MadGraph5_aMC@NLO

aMC@NLO include b b~ in proton definitions

Asked by Harikrishnan Ramani

Hello Authors,
I read thru the aMC@NLO webpage that b,b~ > W+ W- production at 1 loop QCD is not included in aMC@NLO because the bg->WW produces a top quark on shell.
I tried to get around this by changing the top mass parameter to zero in paramcard.dat and tried running it. But it still failed during event merging.
Is there any work around to include b contribution to WW production? it would be great if I could set top to zero to achieve this!

Sincerley
Harikrishnan

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

Dear Harikrishnan,

You'll have to use a model with a massless b quark (e.g. import model loop_sm-no_b_mass) and make them part of the 'p' and 'j' definitions (define p = p b b~ and define j = j b b~). Then you can generate the processes as usual. To get around the problem with the top quark going on-shell, you'll have to set the top mass smaller than the mass of the W-boson (e.g. 20 GeV) and the width of the top quark to zero. You cannot set the mass of the top to zero.

However, even though this will circumvent the problem of the top quarks going on-shell, I'm not so sure if this really makes sense from a physics point of view. The top quark does not have a mass of 20 GeV, so the cross section you'll get is not 'correct'. It might be that the effect is small, but I can't tell beforehand. In particular, if you try to enhance the effects from the b-quarks, the error you are making in this procedure is not really under control.

Best regards,
Rikkert

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