Penguin and Box Diagrams in Unitary Gauge

TL;DR

This paper evaluates one-loop penguin and box diagrams in the unitary gauge for FCNC processes within the Standard Model and its extensions, highlighting divergence cancellations and providing expressions applicable to related models.

Contribution

It explicitly demonstrates divergence cancellations in the unitary gauge for FCNC diagrams and derives consistent expressions with other gauges, applicable to models with similar fermion-gauge interactions.

Findings

Explicit divergence cancellation in SM FCNC diagrams.

Derived expressions consistent with R_xi gauge results.

Results applicable to models with similar fermion-gauge interactions.

Abstract

We evaluate one-loop diagrams in the unitary gauge that contribute to flavor-changing neutral current (FCNC) transitions involving two and four fermions. Specifically, we deal with penguin and box diagrams arising within the standard model (SM) and in nonrenormalizable extensions thereof with anomalous couplings of the W boson to quarks. We show explicitly in the SM the subtle cancelation among divergences from individual unitary-gauge contributions to some of the physical FCNC amplitudes and derive expressions consistent with those obtained using R_xi gauges in the literature. Some of our results can be used more generally in certain models involving fermions and gauge bosons which have interactions similar in form to those we consider.