MadGraph5_aMC@NLO

NLO with a simple BSM model

Asked by Mikael Chala

Dear aMC@NLO team,

I have a MG5 model containing a SM extension with
Wtb anomalous couplings, and I would like to study
it at NLO in QCD using aMC@NLO. However, I see that
for the moment, only SM processes are allowed. However,
given that this is a very simple extension of the SM,
I wonder whether I can perform NLO computations of
ttbar, as it should only affect to the production
(which is SM) and not to the decay of t and t~ (where
the EW new physics is).

Thank you very much in advance.

All the best,

Mikael Chala.

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
Olivier Mattelaer Edit question
Solved by:
Mikael Chala
Solved:
Last query:
Last reply:
Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi Mikael,

What you ask for is more complicated than that, since your anomalous coupling has some particle inside, if you want to go NLO,
then you need to have the associate R2 term associate to this new interactions. The creation of such model will be possible by an extension of the NLOCT module of FeynRules but that part is not validated yet.

One other solution is to generate p p > t t~ @NLO accuracy (in the Standard Model) and make the decay @LO accuracy. Which seems what you have in mind in your model. For this you can use MG5_aMC + MadSpin (arXiv:1212.3460)

MadSpin is actully now shipped internally to MG5_aMC so no need to do any installation/...

in order to use MadSpin, you need to set the flag Madspin=ON when MG5_aMC ask you the question:
The following switches determine which operations are executed:
 1 Perturbative order of the calculation: order=NLO
 2 Fixed order (no event generation and no MC@[N]LO matching): fixed_order=OFF
 3 Shower the generated events: shower=ON
 4 Decay particles with the MadSpin module: madspin=OFF
  Either type the switch number (1 to 4) to change its default setting,
  or set any switch explicitly (e.g. type 'order=LO' at the prompt)
  Type '0', 'auto', 'done' or just press enter when you are done.
 [0, 1, 2, 3, 4, auto, done, order=LO, order=NLO, ... ][60s to answer]

Then at the question:
The following switches determine which operations are executed:
 1 Perturbative order of the calculation: order=NLO
 2 Fixed order (no event generation and no MC@[N]LO matching): fixed_order=OFF
 3 Shower the generated events: shower=ON
 4 Decay particles with the MadSpin module: madspin=ON
  Either type the switch number (1 to 4) to change its default setting,
  or set any switch explicitly (e.g. type 'order=LO' at the prompt)
  Type '0', 'auto', 'done' or just press enter when you are done.
 [0, 1, 2, 3, 4, auto, done, order=LO, order=NLO, ... ][60s to answer]

you have to type 4 to edit the madspin_card
and include in that file the line
import model NAME CARD_PATH

where NAME is the NAME of your LO model and CARD_PATH is the name of the param_card that you want to consider (which should be compatible with the one use for the NLO computation, i.e. same value of the top mass, ...)

Note that MadSpin include the full-spin correlation and take into account the width of the particle.
But that the cross-section is compute by the production cross-section times the branching ratio. (and has therefore an additional theoretical uncertainty of gamma/m)

Cheers,

Olivier

Revision history for this message
Mikael Chala (mikael.chala) said : 		#2

Thank you very much for this excellent answer!

All the best,

   Mikael Chala.