amplitude for a process

Asked by Antara Sarkar on 2020-01-13

Is there any way one can get the values of amplitude for a process generated? If there are several subprocesses, I wanted to see which of those interfere constructively and destructively.

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2020-01-13
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2020-01-13

Hi,

In general, Yes it is possible but you will need to define your-self what you are looking at the fortran level since such quantity are not very well defined (not gauge and lorentz invariant)

So you will obviously need to use the standalone output where you only evaluate the matrix-element on a single phase-space point.
There you can customise the file matrix.f to add the tracking of the quantity that you are interested in.

You will find in that file the following variable

within the function MATRIX
AMP(I): complex number corresponding to the amplitude of a given diagram WITHOUT any color algebra and for a given helicity.
JAMP(I): complex number corresponding to a given color-flow (the color factor is still not included), this for a given helicity
CF(J,I): Is the. color matrix

So the contribution [up to some normalisation] of one color-flow is CF(I,I) * JAMP(I) * DCONJ(JAMP(I))

within the function SMATRIX

This is where we loop over all polarization in order to get the final amplitude.

Cheers,

Olivier

> On 13 Jan 2020, at 12:38, Antara Sarkar <email address hidden> wrote:
>
> New question #687963 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/687963
>
> Is there any way one can get the values of amplitude for a process generated? If there are several subprocesses, I wanted to see which of those interfere constructively and destructively.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Antara Sarkar (antarasarkar) said : #2

I wish to know the matrix element for each of the subprocesses within p p > h z upto one-loop.
So, for tree level, I get it by the following commands:
> generate p p > h z
> output standalone PATH

But for loop level, with loop_qcd_qed_sm model:
> generate g g > h z [noborn=QED]
> output standalone PATH
Command "output standalone_cpp PATH" interrupted with error:
PhysicsObjectError : Usage of get_color_amplitudes is not allowed in a LoopHelasMatrixElement
Please report this bug on https://bugs.launchpad.net/mg5amcnlo
More information is found in 'MG5_debug'.
Please attach this file to your report.

with a SM at one loop model file created with NLOCT within Feynrules, I try:
> generate g g > h z [noborn=QED]
> output standalone PATH
Error detected in "output standalone PATH"
write debug file MG5_debug
If you need help with this issue please contact us on https://answers.launchpad.net/mg5amcnlo
MadGraph5Error : The analytic information wavefunction_rank has not been computed yet for this wavefunction and an alohaModel was not specified, so that the information cannot be retrieved.

Could you please help to get the matrix element value?
 I also wish to see for :
> define q = u c d s u~ c~ d~ s~
> generate q q > h z [noborn=QED]
> output standalone path
Error detected in "output standalone PATH"
write debug file MG5_debug
If you need help with this issue please contact us on https://answers.launchpad.net/mg5amcnlo
MadGraph5Error : The analytic information wavefunction_rank has not been computed yet for this wavefunction and an alohaModel was not specified, so that the information cannot be retrieved.

Antara Sarkar (antarasarkar) said : #3

Also, is there some other way too, to check which diagram interferes destructively or constructively for a given process?

Hi,

You have to use a dedicated syntax to have loop working in standalone:

>> generate g g > h z [sqrvirt=QED]
>> output standalone PATH

Now I do not know how to edit such standalone output to get more detail value about the computation. They are a lot of optimisation performed and therefore this is far from trivial to do it.
The only way is likely to create N-different code computing N different piece rather than tracking the quantity that you want.
One way is do use loop-filtering (some information about that hacking is presented here:
https://answers.launchpad.net/mg5amcnlo/+question/402723 <https://answers.launchpad.net/mg5amcnlo/+question/402723>)

Cheers,

Olivier

> On 13 Jan 2020, at 13:27, Antara Sarkar <email address hidden> wrote:
>
> Question #687963 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/687963
>
> Status: Answered => Open
>
> Antara Sarkar is still having a problem:
> I wish to know the matrix element for each of the subprocesses within p p > h z upto one-loop.
> So, for tree level, I get it by the following commands:
>> generate p p > h z
>> output standalone PATH
>
> But for loop level, with loop_qcd_qed_sm model:
>> generate g g > h z [noborn=QED]
>> output standalone PATH
> Command "output standalone_cpp PATH" interrupted with error:
> PhysicsObjectError : Usage of get_color_amplitudes is not allowed in a LoopHelasMatrixElement
> Please report this bug on https://bugs.launchpad.net/mg5amcnlo
> More information is found in 'MG5_debug'.
> Please attach this file to your report.
>
> with a SM at one loop model file created with NLOCT within Feynrules, I try:
>> generate g g > h z [noborn=QED]
>> output standalone PATH
> Error detected in "output standalone PATH"
> write debug file MG5_debug
> If you need help with this issue please contact us on https://answers.launchpad.net/mg5amcnlo
> MadGraph5Error : The analytic information wavefunction_rank has not been computed yet for this wavefunction and an alohaModel was not specified, so that the information cannot be retrieved.
>
> Could you please help to get the matrix element value?
> I also wish to see for :
>> define q = u c d s u~ c~ d~ s~
>> generate q q > h z [noborn=QED]
>> output standalone path
> Error detected in "output standalone PATH"
> write debug file MG5_debug
> If you need help with this issue please contact us on https://answers.launchpad.net/mg5amcnlo
> MadGraph5Error : The analytic information wavefunction_rank has not been computed yet for this wavefunction and an alohaModel was not specified, so that the information cannot be retrieved.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Antara Sarkar (antarasarkar) said : #5

Also, is there some other way too, to check which diagram interferes destructively or constructively for a given process?

no

> On 13 Jan 2020, at 14:03, Antara Sarkar <email address hidden> wrote:
>
> Question #687963 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/687963
>
> Status: Answered => Open
>
> Antara Sarkar is still having a problem:
> Also, is there some other way too, to check which diagram interferes
> destructively or constructively for a given process?
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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