MadGraph5_aMC@NLO

Low neutrino DIS cross section

Asked by Maksym Ovchynnikov

Hi! I am trying to generate the neutrino DIS cross section, considering a fixed proton:

generate vm~ p > mu+ j

In run_card, I have set

 10.0 = ebeam1 ! beam 1 total energy in GeV
  0.0 = ebeam2 ! beam 2 total energy in GeV
 0.0 = ptj ! minimum pt for the jets
  0.0 = ptl ! minimum pt for the charged leptons
  -1.0 = ptjmax ! maximum pt for the jets
  -1.0 = ptlmax ! maximum pt for the charged leptons
 -1.0 = etaj ! max rap for the jets
  -1.0 = etal ! max rap for the charged leptons

The MadGraph prediction for 10 GeV antineutrinos is
sigma = 2.143e-05 pb = 2.143e-41 cm^2.

Therefore,

sigma/E_nu = 2.143e-42 cm^2/GeV = 2.143e-4 * e-38 cm^2/GeV.

pdg says (https://pdg.lbl.gov/2020/mobile/reviews/pdf/rpp2020-rev-nu-cross-sections-m.pdf) that it should be sigma/E_nu ~ 0.3e-38 cm^2/GeV. Therefore, the MadGraph prediction underestimates the cross section by 3 orders of magnitude.

My question: am I doing something wrong here?

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Olivier Mattelaer
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Maksym Ovchynnikov (name-xxx) said : 		#1

P.S. If I set large E_nu, e.g. 10000 GeV, then MadGraph returns ~30 pb, which agrees well with the data.

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Olivier Mattelaer (olivier-mattelaer) said : 		#2

I'm in holliday for the moment,
so do not have the time to investigate in details,

can you set in the run_card
sde_strategy=1, the mode=2 was designed for high energy and therefore might perform poorly at low energy.

Cheers,

Olivier

> On 11 Aug 2022, at 08:20, Maksym Ovchynnikov <email address hidden> wrote:
>
> Question #702730 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/702730
>
> Maksym Ovchynnikov gave more information on the question:
> P.S. If I set large E_nu, e.g. 10000 GeV, then MadGraph returns ~30 pb,
> which agrees well with the data.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Maksym Ovchynnikov (name-xxx) said : 		#3

I see. Unfortunately, changing sde_strategy to 1 did not help. The result is practically the same.

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#4

Ah then this seems an alphas scale issue.
which crash at too small scale and act as an effective cut

You can set the computation at fix scale for alphas to avoid the issue.

Cheers,

Olivier

> On 11 Aug 2022, at 08:45, Maksym Ovchynnikov <email address hidden> wrote:
>
> Question #702730 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/702730
>
> Status: Answered => Open
>
> Maksym Ovchynnikov is still having a problem:
> I see. Unfortunately, changing sde_strategy to 1 did not help. The
> result is practically the same.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Maksym Ovchynnikov (name-xxx) said : 		#5

Thanks Olivier Mattelaer, that solved my question.