Show that the completeness relation is ∑λ^(λ) * ^(λ)=-gμν+(pμpv)/(M^2) where the sum is over the

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Understand the problem. We are dealing with a massive vector particle, which has three p

Show that the completeness relation is $$ \sum_\lambda \varepsilon_p^{(\lambda)} * \varepsilon_p^{(\lambda)}=-g_{\mu \nu}+\frac{p_\mu p_v}{M^2} $$ where the sum is over the three polarization states of the massive vector particle.