MadGraph5_aMC@NLO

higgs couplings in heft

Asked by zheng

Hello,

My questions is mainly about understanding the higgs couplings in heft.

I wanted to modify the Higgs to top quark couplings. In the file "couplings.f" of SM model, there is only ghtop(1) and ghtop(2) representing left-handed and right-handed couplings respectively. In heft_v4 model, there are two additional couplings, gh3top(1) and gh3top(2), do I need to modify this two couplings defined by gh3top at the same time? I don't quite understand the difference between gh3top and ghtop.

The second question is that after I modified the Higgs to top couplings, do I need to modify the Higgs to gluon gluon couplings accordingly because top loop contributes to the H-g-g coupling? I found the expressions of Higgs to gluon couplings in "couplings.f" but I am not sure if the relation between Higgs to top couplings has be correlated automatically.

Thank you very much.

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MadGraph5_aMC@NLO Edit question
Assignee:
Rikkert Frederix Edit question
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Rikkert Frederix
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Rikkert Frederix (frederix) said : 		#1

Hello,

The gh3top coupling is not used by default, and was implemented to simulate a pseudo-scalar Higgs. Therefore, you don't need to change that coupling. However, both ghtop and gh3top are NOT used for the g-g-H effective vertex, but only for the Higgs Yukawa coupling. The g-g-h and g-g-g-h effective coupling constants are GH and GH4, respectively. They are computed in the m_top->infinity limit, but also some subleading terms in the Taylor expansion around tau=(m_higgs/2*m_top)^2 are kept.

So, you have to change the ghtop, gh and gh4 couplings consistently to change the strength of the Higgs coupling to top quarks.

Best regards,
Rikkert

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zheng (yjzheng218) said : 		#2

Thanks Rikkert Frederix, that solved my question.

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zheng (yjzheng218) said : 		#3

Hi Rikkert,

Thanks to the heft model, I get what I need from your codes.

But I am still curious about the approximation expansion that in the limit of \tau->0 to the 3rd order and \tau_W->0 to the 6th order. The relevant original function is arcsin[sqrt{tau}], which can be directly computed by fortran, I am wondering why don't we just put the original function inside the code rather than make series expansion to a certain order? Thank you.

Best regards,
Ya-Juan

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Best Rikkert Frederix (frederix) said : 		#4

Dear Ya-Juan,
Indeed, if one is just interested in gg-> h (or h-> gamma gamma) then one could use the full form factor (the arcsin function). However, that is not the correct factor for Higgs+one jet production. Therefore we have decided to use an intermediate solution which is to expand the coupling to the first couple of orders.
Kind regards,
Rikkert

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zheng (yjzheng218) said : 		#5

Thanks Rikkert Frederix, that solved my question.