Question related to completeness relation for photons

[Dhmht_Kr]

Homework Statement

Question is posted below.

Relevant Equations

completeness relation

Hi
Would you explain to me what is the q^ and how they are related to completeness.How can i solve this exercise?It is from "Quarks and leptons An Introductory course in Modern Particle Physics" of Halzen and Alan D.Martin.Also, can you point me to a useful bibliography?

 

[vanhees71]

Just write down the definition of the $\epsilon_{\lambda}$ and do the calculation by "brute force" (it's not too difficult).