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2Pedro,

Btw. do not forget to rename Lorentz indices in the conjugated matrix

element after contraction from a to b.

2018-02-27 22:29 GMT+03:00 Pedro <noreply@feyncalc.org>:

*> Dear all,
*

*>
*

*> I am calculating a simple diagram and the program is returning the wrong
*

*> answer.
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*>
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*> I have looked up my code thousand times and I don't think it has an error
*

*> (plus I run the EWMuonDecayTree example and it gave me the same wrong
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*> answer....)
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*>
*

*> Is this a bug or am I missing something very trivial?
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*>
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*> The proble is the following:
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*>
*

*> I am calculating the muon decay matrix element, using the following code:
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*>
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*> (* Beging of the code *)
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*>
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*>
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*> (*This is Gf/Sqrt[2]Overscript[u, _](p3)\[Gamma]^a(1-\[Gamma]^5)u(p1)
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*> (muon curent) *)
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*>
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*> j\[Mu] = Gf/Sqrt[2]*Spinor[p3, 0].GA[a].(1 - GA[5]).Spinor[p1, m\[Mu]];
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*>
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*>
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*> (*This is Gf/Sqrt[2]Overscript[u, _](p4)\[Gamma]^a(1-\[Gamma]^5)v(p2)
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*> (electron curent) *)
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*>
*

*> je = Gf/Sqrt[2] Spinor[p4, me].GA[b].(1 - GA[5]).Spinor[-p2, 0];
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*>
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*>
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*> (*This is to contract the \[Gamma] matrices*)
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*>
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*> M = (j\[Mu] MT[a, b] je // Contract);
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*>
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*>
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*> (* Matrix element <|M|^2>. Should be 64 Gf^2 (p1.p2)(p3.p4), see e.g.
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*> Griffths *)
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*>
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*> FermionSpinSum[M*ComplexConjugate[M]] /. DiracTrace -> TR
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*>
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*>
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*> (*End of the code *)
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*>
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*>
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*> The result it is giving is: 64 Gf^2 (p1.p3)(p2.p4) and not the well known
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*> 64 Gf^2 (p1.p2)(p3.p4).
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*>
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*>
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*> Now, if I run the EWMuonDecayTree example, it will show in the last line
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*>
*

*> Check with the Okun, Chapter 3.2:CORRECT.
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*>
*

*> But only because it is checking the final total decay rate and not the
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*> intermediate step (by lucky, it gave the correct result). If you run the
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*> EWMuonDecayTree, in the third output, after the Feynman Diagram, you will
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*> find the equivalent expression. Notice that the program gives ~(k.q1)(p.q2)
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*> which should be ~(k.q2)(p.q1)
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*>
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*>
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*> I am currently using version 9.2.0 and I also tested it on 8.2.0 and it
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*> gave the same results.
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*>
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*> Is there something wrong with my code and the example?
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*>
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*>
*

*> thanks
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*>
*

*>
*

-- Regards, Aliaksandr Dubrouski

**Next message:**V. Shtabovenko: "Re: Problem in Calculating matrix element"**Previous message:**Aliaksandr Dubrouski: "Re: Problem in Calculating matrix element"**In reply to:**Pedro: "Problem in Calculating matrix element"**Next in thread:**V. Shtabovenko: "Re: Problem in Calculating matrix element"**Reply:**V. Shtabovenko: "Re: Problem in Calculating matrix element"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]**Mail actions:**[ respond to this message ] [ mail a new topic ]

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