Cross sections for new physics

Asked by Christopher Marino on 2012-03-08

I am trying to define the couplings for a standard model extension. I want to define qballs (i.e. multi-charged, massive exotic particles) produced via Drell-Yan. I can set the user defined coupling in a version 4 model similar to that of muons produced via Drell-Yan but then the cross-section does not change with higher charges. Is there a more elegant or official way of producing these samples than to enter a factor of charge into the user defined coupling? Is there any to determine when multiplying by a factor of the charge ceases to valid, ie for charges 5 or 6?

Thanks for your help.

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Johan Alwall
Solved:
2012-03-15
Last query:
2012-03-15
Last reply:
2012-03-15
Johan Alwall (johan-alwall) said : #1

Dear Christopher,

I would suggest to use FeynRules to define your model, by starting from the Standard Model implementation and adding your new particles and Lagrangian terms. FeynRules can then write out all necessary model files for MadGraph 5 (using the WriteUFO command) or for any of several other matrix element generators. This is by far the most elegant way of implementing a new model. Effectively, however, you will be doing the same thing - including a user-defined factor in the charge of the new particles.

All the best,
Johan

Dear Johan,

Thank you for your response. I was actually using the MadGraph version 4 model in MadGraph 5 as I knew better how to implement that. If it will be effectively the same thing I don't know if it is worth starting over with the MadGraph 5 model. One question is could an expert confirm one data point for me so I can feel more confident that I am doing this correctly? For instance, I use Drell-yan produced 200 GeV doubly charged particles and get a cross section of 0.106 pb.

Thanks,
Christopher

-----Original Message-----
From: <email address hidden> [mailto:<email address hidden>] On Behalf Of Johan Alwall
Sent: 09 March 2012 04:31
To: Christopher Marino
Subject: Re: [Question #190073]: Cross sections for new physics

Your question #190073 on MadGraph5 changed:
https://answers.launchpad.net/madgraph5/+question/190073

    Status: Open => Answered

Johan Alwall proposed the following answer:
Dear Christopher,

I would suggest to use FeynRules to define your model, by starting from the Standard Model implementation and adding your new particles and Lagrangian terms. FeynRules can then write out all necessary model files for MadGraph 5 (using the WriteUFO command) or for any of several other matrix element generators. This is by far the most elegant way of implementing a new model. Effectively, however, you will be doing the same thing - including a user-defined factor in the charge of the new particles.

All the best,
Johan

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Best Johan Alwall (johan-alwall) said : #3

Hello Christopher,

The cross section should simply be a factor 4 larger than the corresponding cross section for a singly charged particle. The actual cross section depends on the spin of the particle as well as its SU(2) quantum number (if it connects to the Z boson). I would suggest that you refer to standard literature or colleagues at your institution to check your calculation (perhaps starting from e+e- with fixed energy, to avoid the difficulties relating to PDFs etc). Another way is to run for the corresponding masses with particles from well-tested models such as SUSY or UED. Many such models are available on the FeynRules wiki page or already inside MadGraph 5.

All the best,
Johan

Thanks Johan Alwall, that solved my question.

Hi Christopher,

> For instance, I use Drell-yan produced 200 GeV doubly
> charged particles and get a cross section of 0.106 pb.

You should compare this to paper computing such cross-section
theoretically.

In order to ensure that you didn't do bullshit.
you should use the command "check"
for example:
check p p > e+ e-
this check the
lorentz invariance
gauge invariance
coherence of different way to build the square matrix element.

On 15-mars-12, at 09:00, Christopher Marino wrote:

> Question #190073 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/190073
>
> Status: Answered => Open
>
> Christopher Marino is still having a problem:
> Dear Johan,
>
> Thank you for your response. I was actually using the MadGraph
> version
> 4 model in MadGraph 5 as I knew better how to implement that. If it
> will be effectively the same thing I don't know if it is worth
> starting
> over with the MadGraph 5 model. One question is could an expert
> confirm
> one data point for me so I can feel more confident that I am doing
> this
> correctly? For instance, I use Drell-yan produced 200 GeV doubly
> charged particles and get a cross section of 0.106 pb.
>
> Thanks,
> Christopher
>
> -----Original Message-----
> From: <email address hidden> [mailto:<email address hidden>] On Behalf
> Of Johan Alwall
> Sent: 09 March 2012 04:31
> To: Christopher Marino
> Subject: Re: [Question #190073]: Cross sections for new physics
>
> Your question #190073 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/190073
>
> Status: Open => Answered
>
> Johan Alwall proposed the following answer:
> Dear Christopher,
>
> I would suggest to use FeynRules to define your model, by starting
> from
> the Standard Model implementation and adding your new particles and
> Lagrangian terms. FeynRules can then write out all necessary model
> files
> for MadGraph 5 (using the WriteUFO command) or for any of several
> other
> matrix element generators. This is by far the most elegant way of
> implementing a new model. Effectively, however, you will be doing the
> same thing - including a user-defined factor in the charge of the new
> particles.
>
> All the best,
> Johan
>
> --
> If this answers your question, please go to the following page to
> let us know that it is solved:
> https://answers.launchpad.net/madgraph5/+question/190073/+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/madgraph5/+question/190073
>
> You received this question notification because you asked the
> question.
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.