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I have the following charged current interaction, at quark level, by the process: $$\nu_e(p_1)+d(p_2)\rightarrow e^-(p_3)+u(p_4)$$ By assuming that the energy is such that I can neglect the lepton and quark masses, I obtained a certain amplitude $M$. I want to determine $\langle|M|^2\rangle$ and since in charged current interactions only left-handed chiral particle states participate, I have that $M\equiv M(\downarrow\downarrow,\downarrow\downarrow)$

enter image description here

My question is the following: the solution to the problem gives that $\langle|M|^2\rangle=M\equiv \frac{1}{2}|M(\downarrow\downarrow,\downarrow\downarrow)|^2$. Why the factor $\frac{1}{2}$? Don't I only have one possible combination for the helicities, which is the one in the image? Why not only $\langle|M|^2\rangle=M\equiv |M(\downarrow\downarrow,\downarrow\downarrow)|^2$?

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