Homework Help: W boson decay

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1. Feb 1, 2016

andrex904

Hi, i'm trying to compute the differential decay of W boson at rest
$$W^+ \rightarrow e^+ \nu_e$$
where the boson has fixed spin along z axis, and so specific cirular polarization. Using Feynman rules i get
$$M=\frac{ig_w}{\sqrt{2}}\bar{u}(q)\gamma^\mu(\frac{1-\gamma_5}{2})v(k)\epsilon_\mu^\lambda(p)$$
and i'm in trouble with a term coming from trace
$\varepsilon^{\alpha \mu \beta \nu} q_\alpha k_\beta$ $\epsilon^\lambda_\mu$ $\epsilon^{*\lambda}_\nu$
where $\varepsilon$ is the Levi-Civita symbol.

2. Feb 6, 2016