Neutrino signals from electroweak bremsstrahlung in solar WIMP annihilation

Abstract

Bremsstrahlung of $W$ and $Z$ gauge bosons, or photons, can be an important dark matter annihilation channel. In many popular models in which the annihilation to a pair of light fermions is helicity suppressed, these bremsstrahlung processes can lift the suppression and thus become the dominant annihilation channels. The resulting dark matter annihilation products contain a large, energetic, neutrino component. We consider solar WIMP annihilation in the case where electroweak bremsstrahlung dominates, and calculate the resulting neutrino spectra. The flux consists of primary neutrinos produced in processes such as $\chi\chi\rightarrow \bar{\nu}\nu Z$ and $\chi\chi\rightarrow \bar{\nu}\ell W$, and secondary neutrinos produced via the decays of gauge bosons and charged leptons. After dealing with the neutrino propagation and flavour evolution in the Sun, we consider the prospects for detection in neutrino experiments on Earth. By comparing our signal with that for annihilation to $W^+W^-$, we show that the detection prospects for the bremsstrahlung annihilation channel are favourable.