MadGraph5_aMC@NLO

MadSpin configuration of Higgs rare decay mode

Asked by DanningLiu

Dear experts,

I'm currently working on simulating a Higgs boson production via gluon-gluon fusion with HC_NLO_X0_UFO model.
And what we want to do is to model the Higgs boson decaying into one photon and one off-shell photon, with the off-shell photon subsequently decaying into a pair of charged leptons

The processes I'm using are as follows :
p p > x0 / t aEQ=0 aS=2 QNP=1
p p > x0 j / t aEQ=0 aS=3 QNP=1
p p > x0 j j / t aEQ=0 aS=4 QNP=1

I want to model the decay chain where x0 > a a*, with "a*" representing an off-shell photon and distinct from a real photon "a", and "a* -> l+ l-".
However, I am unsure how to correctly configure the decay syntax in MadSpin.
Would the syntax "decay x0 > a a, a > l+ l-" be appropriate for representing the off-shell photon in this context ?
Here I use "x0 > a a" not "x0 > a a*" due to the fact that off-shell photon "a*" is not defined in this UFO model, the only symbol I can find for particle ID = 22 is "a". Therefore, I'm concerned whether an off-shell photon can be explicitly defined within the UFO model or if additional configuration is necessary?

I would be appreciate any guidance or suggestions on how to accurately implement this decay mode within MadGraph and MadSpin

Many Thanks,
Danning

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

They are likely various consideration/option that you can take into account here. So my answer is just my approach to the problem that I do understand from your question. Depending of your exact need you might need to tune it obviously.

Here is my thought:
1) You can not force an offshell photon since such final state can also be produced by offshell Z and that they interfere significantly. (but off course if you want to consider this interference in a separate template/...)
2) I'm not sure how accurate is the Higgs Effective Theory (valid in the infinite top mass limit here) since such decay can involve weak boson where the approximation is not valid. Now the dominant loop is likely the top one so maybe it is valid, but I would personally move to EW loop computation since it is not that slow and available (for the standard model)
3) Since decay is then a loop computation, madspin is the method to use (also because you need to switch model)
4) I do not think that such process is integrable without cuts, so one need to implement some minimal cut to make the avoid divergence and allow the integrator to generate events.

Given those considerations I would
1) use the loop_qcd_qed_sm model for handling the decay (within madspin)
2) set madspin on "no spin-correlation/no BW effect" (which are both fine for Higgs decay)
3) set madspin to consider cuts on the final state (I did use ptl=1, pta=1, mmll=1).

So my madspin card will look at something like this:
set spinmode none
import model loop_qcd_qed_sm ./path_to_param_card_to_use_for_loop_qcd_qed_model.dat
set run_card ./path_to_run_card_to_set_cut_on_the_decay.dat
decay x0 > e+ e- a [noborn=QED]

Cheers,

Olivier

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