Comment 1 for bug 921487
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| Ben O'Leary (ben-oleary) wrote Re: cross-section highly dependent on convention for PDG particle code | #1 |
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bbfa235
(Get the code!)
Well, since nobody form the MadGraph team seems to be working on this, I'll just post a little more information that I have gathered about this bug:
1) Um, I wasn't really thinking when I made the statement about "the contribution from each of the 3 diagrams", since there are clearly interference terms.
2) The interference terms seem to be the crucial part, since apparently the results are the same if one only considers one diagram at a time:
"generate e+ e- > a > x1+ x1- @1
add process e+ e- > z > x1+ x1- @2
add process e+ e- > x1+ x1- / a z @3
the results for each individual process is the same with or without the minus sign." (thanks to Florian Bonnet)
3) It may be that the problem is MG getting the helicities wrong inconsistently, because one can take just coherent sum of the photon & Z diagrams, & then the result does not depend on which sign of code the chargino has; however, in these diagrams, there are no clashing arrows of fermion flow.
4) Certainly one cannot state that it's just the result of a perverse choice of sign of code: if the down-antidown annihilation to charginos is correct, then the up-antiup equivalent has the problem, & vice-versa.
Maybe a little update from the MG team about the status of this bug would be appreciated? It's a rather serious issue: I'd like to be sure that all my cross-sections are not wrong by a factor of 6, & it has been almost a week since I reported this bug...
Thank you,
Ben