# Defining custom couplings

Hi MadGraph Team,

I am working on Milli-Charged particles (MCP) and I want to define them as follows: Suppose k is the fractional charge (i.e. 1/10e or something like that).

I want the MCP to couple to the photon and the Z boson with charge k*cos(weinberg angle) and -k*sin(weinberg angle) respectively. The fractional charge E in units of the electric charge e is therefore E = k*cos(weinberg angle)/e.

I understand you can create a custom model and edit the interactions.dat file to include the coupling to the photon and z boson but is there a way to define this so called 'custom' coupling above instead of the predefined couplings that occur when one edits the interaction.dat file?

(If necessary, let us assume k = 0.1)

Many thanks,

Ameir

## Question information

- Language:
- English Edit question

- Status:
- Solved

- Assignee:
- No assignee Edit question

- Solved by:
- Olivier Mattelaer

- Solved:
- 2018-01-13

- Last query:
- 2018-01-13

- Last reply:
- 2018-01-13

## This question was reopened

- 2018-01-13 by Ameir Shaa

Hi,

Recent model does not have any file interactions.dat anymore.

That format of model is outdated and is support is going to be stopped in a very near future.

In short you should not invest time to create a new model in that format (and I do not have the expertise to help you in such type of model).

MG5aMC, is now fully supporting the UFO format which is now used by mainly all the monte-carlo code.

So I would suggest to create your model within that format and/or to use a tool (feynrules/

In UFO, what you ask seems trivial to do.

Cheers,

Olivier

> On 11 Jan 2018, at 20:17, Ameir Shaa <email address hidden> wrote:

>

> New question #662970 on MadGraph5_aMC@NLO:

> https:/

>

> Hi MadGraph Team,

>

> I am working on Milli-Charged particles (MCP) and I want to define them as follows: Suppose k is the fractional charge (i.e. 1/10e or something like that).

>

> I want the MCP to couple to the photon and the Z boson with charge k*cos(weinberg angle) and -k*sin(weinberg angle) respectively. The fractional charge E in units of the electric charge e is therefore E = k*cos(weinberg angle)/e.

>

> I understand you can create a custom model and edit the interactions.dat file to include the coupling to the photon and z boson but is there a way to define this so called 'custom' coupling above instead of the predefined couplings that occur when one edits the interaction.dat file?

>

> (If necessary, let us assume k = 0.1)

>

> Many thanks,

>

> Ameir

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

Ameir Shaa (ameirshaa) said : | #2 |

Hi Olivier,

Many thanks for your reply! I will look up FeynRules!

Many thanks,

Ameir

Ameir Shaa (ameirshaa) said : | #3 |

Hi Olivier,

Sorry to bother you again. I have one question on FeynRules and I apologize if it seems trivial but I am extremely new to this!

So, as I understand it, FeynRules takes in two inputs from the user - firstly, a .fr file with the description of the particles and secondly, the lagrangian.

Here I have the lagrangian: L = LSM - 0.25*[A,

where phi is a dirac fermion of mass M_MCP and charged under the new U(1) field [A,mu] with charge e' and the field strength is defined as [A,mu,nu] = [del,mu]*[A,nu] - [del,nu]*[A,mu].

By redefining [A,mu] -> [A,mu] +k*[B,mu], we eliminate the mixing term (last term) and it results in the coupling of phi to the hypercharge ke'. The lagrangian then becomes:

L = LSM - 0.25*[A,

where i is the square root of -1, (slash) refers to the Feynman slash notation, phi acts as a field charged under hypercharge with a charge ke' which is a milli charge of mass M_MCP where MCP stands for milli charged particle.

The MCP couples to the photon and the Z boson with charge k*cos(weinberg angle) and -k*sin(weinberg angle) respectively. The fractional charge E in units of the electric charge e is therefore E = k*cos(weinberg angle)/e.

So going by the rules of FeynRules, I would first have to define a .fr file with all the parameters and particles listed.

So, I would have to define A, B, and phi like so:

M$ClassesDescri

V[100] == {

ClassName -> A,

Indices -> {},

},

V[101] == {

ClassName -> B,

Indices -> {},

},

F[100] == {

ClassName -> phi,

Indices -> {},

Mass -> {Mmcp, 100},

Width -> {Wmcp,1}

}

};

Is this correct? Do I have to define e' and ke' as well? Also where do I define the coupling to the photon and Z boson as stated above?

Much apologies if these questions are too long and/or too trivial..

Many thanks,

Ameir

Hi,

Sorry but I'm not a FeynRules author. I have a very limited knowledge on that code.

> Is this correct?

That I have no clue.

> Do I have to define e' and ke' as well? Also where do I define the coupling to the photon and Z boson as stated above?

Yes you shoud define those.

Cheers,

Olivier

Ameir Shaa (ameirshaa) said : | #5 |

Hi Olivier,

Many thanks for your reply!

Many thanks,

Ameir

Ameir Shaa (ameirshaa) said : | #6 |

Thanks Olivier Mattelaer, that solved my question.