Double Spin Asymmetry in Wide-Angle Charged Pion Photoproduction - $A_{LL}$ experiment

The measured differential cross sections for the single neutral/charge pion photoproduction from a nucleon in the wide angle regime (s,-t,-u much larger hadronic scale) disagree with theoretical calculations by almost two orders of magnitude in spite of many years of investigation. A solution was proposed by P. Kroll and K. Passek-Kumericki, whose GPD-based theory includes both twist-2 and twist-3 amplitudes. The signatures of the twist-3 amplitude (compared to twist-2) is a increase in the cross section and has opposite signs of the double polarization observables $K_{LL}$ and $A_{LL}$ at backward production angles. An experimental check of the prediction would provide valuable information on the validity of the handbag mechanism in the GPD framework in the accessible energy range. The proposed experiment will measure the double  spin  asymmetry $A_{LL}$~for  charged  pion  photoproduction in the wide angle regime using the reaction ($\vec \gamma \vec n \rightarrow \pi^- p$).

We plan to test three aspects of the theoretical prediction:

\begin{itemize}

\item Similar absolute values and opposite signs of \KLL~and \ALL. 

\item Dependence of \ALL~with a slight variation of the pion centre of mass angle.  

\item Stability of \ALL~in the wide angle regime as a function of Mandelstam variable $s$ in the energy range 9 to 15 GeV$^2$.

\end{itemize}

This experiment aims to measure helicity correlation observables that have not been measured before for wide angle pion photoproduction. Such a pioneering measurement will help to uncover the nature of the interaction mechanism that underlies exclusive single pion photoproduction from the nucleon in the wide angle regime. Overview and projected results will be presented in this talk.