Feynrules Model Generator

Step 2: Map Symbols to FeynRules Conventions

• Use the SM symbol conventions built into FeynRules (see reference doc):
  • ee = electromagnetic coupling, gw = weak SU(2)_L coupling, gs = strong SU(3)_C coupling
  • g1 = hypercharge U(1)_Y coupling, sw/cw = sine/cosine of Weinberg angle
• Assign PDG codes > 9000000 for generic BSM particles; use established PDG reservations where they exist (e.g. 5000039 for KK graviton)
• Define new coupling parameters with InteractionOrder -> {NP, 1}

Step 3: Create the .fr file from the skeleton

Copy templates/skeleton.fr into models/<Model>.fr (e.g. models/HeavyN.fr). Fill in M$ModelName and M$Information with the model name and metadata. Add any needed index definitions.

Output paths (all relative to working directory):

• FeynRules model file: models/<Model>.fr
• UFO directory (after validation + generation): models/<Model>_UFO/

Step 4: Define particle classes

Edit the M$ClassesDescription section. For each BSM particle, define the appropriate class (F[], S[], V[], T[]) with all required attributes. See references/feynrules_syntax.md section 5 for the full attribute list for each particle type.

Step 5: Define parameters

Edit the M$Parameters section. For each new coupling constant:

• Set ParameterType -> External with BlockName, OrderBlock, Value
• Include InteractionOrder -> {NP, 1} for every new physics coupling (both External and Internal)
• Do NOT add Mass or Width symbols here — they are already defined by the particle class

For derived parameters, use ParameterType -> Internal with a Value expression.

See references/feynrules_syntax.md section 6 for External/Internal parameter examples, including mixing matrices with Unitary -> True.

Step 6: Write the Lagrangian

Edit the Lagrangian section. Key rules:

• Use Block[{indices}, ...] to declare dummy indices
• Use explicit * for all multiplication
• If the Lagrangian has "+ h.c.", use the Ltmp pattern:

  LNPtmp := Block[{...}, <non-hermitian part only>];
  LNP := LNPtmp + HC[LNPtmp];
  

• If there is NO "+ h.c.", write the Lagrangian directly (do NOT add HC)

See references/feynrules_syntax.md sections 7-8 for Lagrangian syntax and a complete BSM example.

Step 7: Validate

After writing the .fr file, use the validate-feynrules Magnus blueprint (see feynrules-model-validator skill) to validate the model:

magnus run validate-feynrules -- --model <path-to-fr-file> --lagrangian <lagrangian-symbol>

Additionally, manually verify:

• Every spinor index appears exactly twice per monomial (proper contraction)
• Every monomial has zero total electric charge
• HC[] is only used when the Lagrangian explicitly contains "+ h.c."

Key Conventions

• BSM ClassName must be at least 2 characters (e.g., Snew, Zp, N1)
• Do NOT generate the SM part — only the BSM extension
• Use explicit multiplication (*) instead of implicit juxtaposition
• Fermion bilinears: psibar[sp1].Ga[mu,sp1,sp2].psi[sp2] or dot notation psibar.Ga[mu].psi
• Projectors: ProjM = (1-gamma5)/2 (left), ProjP = (1+gamma5)/2 (right)
• HC[] usage: Define non-Hermitian part as Ltmp, then L := Ltmp + HC[Ltmp]
• Non-selfconjugate fields: X = positive charge, Xbar = negative charge (canonical convention)
• (L -> R) substitution: Replace ALL left-handed objects simultaneously — projectors, fields, AND coupling parameters
• Width of BSM particles: If the particle is expected to decay, define Width as an external parameter (e.g. Width -> {WSnew, 0.04}) or use Width -> {WSnew, Internal}. Do NOT use Width -> 0 — this tells MG5 the particle is stable, and MG5 will silently override any set param_card DECAY attempt to match the model's zero-width expression

Reference Documentation

• See references/feynrules_syntax.md for the complete FeynRules .fr file syntax specification

Templates

• templates/skeleton.fr — Copyable starting point for a BSM extension .fr file with all required sections and TODO markers