# How to modify the matrix element in Madgraph?

How to modify the matrix element in MadGraph?

What I would like to do is below:

For a photon electron scattering, the xsec ~ g^\mu\nu L_\mu\nu where g^\mu\nu is the spin sum of the photon while L_\mu\nu is the lepton tensor. How could I, in MadGraph, change g^\mu\nu to, say, (1,0,0,1)^\mu (1,0,0,1)^\nu?

Thanks!

Best regards

## Question information

- Language:
- English Edit question

- Status:
- Solved

- Assignee:
- No assignee Edit question

- Solved by:
- Qiji Xin

- Solved:
- 2018-11-26

- Last query:
- 2018-11-26

- Last reply:
- 2018-11-25

Hi,

If I understand correctly your question, you should change the vxxxxx function.

This function is described in all details in the HELAS manual (just note that we might not use the exact same convention (sign of the momemtum/ ordering of the component/...) but the rest is still the same.

Cheers,

Olivier

> On 17 Nov 2018, at 04:43, Qiji Xin <email address hidden> wrote:

>

> New question #676227 on MadGraph5_aMC@NLO:

> https:/

>

> How to modify the matrix element in MadGraph?

> What I would like to do is below:

> For a photon electron scattering, the xsec ~ g^\mu\nu L_\mu\nu where g^\mu\nu is the spin sum of the photon while L_\mu\nu is the lepton tensor. How could I, in MadGraph, change g^\mu\nu to, say, (1,0,0,1)^\mu (1,0,0,1)^\nu?

>

> Thanks!

>

> Best regards

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

Qiji Xin (xinqijisuper) said : | #2 |

Hi Olivier,

Thanks!

I think you understand my question well. But let me say more about it.

Actually I am doing the electron nucleus scattering (mediated by a photon).

And the interaction between the virtual photon and electron can be broken down into g^\mu\nu L_\mu\nu part (transverse component) and pe^\mu pe^\nu L_\mu\nu part (longitudinal component, because it’s virtual photon).

(Here L_\mu\nu is the leptonic tensor of the electron part. p_e=(Ee,0,0,Ee) is the four momentum of the electron, Ee is electron energy.)

So could I directly use the tip here. https:/

And if must modify vxxxxx(), after my better description of my question, any more suggestions on this?

Thanks a lot!

Best regards

Yeah following those instructions sounds perfect if you are interested in polarization.

Cheers,

Olivie

> On 19 Nov 2018, at 04:08, Qiji Xin <email address hidden> wrote:

>

> Question #676227 on MadGraph5_aMC@NLO changed:

> https:/

>

> Status: Answered => Open

>

> Qiji Xin is still having a problem:

> Hi Olivier,

>

> Thanks!

> I think you understand my question well. But let me say more about it.

>

> Actually I am doing the electron nucleus scattering (mediated by a photon).

> And the interaction between the virtual photon and electron can be broken down into g^\mu\nu L_\mu\nu part (transverse component) and pe^\mu pe^\nu L_\mu\nu part (longitudinal component, because it’s virtual photon).

> (Here L_\mu\nu is the leptonic tensor of the electron part. p_e=(Ee,0,0,Ee) is the four momentum of the electron, Ee is electron energy.)

>

> So could I directly use the tip here. https:/

> And if must modify vxxxxx(), after my better description of my question, any more suggestions on this?

>

> Thanks a lot!

>

> Best regards

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

Qiji Xin (xinqijisuper) said : | #4 |

Hi Olivier,

A related question, can I put the matrix element by hand and ask MadGraph to do the phase space integral?

I already got the analytic form of the matrix element from FeynCalc, then I can easily modify that analytically, though there are a few tens of terms.

But FeynCalc doesn't do the phase space integral.

So is it possible that I put matrix element by hand and ask MadGraph to do the phase space integral?

Thanks!

Best regards

No this is highly non trivial.

In top of the full matrix-element, you also need to provide the amplitude in the color-flow basis.

This is of course possible (everything is possible) but I would not recommend a non author of MG5aMC of doing it.

Cheers,

Olivier

Qiji Xin (xinqijisuper) said : | #6 |

Thank you so much Olivier. The answers are very helpful.