MadGraph5_aMC@NLO

"p p > W+ H" times BR(H->W+W-) or directly p p > W+ W+ W- ?

Asked by yanwenliu

Dear MadTeam,
I am confused by the following numbers that I got from madgraph 5:
1) for p p collisions at sqrts = 7 TeV, mH=125 GeV, cteq6l1,
the cross section of p p > W+ H is 0.298pb
that of p p > W- H is 0.160 pb
So, the total WH production times BR(H->WW) is 0.0989 pb.

2) same settings, for p p > W+ W+ W- , the cross section is 0.02832 pb
          p p > W- W+ W-, the cross section is 0.01283 pb.
This totals to 0.045 pb, which is only about 50% of 1), while 1) should only be a sub-channel of 2). I would expect the total of 2) be larger than 1). Do you understand the opposite direction?

Thanks,

Yanwen

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Johan Alwall
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Johan Alwall (johan-alwall) said : 		#1

Hello Yanwen,

Do you decay the Ws? Otherwise you can't have the Higgs onshell, and it will give a negligible contribution. If yes, can you double-check that the Higgs width you are using in the param_card is the correct (leading-order) width for the chosen mass value?

All the best,
Johan

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yanwenliu (yanwenliu) said : 		#2

Hi Johan. Thanks for your reply. I decay Ws with PYTHIA later. So, I should do something like :
generate p p > e+ e+ mu- ve ve vm~
in order to decay the Ws in madgraph?
This produces a lots of diagrams. Is there a way to separate the processes relevant to Higgs?

Thanks!

yanwen

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Best Johan Alwall (johan-alwall) said : 		#3

Hi Yanwen,

First, note that final-state particles in MadGraph are exactly onshell (since MG is a matrix element generator, and there is no well-defined matrix element with an offshell particle in the final state). So unless m_H > 2m_W, you can't get an onshell Higgs if you have Ws in the final state.

For decayed Ws, you have several options. If you want an on-shell Higgs in all your events and you want all Ws to decay to electron and muon, you simply do

define w = w+ w-
define l = l+ l- vl vl~
p p > h w, h > l+ vl l- vl~, w > l l
(or, if you want to also include leptonic and invisible Z as associated particle, you do
p p > h l l, h > l+ vl l- vl~
)

If you want also diagrams without a Higgs, but with intermediate w+ w- propagators, you do

define w = w+ w-
define l = l+ l- vl vl~
p p > w+ w- > l+ vl v- vl~ w, w > l l

(note that with this syntax, you are not requiring the w+ w- to go onshell). Feel free to explore the consequences of different MadGraph syntaxes, but make sure you check the resulting diagrams so you actually get what you wanted.

All the best,
Johan

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yanwenliu (yanwenliu) said : 		#4

Thanks Johan Alwall, that solved my question.