Mismatch between cross-sections in different mg5 version

Asked by rojalin padhan on 2021-03-04

Dear mg5 author,

   I am getting a mismatch between the results  obtained in  two different versions of mg5. Cross section  using MG5_aMC_v2_9_2  is 144 fb and using MG5_aMC_v2_7_3 (and also in MG5_aMC_v2_6_7) is 128 fb. I am using the same parameters for both runs.

The process I generate is  e- pb > pb n1, which is mediated by a scalar Leptoquark and also by W boson. Here pb = p b b~, n1 is a gauge singlet right handed neutrino. The difference is there for W mediated channels. Even if the coupling at  W-e-n1 vertex is switched off the contribution from this W mediated channel is non-zero for   MG5_aMC_v2_9_2 . Whereas, for  MG5_aMC_v2_7_3 it is zero. The result of MG5_aMC_v2_9_2 is not reasonable.

Is anything wrong with MG5_aMC_v2_9_2 ? Or I am missing something?

It will be  helpful if I can get any suggestions from your side. Thank you very much.


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Solved by:
Olivier Mattelaer
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Dear Rojalin,

As stated in 2102.00773, the multi-channel algorithm has deeply changed between 2.8.x and 2.9.x for some processes.
The meaning of the channel of integration is in particular quite different between 2.8.x and 2.9.x and indeed even if you switch off a coupling the associated channel of integration will not be turned off anymore since the channel weight only depends of the propagator and not of the associated coupling anymore.
You do have the possibility to choose between the two multi-channel strategy via the new parameter of the run_card "sde_strategy".

As stated in the paper, this change of strategy does have impact on the default scale associated to each phase-space point and can therefore modify your cross-section within theoretical uncertainty. Using a simpler dynamical scale like HT/2 (dynamical_scale_choice 3) would allow to check the two strategy with a fix strategy and therefore hide that theoretical uncertainty.

Another big potential change that you did not mention is the drop of the width for the T-channel particles. This was introduced in 2.8.0 since we realize that it sometimes was introducting breaking of gauge and of unitarity. This new mode is slightly better but if it is responsible for the change that you observe it is typically important to use complex-mass scheme since you are then very sensitive to the gauge and neither zero-width for T-channel or fix-width are then a good description.



rojalin padhan (rossy.mg) said : #2

Thanks Olivier Mattelaer, that solved my question.