Hi,
As far as I concerned, there are basically two ways to evaluate any process with explicit Fermion spin.
First, you can just write down the amplitude by hand and make it to matrix elements squared further, when summing over spins, you just replace the spinor part of explicit spin with (DiracSlash[p1]-m).(1+GA[5].DiracSlash[sm]) in your case. Here p1 is the 4-momentum of muon in center of mass frame and sm is the spin vector of muon.
Second, you can work with amplitude directly by inserting (1+GA[5].DiracSlash[sm])/2 after SpinorVBar[p1,mm] and then sum over spins "normally", you will get the exact same answer.
Further, Here is an attachment I've done not far before for muon decay with polarization allowed. It is almost the same with your case. For simplicity, I have just used Fermi Theory instead of standard electroweak theory. I hope it will help.
Regards,
Lingxiao Xu
------------------ 原始邮件 ------------------
发件人: "Kasi Jaswin.";<noreply@feyncalc.org>;
发送时间: 2014年12月10日(星期三) 凌晨0:48
收件人: "feyncalc"<feyncalc@feyncalc.org>;
主题: How to have a specific spin in the amplitude.
Hi,
I was interested in seeing a electro-weak decay of mu+ -> e+ ve vu, where mu+ are at rest and a specific alignment. What should I do to the spinor corresponding to mu ? I found a matrix for mu mu-bar, how do I include that in the amplitude ?
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