The definition of transverse amplitude in the process: ep→epπ

[zhangyang]

When CLAS of Jlab describe the result of the process ep→epπ,the use the terminal "transverse amplitude" M_l±(W,Q²), E_l±(W,Q²),and "scalar(longitudinal) amplitudes"S_l±(W,Q²),they correspond to photons of the magnetic, electric, and Coulombic type.Are they observables?
The index of polarization vector of photon is :0,1,2,3.0 is scalar photon ;1 and 2 are transvers one;3 is longitudinal.But in experiment ,could observables be divided into scalar, transvers and longitudinal?
Thanks.

 

[RGevo]

What is observed is related to the squared amplitude, integrated over the phase space. So not directly no, but these squared amplitudes will result in different kinematics of the final state, and can generally be inferred with a fit to data. If you provide a reference, I can check explicitly.

 

[zhangyang]

What is observed is related to the squared amplitude, integrated over the phase space. So not directly no, but these squared amplitudes will result in different kinematics of the final state, and can generally be inferred with a fit to data. If you provide a reference, I can check explicitly.

http://www.sciencedirect.com/science/article/pii/S0146641011001086
3.2. Expansion over multipole amplitudes

Thank you!