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SARAH
SARAH

FeynArts

Last edited by Martin Gabelmann
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FeynArts

Generate model files for FeynArts 

A model file for “ FeynArts ” is created by

ModelOutput[$EIGENSTATES, WriteFeynArts->True]

or, if ModelOutput was used before and the vertices are already calculated, by

MakeFeynArts[Options]

As options can AddCounterTerms -> True be used to add counter terms to each parameter: in that case all appearing parameters are replaced by x → x + δ**x. Note, this option has to be used carefully because the routines are not yet very sophisticate. For instance, the shift is applied for mixing angles like the Weinberg angle itself and not for trigonometric functions as it is usually done. This will be improved in the future.

The following things are done:

  1. A list of all particles present in the models is generated. The particles in FeynArts are named

    1. S[X]: For scalars, with some integer X
    2. F[X]: For fermions, with some integer X
    3. V[X]: For vector bosons, with some integer X
    4. U[X]: For ghosts, with some integer X

    X can be defined in the particle definitions file of SARAH or is chosen automatically. FeynArts also supports labels for particles which are easier to read for humans eyes by using a TeX-like output. The label for each particle is generated from the defined LaTeX name by SARAH .

  2. A list with all appearing indices is written.

  3. The list with interactions is written. If the theory contains several non-Abelian, unbroken gauge groups, the generators of these gauge groups will appear in the vertices. By default, the generators of S**U(3) are associated to SUNT and automatically simplified when using FormCalc .

Dependences, numerical values and special abbreviations for FormCalc 

A second file is generated by SARAH together with the model file for FeynArts . This file is called Substitutions-<> $EIGENSTATES <>.m and contains additional information which might be useful for calculating diagrams:

  1. Replacement rules with the defined dependences in SARAH , see parameters.m

    1. Dependences
    2. DependencesOptional
    3. DependencesNum

  2. The definitions of the masses: Masses in SARAH 

  3. The numerical values for the parameters in SARAH : NumericalValues

  4. Special abbreviations for FormCalc like those are also defined for the MSSM and SM in the FormCalc package:

    1. A complex conjugation is replaced by C: Conjugate[X] ->XC
    2. A square is replaced by 2: X^2 -> X2
    3. The names of soft-breaking couplings are merged: T[X] -> TX

    It is recommended to use this definitions to speed up the calculations with FormCalc .

See also