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As in Ta-Pei Cheng and Li's book for example the W boson propagator is given by :

## \frac{-i}{k^2-M^2} [g_{\mu\nu}+ (\zeta-1) k_\mu k_\nu/(k^2 - \zeta M^2)] ##

At the unitary gauge ## \zeta = \infty ##, where the W propagator becomes :

## \frac{-i}{k^2-M^2} [g_{\mu\nu}- k_\mu k_\nu/ M^2 ] ##

It is not clear for me how when we set ## \zeta ## to ## \infty ##, we got the last formula in the unitary gauge ?

Bests.

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# W boson propagator

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