How far do Madgraph integrate offshell for virtual particles
Dear Madgraph experts,
This is probably a really simple question but here goes:
I'm attempting to generate:
generate p p > W- W- W+ [QCD]
I can see the pp > W H > WWW diagram in addition to many other diagrams in the html output of madgraph.
However, when I plot the WW mass, I do not see a peak at mH = 125.
Initially, I thought this was because my BW_cut is set to the default 15 so we are only allowing W* to go offshell to ~50 GeV. So I set BW_cut to 40 to allow the full W mass window.
I thought this would allow the pp > WH > WWW* process and therefore the mH = 125 GeV peak. However, this is not what I see.
On top of this, the calculated cross-section of the pp > WWW process did not change when I changed BW_cut from 15 to 40, even if doing so should have allowed another resonant production pp > WH > WWW* to be included.
I do see events with low W boson mass when I increase the BW_cut.
So my question is what is madgraph generating and how is it calculating the cross-section? It's obviously not integrating over the full W mass line shape for all diagrams. Doing so would include the contribution from pp > WH > WWW* where the H is on-shell.
If it's not including the offshell components what exactly is meant by having the pp > WH > WWW* diagram? Is it somehow only calculating pp > WH* > WWW where all 3 W's are on-shell and the tail of the W BW is added later just by tweaking the WWW phase space somehow without actually calculating the exact W line shape and interference from pp> WH > WWW*?
Thank you for your help,
-Sun
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- Olivier Mattelaer
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