Speaker
Description
Dark photons with masses below the electron-positron pair production threshold, $m_{A'} < 2m_e$, occupy a qualitatively distinct regime of hidden sector physics. In this mass range, visible decays into charged leptons are kinematically forbidden
Two competing decay modes become relevant, which exhaust the allowed decay channels. The first is the loop-induced visible decay $A' \to 3\gamma$, yielding a highly suppressed but experimentally distinctive multi-photon signal.
The second arises from mixing with the Standard Model $Z$ boson and leads to an invisible decay channel, $A' \to \nu \bar{\nu}$. Despite their importance, the interplay between these two decay modes has not previously been studied in a systematic way.
We present the first framework that consistently compares three-photon and neutrino decays of sub-$2m_e$ dark photons and analyses their distinct implications for early-Universe cosmology and for experimental searches for long-lived particles. In addition to collider probes, energy injection from the $A' \to 3\gamma$ channel can lead to spectral distortions of the Cosmic Microwave Background, providing complementary cosmological sensitivity to this otherwise elusive decay mode.
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