurement of the helicity asymmetry Ε for the γ p → pπ0 reaction in the resonance region

Abstract

Abstract The double-spin-polarization observable

$${\mathbb {E}}$$

E

for

$$\vec {\gamma }\vec {p}\rightarrow p\pi ^0$$

γ →

p →

→ p

π 0

has been measured with the CEBAF Large Acceptance Spectrometer (CLAS) at photon beam energies

$$E_\gamma $$

E γ

from 0.367 to

$$2.173~\textrm{GeV}$$

2.173

GeV

(corresponding to center-of-mass energies from 1.240 to

$$2.200~\textrm{GeV}$$

2.200

GeV

) for pion center-of-mass angles,

$$\cos \theta _{\pi ^0}^{c.m.}$$

cos

θ

π 0

c . m .

, between

$$-$$

-

0.86 and 0.82. These new CLAS measurements cover a broader energy range and have smaller uncertainties compared to previous CBELSA data and provide an important independent check on systematics. These measurements are compared to predictions as well as new global fits from The George Washington University, Mainz, and Bonn-Gatchina groups. Their inclusion in multipole analyses will allow us to refine our understanding of the single-pion production contribution to the Gerasimov-Drell-Hearn sum rule and improve the determination of resonance properties, which will be presented in a future publication.