Changes between Version 5 and Version 6 of FormFactors


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Timestamp:
12/12/13 10:09:24 (6 years ago)
Author:
omatt
Comment:

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  • FormFactors

    v5 v6  
    3939=== Add the vertex dependencies in your form factor:
    4040
    41 For later...
     41Go in the file vertices.py and identify the interactions that you want to modify:
     42Let assume that you want to modify the following interactions:
     43{{{
     44V_2 = Vertex(name = 'V_2',
     45             particles = [ P.W__minus__, P.W__plus__, P.H ],
     46             color = [ '1' ],
     47             lorentz = [ L.VVS1, L.VVS2 ],
     48             couplings = {(0,0):C.GC_1,(0,1):C.GC_3})
     49}}}
     50Then youhave to modify the lorentz structure associate to it:
     51{{{
     52V_2 = Vertex(name = 'V_2',
     53             particles = [ P.W__minus__, P.W__plus__, P.H ],
     54             color = [ '1' ],
     55             lorentz = [ L.FF1, L.FF2 ],
     56             couplings = {(0,0):C.GC_1,(0,1):C.GC_3})
     57}}}
     58Then you have to define those lorentz structure in the file lorentz.py
     59if they were define as:
     60{{{
     61VVS1 = Lorentz(name = 'VVS1',
     62               spins = [ 3, 3, 1 ],
     63               structure = 'Metric(1,2)')
     64
     65VVS2 = Lorentz(name = 'VVS2',
     66               spins = [ 3, 3, 1 ],
     67               structure = 'P(-1,1)*P(-1,2)*Metric(1,2)')
     68}}}
     69then you need to add the following lines:
     70{{{
     71FF1 = Lorentz(name = 'FF1',
     72               spins = [ 3, 3, 1 ],
     73               structure = 'AAA*Metric(1,2)',
     74               formfactors=[ForFac.AAA])
     75
     76VVS2 = Lorentz(name = 'VVS2',
     77               spins = [ 3, 3, 1 ],
     78               structure = 'AAA*P(-1,1)*P(-1,2)*Metric(1,2)',
     79               formfactors=[ForFac.AAA])       
     80}}}
     81  1. You need to be sure to have the line "import formfactors as ForFac" at the beginning of the lorentz.py file
     82  2. You can include more than one formfactor for a single lorentz structure.
     83  3. You are not force to define a new Lorentz structure if the old one will not be use anymore. 
     84
     85
     86=== For More complex formfactor (Those who cann't be written in one line).
     87
     88If the formfactor is a very complex function with a lot of "if" statement (like those obtained if when you add a form-factor to have the vertex associated to a full loop).
     89The above procedure is not the most clear one. They are therefore the possibility to code your form-factor directly in Fortran. By providing a unique file containing the definition of the functions.
     90Even if this method is usually easier to implement, this is not compatible with the C++/Python output, and you can not validate your implementation with the "check" command and/or produce any type of code related to the C++/Python output provided by MG5_aMC@NLO.
     91
     92In that case you need to create a directory Fortran insice the UFO directory
     93and create a file functions.f
     94You are free to define whatever you want in that file (if this doesn't create conflict with another functions of mg5. I advice to start all your routine/function by "mymdl_" to avoid any problem).
     95
     96Assuming that this Fortran file define the function AAA. Then you can modify the lorentz.py structure to the following:
     97VVS2 = Lorentz(name = 'VVS2',
     98               spins = [ 3, 3, 1 ],
     99               structure = 'AAA(P(-1,1)*P(-1,3))*P(-1,1)*P(-1,2)*Metric(1,2)') 
     100}}}
     101   1. All functions defined liked that need to return a complex number
     102   2. All argument of such function need to be a complex number, or an expression returning a complex number. It is therefore not possible to provide to the function the set of momenta as argument (since this is a vector).   
    42103
    43104