Luminous solar neutrinos. I. Dipole portals.

Solar neutrinos upscattering inside the Earth can source unstable particles that subsequently decay inside large volume detectors (e.g., neutrino experiments). Contrary to naive expectations, when the decay length is much shorter than the radius of the Earth (rather than the length of the detector), the event rate is independent of the decay length. In this paper, we study a neutrino-dipole portal (transition dipole operator) and show that existing data from Borexino and Super-Kamiokande probes previously untouched parameter space in the 0.5-20 MeV regime, complementing recent cosmological and supernova bounds. We discuss similarities and differences with luminous dark matter and comment on future prospects for analogous signals stemming from atmospheric neutrinos. A companion paper explores an analogous mass-mixing portal. [ABSTRACT FROM AUTHOR]