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Consistent Extensions and Portals

A portal couples new fields to the Standard Model, but a coupling alone does not specify a consistent extension. This chapter connects field content, anomalies, mass generation, and vacuum structure to the choice between a low-energy EFT and a resolved mediator, then to the interpretation of searches.

Enter this chapter

Section titled “Enter this chapter”

Begin with Consistency Checklist for Standard Model Extensions to test a proposed extension’s fields, symmetries, and mass generation.

Topics

Section titled “Topics”
  1. Consistency Checklist for Standard Model Extensions — Identifies the consistency conditions needed before phenomenology.
  2. Consistent New Matter and Gauge Sectors — Checks representations, anomalies, and gauge-invariant mass terms.
  3. Higgs-Singlet Scalar Portals — Derives singlet mixing and vacuum constraints.
  4. Extended Higgs Sectors: Alignment, Custodial Symmetry, and Decoupling — Separates alignment, custodial symmetry, and decoupling.
  5. Vector Portals and Kinetic Mixing — Canonicalizes kinetic mixing before reading physical couplings.
  6. Fermion and Neutrino Portals — Relates neutral-fermion interactions to mass and mixing structure.
  7. Axionlike and Pseudoscalar Portals — Tracks derivative and anomalous pseudoscalar couplings.
  8. Effective, Simplified, and Mediator Descriptions — Chooses a description from the actual momentum transfers.
  9. Search Validity and Reinterpretation for Portal Models — Restricts reinterpretations to valid theory and measurement domains.

How the ideas fit together

Section titled “How the ideas fit together”

Integrating out a mediator expands its propagator in momentum transfer over its mass. Near a pole or threshold that expansion fails even if a coefficient fit appears numerically good. A resolved mediator must then include its mass-generation mechanism, width, interference, and any states needed for consistency. Search reinterpretation adds another condition: the predicted events must satisfy the observable and likelihood definitions used by the experiment. Applying these restrictions before fitting prevents an exclusion from being driven by events the proposed description cannot predict. Abdallah et al. 2020, §§ 2–5.

Where to go next

Section titled “Where to go next”

Continue to Matching, Decoupling, and Threshold Evolution to derive the low-energy coefficients of a chosen consistent extension.

References

Section titled “References”
  • Abdallah, Waleed, et al. “Reinterpretation of LHC Results for New Physics: Status and Recommendations after Run 2.” SciPost Physics 9 (2020): 022. DOI.