In the DFSZ axion model, the b to s a transition receives an unsuppressed two-loop contribution proportional to the scalar coupling lambda, which becomes important for tan beta above about 5.
Calculation of Feynman Diagrams from their Small Momentum Expansion
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abstract
A new powerful method to calculate Feynman diagrams is proposed. It consists in setting up a Taylor series expansion in the external momenta squared (in general multivariable). The Taylor coefficients are obtained from the original diagram by differentiation and putting the external momenta equal to zero, which means a great simplification. It is demonstrated that it is possible to obtain by analytic continuation of the original series high precision numerical values of the Feynman integrals in the whole cut plane. For this purpose conformal mapping and subsequent resummation by means of Pad\'{e} approximants or Levin transformation are applied.
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$B\rightarrow K + \text{axion-like particles}$: effective versus UV-complete models and enhanced two-loop contributions
In the DFSZ axion model, the b to s a transition receives an unsuppressed two-loop contribution proportional to the scalar coupling lambda, which becomes important for tan beta above about 5.