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I am trying to calculate the matrix element of the process where an incoming upp quark and gluon interact via an upquark and emit a photon and an upquark.

---u(p1)->--~~gamma(p3)~~

||u(q)|~~g(p2)~~~-------u(p4)-------|

from the gluon vertex I get a color factor [tex]t^a[/tex] which I am not sure how to handle. My matrix element then looks like this

[tex]\bar{u}^s(p_4)ig_s \gamma^{mu}t^a\epsilon_{\mu}(p_2)\frac{\gamma^{\alpha}q_{\alpha} + m}{q^2-m^2}\epsilon^*_{\nu}(p_3)iQ_ue\gamma^{\nu}u^s(p_1)[/tex]

But there must be something that I am missing, since [tex]t^a[/tex] is a matrix it seems like my matrix element has the matrix dimensions of [tex]t^a[/tex]. Should I include a color part of the u quark? And if so, how does that work? Any reference where I could read more about it?

Thanks alot for any imput!

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# Color in matrix element calculation

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