Simplified leptoquark models for precision $l_i \rightarrow l_f \gamma$ experiments: two-loop structure of $O(\alpha_S Y^2)$ corrections

We put forward the framework of simplified leptoquark models: simple extensions of the Standard Model that serve as benchmarks to test the interactions of leptons with new colored degrees of freedom considered, for instance, in leptoquark or grand unification models. As a first application of the scheme, we analyze the power of precision lepton observables by computing gauge invariant two-loop radiative corrections to the lepton-photon vertex, generated here by Yukawa interactions between the lepton and new colored degrees of freedom. The result, detailed in explicit expressions for the involved form factors, improves on the literature for the higher loop order considered and highlights the existence of regions in the parameter space of the model where two-loop corrections cannot be neglected.
Comment: 17 pages, 12 figures. Ancillary file attached: Diags.m v2: Minor changes to text and references updated. Version accepted in PRD