Changes between Version 5 and Version 6 of FormFactors

Timestamp:

Dec 12, 2013, 10:09:24 AM (11 years ago)

Author:

Olivier Mattelaer

Comment:

--

FormFactors

@@ -39 +39 @@
 === Add the vertex dependencies in your form factor:
 
-For later...
+Go in the file vertices.py and identify the interactions that you want to modify:
+Let assume that you want to modify the following interactions:
+{{{
+V_2 = Vertex(name = 'V_2',
+             particles = [ P.W__minus__, P.W__plus__, P.H ],
+             color = [ '1' ],
+             lorentz = [ L.VVS1, L.VVS2 ],
+             couplings = {(0,0):C.GC_1,(0,1):C.GC_3})
+}}}
+Then youhave to modify the lorentz structure associate to it:
+{{{
+V_2 = Vertex(name = 'V_2',
+             particles = [ P.W__minus__, P.W__plus__, P.H ],
+             color = [ '1' ],
+             lorentz = [ L.FF1, L.FF2 ],
+             couplings = {(0,0):C.GC_1,(0,1):C.GC_3})
+}}}
+Then you have to define those lorentz structure in the file lorentz.py
+if they were define as:
+{{{
+VVS1 = Lorentz(name = 'VVS1',
+               spins = [ 3, 3, 1 ],
+               structure = 'Metric(1,2)')
+
+VVS2 = Lorentz(name = 'VVS2',
+               spins = [ 3, 3, 1 ],
+               structure = 'P(-1,1)*P(-1,2)*Metric(1,2)')
+}}}
+then you need to add the following lines:
+{{{
+FF1 = Lorentz(name = 'FF1',
+               spins = [ 3, 3, 1 ],
+               structure = 'AAA*Metric(1,2)',
+               formfactors=[ForFac.AAA])
+
+VVS2 = Lorentz(name = 'VVS2',
+               spins = [ 3, 3, 1 ],
+               structure = 'AAA*P(-1,1)*P(-1,2)*Metric(1,2)',
+               formfactors=[ForFac.AAA])        
+}}}
+  1. You need to be sure to have the line "import formfactors as ForFac" at the beginning of the lorentz.py file
+  2. You can include more than one formfactor for a single lorentz structure.
+  3. You are not force to define a new Lorentz structure if the old one will not be use anymore.  
+
+
+=== For More complex formfactor (Those who cann't be written in one line).
+
+If 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).
+The 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.
+Even 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.
+
+In that case you need to create a directory Fortran insice the UFO directory
+and create a file functions.f
+You 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).
+
+Assuming that this Fortran file define the function AAA. Then you can modify the lorentz.py structure to the following:
+VVS2 = Lorentz(name = 'VVS2',
+               spins = [ 3, 3, 1 ],
+               structure = 'AAA(P(-1,1)*P(-1,3))*P(-1,1)*P(-1,2)*Metric(1,2)')  
+}}}
+   1. All functions defined liked that need to return a complex number
+   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).