# W boson propagator

1. Mar 15, 2015

### Safinaz

Hi there,

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.

2. Mar 15, 2015

### Staff: Mentor

You can divide numerator and denominator by ζ and then simply take the limit of both.