Speaker
Description
The rare radiative $K^+\to\pi^+\ell^+\ell^-$ decays ($\ell=e,\mu$) provide experimental access to the $K^+\to\pi^+\gamma^*$ transition. The relevant form factor is conventionally written in terms of two hadronic parameters, $a_+$ and $b_+$, which are being measured by NA62 in both electron and muon channels. Comparing the two channels allows for a stringent test of lepton-flavor universality. However, appropriate experimental analysis requires adequate theory inputs: Although the $K^+\to\pi^+\gamma^*$ conversion has been studied extensively, radiative corrections involve the $K^+\to\pi^+\gamma^*\gamma^{(*)}$ transitions (with up to two virtual photons), not fully addressed in the literature. At the same time, the $K^+\to\pi^+\gamma^*\gamma^*$ transition is essential for the description of the $K^+\to\pi^+e^+e^-\ell^+\ell^-$ decays, which represent a background to new-physics searches.
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