Why are KμKμ and KνKν terms absent from equation 3.11?

[Fredrik]

Top of page 69. I get six second order terms. Two of them cancel. Two are the ones on the right of 3.11. The other two involve K_μK_μ and K_νK_ν. Why are there no such terms on the right of 3.11? I'm probably just missing something painfully obvious.

 

[meopemuk]

Actually, there are more than 6 second-order terms. In order to see all of them, write each exponent as

$$e^{i \epsilon K_{\mu}} = 1 + i \epsilon K_{\mu} -1/2 \epsilon^2 K_{\mu}^2 + \ldots$$

 

[Fredrik]

Thank you. At least I was right about something: I did miss something painfully obvious.