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

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.