On the definitions of the gamma*N -> N* helicity amplitudes

Understand

We present definitions and formulas that may be useful for a consistent computation of helicity amplitudes for the process gamma* N -> N* in theoretical approaches.

• Of particular importance is the correct determination of the common sign of the amplitudes and of the relative sign between the transverse (A12,A32) and longitudinal (S12) amplitudes.
• This clarification is necessary to clear up confusions present in theoretical works.
• Using the definitions presented in this paper will enable a direct comparison with amplitudes extracted from experimental data.