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I started using CalcHEP for some analytical calculations, and in looking at Higgs decays, I noticed that the default model file (as well as some other files available online) mention Higgs coupling to fermion pairs for the muon-antimuon and the tau-antitau (so hmM and hlL couplings according to the CalcHEP syntax), but not with the electron-positron (no heE term). Is that by design (kinematics as the electron is too light)?

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  • $\begingroup$ I'm voting to close this question as off-topic because it deals with details of software files and associated choice of the original programmers rather than physics. $\endgroup$ – ZeroTheHero Aug 24 '17 at 8:30
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    $\begingroup$ @ZeroTheHero: It seems to me that the design choice is tied to physics, so it should stay open. $\endgroup$ – Kyle Kanos Aug 24 '17 at 10:08
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I see that the default Lagrangian contains only five Yukawa interactions for the muon, tau and three heavy quarks. This is an approximation in which the electron and light quarks are massless and hence have no Yukawa interactions because $y \propto m$.

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  • $\begingroup$ This is weird, as I thought that this would be more of a kinematic approximation, to be performed at the level of the symbolic $|\mathcal{M}|^2$ level, while the input parameters would be kept regardless of how small they really are. $\endgroup$ – N.E. Aug 24 '17 at 3:42
  • $\begingroup$ Dynamics usually refers to the $|M|^2$ factor. The Lorentz invariance phase space factor $d\Phi$ is usually referred to as kinematic. $\endgroup$ – innisfree Aug 24 '17 at 3:44
  • $\begingroup$ I must have mixed up my terminology, thanks. I meant that I thought that any approximation (for example, the relativistic limit, where masses vanish) are applied at the "dynamics" level, as in after the calculation for $|\mathcal{M}|^2$ is performed. $\endgroup$ – N.E. Aug 24 '17 at 15:01
  • $\begingroup$ Oh I see. Computationally it makes sense to do it before calculating $|M|^2$ as you can avoid calculating diagrams that ultimately contribute nothing. $\endgroup$ – innisfree Aug 25 '17 at 4:19

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