Defining custom couplings

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That format of model is outdated and is support is going to be stopped in a very near future.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/lanhep/...) to create the UFO model for you (they ask the Lagrangian and write the model in UFO format).

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://answers.launchpad.net/mg5amcnlo/+question/662970
>
> 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
>
> --
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Hi Olivier,

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

Many thanks,

Ameir

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,mu,nu]*[A,mu,nu] + i*phibar*[del(slash) + i*e'*A(slash)- i*ke'*B(slash) + i*M_MCP]*phi - 0.5*k*[A,mu,nu]*[B,mu,nu]

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,mu,nu]*[A,mu,nu] + i*phibar*[del(slash) + i*e'*A(slash)- i*ke'*B(slash) + i*M_MCP]*phi

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$ClassesDescription = {

    V[100] == {
          ClassName -> A,
          SelfConjugate -> True,
          Indices -> {},
          Unphysical -> True,
         },

    V[101] == {
          ClassName -> B,
          SelfConjugate -> True,
          Indices -> {},
          Unphysical -> True,
         },

      F[100] == {
          ClassName -> phi,
          SelfConjugate -> False,
          Indices -> {},
          QuantumNumbers -> {Q -> 1/10},
          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 Olivier,

Many thanks for your reply!

Many thanks,

Ameir

Thanks Olivier Mattelaer, that solved my question.