MadGraph5_aMC@NLO

Heavy auxiliary propagator in contact interactions

Asked by Matt

Dear Madgraph team,

I am managing with a SM extensions that introduces a new kind of four-fermions contact interactions with a heavy Majorana neutrino.
I know that in Madgraph is necessary to introduce an auxiliary heavy boson to split the vertex, and that is what I did by adding the field:
V[13] == {
    ClassName -> Aux,
    SelfConjugate -> False,
    Mass -> {MAux, 10000},
    Width -> {WAux,0},
    ParticleName ->"Aux+",
    AntiParticleName -> "Aux-",
    QuantumNumbers -> {Q -> 1},
    FullName -> "Auxiliary"
  },
and by substituting the Lagrangian:

Lcont=g^2 (uqbar.ProjM[mu].dq)(lbar.ProjM[mu].Maj)

(where "Maj" obviously labels the heavy neutrino) with:

Lcont_split=g (uqbar.ProjM[mu].dq)Aux[mu]+Aux[mu] (Majbar.ProjM[mu].l) + h.c.

being "g^2" a new coupling constant.

However, when I try to compute a process like p p > Maj e+ I get too small values of the cross sections (compared to the those obtained with other numerical methods).
The same happens if I add the condition $aux+.

Could you please suggest me anything to overcome this problem?

Thanks in advance,
Matt.

Question information

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Status:
Solved
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MadGraph5_aMC@NLO Edit question
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Solved by:
Olivier Mattelaer
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Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi Matt,

Are you sure that such difference is not related to the presensece fo the the default cuts in the run_card?
Or that they are not linked to the PDF and/or scale uncertainty?

Beyond those quite simple remark, the only thing that I can advise you is to check the Feynman diagram and how they are computed to see what the problem can come from. In particular, did you define a specific propagator form for your auxiliary field? If not did you rescale your cross-section accordingly?

Cheers,

Olivier

> On 19 Oct 2017, at 13:15, Matt <email address hidden> wrote:
>
> New question #659650 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/659650
>
> Dear Madgraph team,
>
> I am managing with a SM extensions that introduces a new kind of four-fermions contact interactions with a heavy Majorana neutrino.
> I know that in Madgraph is necessary to introduce an auxiliary heavy boson to split the vertex, and that is what I did by adding the field:
> V[13] == {
> ClassName -> Aux,
> SelfConjugate -> False,
> Mass -> {MAux, 10000},
> Width -> {WAux,0},
> ParticleName ->"Aux+",
> AntiParticleName -> "Aux-",
> QuantumNumbers -> {Q -> 1},
> FullName -> "Auxiliary"
> },
> and by substituting the Lagrangian:
>
> Lcont=g^2 (uqbar.ProjM[mu].dq)(lbar.ProjM[mu].Maj)
>
> (where "Maj" obviously labels the heavy neutrino) with:
>
> Lcont_split=g (uqbar.ProjM[mu].dq)Aux[mu]+Aux[mu] (Majbar.ProjM[mu].l) + h.c.
>
> being "g^2" a new coupling constant.
>
> However, when I try to compute a process like p p > Maj e+ I get too small values of the cross sections (compared to the those obtained with other numerical methods).
> The same happens if I add the condition $aux+.
>
> Could you please suggest me anything to overcome this problem?
>
> Thanks in advance,
> Matt.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Matt (elvin.j) said : 		#2

Hi Olivier,

Thanks for your answer.
I tried again defining a propagator with no momentum dependence like this one

V0 = Propagator(name = "V0",
                numerator = "complex(0,-1) * Metric(1, 2)",
                denominator = "2 * Mass(id) * Mass(id)"
               )

And now seems to work.

Best,
Matt

> On 20 Oct 2017, at 10:59, Olivier Mattelaer <email address hidden> wrote:
>
> Your question #659650 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/659650 <https://answers.launchpad.net/mg5amcnlo/+question/659650>
>
> Status: Open => Answered
>
> Olivier Mattelaer proposed the following answer:
> Hi Matt,
>
> Are you sure that such difference is not related to the presensece fo the the default cuts in the run_card?
> Or that they are not linked to the PDF and/or scale uncertainty?
>
> Beyond those quite simple remark, the only thing that I can advise you
> is to check the Feynman diagram and how they are computed to see what
> the problem can come from. In particular, did you define a specific
> propagator form for your auxiliary field? If not did you rescale your
> cross-section accordingly?
>
> Cheers,
>
> Olivier
>
>> On 19 Oct 2017, at 13:15, Matt <<email address hidden> <mailto:<email address hidden>>> wrote:
>>
>> New question #659650 on MadGraph5_aMC@NLO:
>> https://answers.launchpad.net/mg5amcnlo/+question/659650
>>
>> Dear Madgraph team,
>>
>> I am managing with a SM extensions that introduces a new kind of four-fermions contact interactions with a heavy Majorana neutrino.
>> I know that in Madgraph is necessary to introduce an auxiliary heavy boson to split the vertex, and that is what I did by adding the field:
>> V[13] == {
>> ClassName -> Aux,
>> SelfConjugate -> False,
>> Mass -> {MAux, 10000},
>> Width -> {WAux,0},
>> ParticleName ->"Aux+",
>> AntiParticleName -> "Aux-",
>> QuantumNumbers -> {Q -> 1},
>> FullName -> "Auxiliary"
>> },
>> and by substituting the Lagrangian:
>>
>> Lcont=g^2 (uqbar.ProjM[mu].dq)(lbar.ProjM[mu].Maj)
>>
>> (where "Maj" obviously labels the heavy neutrino) with:
>>
>> Lcont_split=g (uqbar.ProjM[mu].dq)Aux[mu]+Aux[mu] (Majbar.ProjM[mu].l) + h.c.
>>
>> being "g^2" a new coupling constant.
>>
>> However, when I try to compute a process like p p > Maj e+ I get too small values of the cross sections (compared to the those obtained with other numerical methods).
>> The same happens if I add the condition $aux+.
>>
>> Could you please suggest me anything to overcome this problem?
>>
>> Thanks in advance,
>> Matt.
>>
>> --
>> You received this question notification because you are an answer
>> contact for MadGraph5_aMC@NLO.
>
> --
> If this answers your question, please go to the following page to let us
> know that it is solved:
> https://answers.launchpad.net/mg5amcnlo/+question/659650/+confirm?answer_id=0 <https://answers.launchpad.net/mg5amcnlo/+question/659650/+confirm?answer_id=0>
>
> If you still need help, you can reply to this email or go to the
> following page to enter your feedback:
> https://answers.launchpad.net/mg5amcnlo/+question/659650 <https://answers.launchpad.net/mg5amcnlo/+question/659650>
>
> You received this question notification because you asked the question.

Revision history for this message
Matt (elvin.j) said : 		#3

Thanks Olivier Mattelaer, that solved my question.