A charged pion decays to a charged antilepton, its associated neutrino, and a photon. Inner bremsstrahlung from charged external states interferes with structure-dependent radiation. An experimentally resolved photon needs an energy cut: the real-radiation rate alone diverges in the soft limit, whereas a suitably inclusive combination with virtual radiative corrections is finite.[ref]
Radiative pion and kaon decay form factors in the 2025 Review of Particle Physics. Section 62.1, especially equations (62.1)–(62.4)
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