top Yukawa coupling

Asked by stefano on 2018-05-01

Dear MG5 team,

I am calculating cross sections and partial widths for different loop-induced processes involving the higgs bosons and top quarks. We are currently importing the loop_qcd_qed_SM_Gmu UFO model and we are running the code for different values of the top-Yukawa coupling. We noticed that MG5 gives a wrong result for the H -> gamma gamma partial width (by setting yt to 0 the partial width gets smaller than in the SM case, while it is supposed to increase). We dig into the code and we found that the problem seems to be in the definition of the rational term R2_AAHt in the file, which is given in terms of the top mass instead of the top Yukawa coupling. We temporary fixed the bug by defining a new external parameter which rescales yt everywhere in the model and it seems working. I wanted to let you know about this issue. Since we need to run the code also for processes involving a loop-induced hZgamma vertex I wanted also to ask you the following. Is it appropriate to rescale the R2_AZHt coupling as well? Are there other changing that we are still missing?



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Hua-Sheng Shao Edit question
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Hua-Sheng Shao
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Best Hua-Sheng Shao (erdissshaw) said : #1

Dear Stefano, yes, in the SM, we have assigned ymt = MT in counter terms (UV and R2). For you study, I think it is sufficient to rescale R2_AAHt and R2_AZHt with ymt/MT. Best regards, Hua-Sheng

stefano (stefano-boselli01) said : #2

Thanks Hua-Sheng Shao, that solved my question.