I was reading Schwartz's qft book. I saw the proof of ward identity taking pair annihilation as an example. he claimed he didn't assume that photon is massless in this derivation. but i have confusion with this statement. gauge invariance is a fact related to massless particles. now he has replaced $$\epsilon^{\mu}$$ with $$p^{\mu}$$ which is possible because of gauge invariance. it's like shifting $$\epsilon^{\mu}$$ to $$\epsilon^{\mu} + p^{\mu}$$ such a way so that it becomes equal to momentum of that particle. therefore in this derivation he has used gauge invariance and therefore assumed masslessness of photon. So why did he claim that it doesn't matter involved photon is physical or not??? wheredid i miss thepoint ??is there any other reason behind the above replacement??(adsbygoogle = window.adsbygoogle || []).push({});

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# Ward identity

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