I Relation between tensor decomposition and helicity amplitude

CAF123

Gold Member
It is common to write e.g photon two point function in terms of manifest transverse and longitudinal form factors with lorentz structure factored out, e.g $$\Pi^{\mu \nu} = (g^{\mu \nu} - q^{\mu} q^{\nu}/q^2)T_T + q^{\mu} q^{\nu}T_L,$$ where mu and nu are polarisation indices.

How do I relate such expressions to helicity amplitudes which are usually given in terms of e.g ++, -- for transverse degrees of freedom or 00 for longitudinal?

If I take two transverse photons, then mu and nu are both +, but then what is $g^{++}$? Looks like light cone components but I am not sure if this is the correct notation.

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CAF123

Gold Member
Perhaps put another way, how to go from helicity summed expressions to the individual helicity components?

"Relation between tensor decomposition and helicity amplitude"

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