MadGraph5_aMC@NLO

"There should only be one possible Born order for FxFx" when generating BSM interference with SM at NLO+PS

Asked by matteo maltoni

Dear MG5 experts,

I'm trying to generate, with v3.4.2, events at NLO+PS with the following syntax:

import model SMEFTatNLO
generate p p > l+ vl a QCD=0 QED=3 NP^2==2 [QCD] @0
add process p p > l- vl~ a QCD=0 QED=3 NP^2==2 [QCD] @1
add process p p > l+ vl a j QCD=1 QED=3 NP^2==2 [QCD] @2
add process p p > l- vl~ a j QCD=1 QED=3 NP^2==2 [QCD] @3

The generation for the same process in the SM (NP=0) works fine and produces and showers the events, but in this case the generation stops and the log files end with this lines:

REAL 1: keeping split order 1
 REAL 1: keeping split order 2
 REAL 1: not keeping split order 3
 REAL 10: keeping split order 1
 REAL 10: keeping split order 2
 REAL 10: not keeping split order 3
 REAL 2: keeping split order 1
 REAL 2: keeping split order 2
 REAL 2: not keeping split order 3
 REAL 3: keeping split order 1
 REAL 3: keeping split order 2
 REAL 3: not keeping split order 3
 REAL 4: keeping split order 1
 REAL 4: keeping split order 2
 REAL 4: not keeping split order 3
 REAL 5: keeping split order 1
 REAL 5: keeping split order 2
 REAL 5: not keeping split order 3
 REAL 6: keeping split order 1
 REAL 6: keeping split order 2
 REAL 6: not keeping split order 3
 REAL 7: keeping split order 1
 REAL 7: keeping split order 2
 REAL 7: not keeping split order 3
 REAL 8: keeping split order 1
 REAL 8: keeping split order 2
 REAL 8: not keeping split order 3
 REAL 9: keeping split order 1
 REAL 9: keeping split order 2
STOP 1
Thanks for using LHAPDF 6.3.0. Please make sure to cite the paper:
  Eur.Phys.J. C75 (2015) 3, 132 (http://arxiv.org/abs/1412.7420)
 REAL 9: not keeping split order 3
 There should only be one possible Born order for FxFx

I'm not sure of what this means, as FxFx returns no complaints in the SM case: can you give me your opinion?
Let me know if you need more info.

Cheers,

Matteo

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
Rikkert Frederix Edit question
Solved by:
Rikkert Frederix
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Revision history for this message
matteo maltoni (matteo-maltoni) said : 		#1

Hi MG5 experts,

Any updates?

The issue occurred even when I tried with NP^2<=2 (which should include SM and interference with BSM) and NP=2 (which should include everything up to the new physics squared).

Since the NP=0 case works, this seems to be related to the NP flag, but I can't see any link with the FxFx error message.

Cheers,

Matteo

Revision history for this message
Rikkert Frederix (frederix) said : 		#2

Dear Matteo,

Sorry for the delay.

The problem is exactly what it says it is. There are two different Born coupling orders contributing, one with and one without the NP interaction. FxFx cannot work if there is more than one Born coupling order.

best,
Rikkert

Revision history for this message
matteo maltoni (matteo-maltoni) said : 		#3

Hi Rikkert,

Thank you for your reply!

Just to make sure I understood, does this mean that FxFx cannot be used for SM-BSM interference?

What can be done in this case, if so?

Cheers,

Matteo

Revision history for this message
Best Rikkert Frederix (frederix) said : 		#4

Dear Matteo,

That is right. In general, you cannot do FxFx for SM-BSM interference.

There are a couple of exceptions, though. For example, if your BSM only affects the EW part of the SM, than there is no reason why FxFx would not be correct. But, to get this running, you'll need to hack the MG5_aMC code and know what your are doing.

Alternatively, simply using NLO+PS is also an option.

best,
Rikkert

Revision history for this message
matteo maltoni (matteo-maltoni) said : 		#5

Thanks Rikkert Frederix, that solved my question.