  We analyze the Sun as a source for the indirect detection of dark matter
through a search for gamma rays from the solar disk. Capture of dark matter by
elastic interactions with the solar nuclei followed by annihilation to
long-lived mediators can produce a detectable gamma-ray flux. We search three
years of data from the High Altitude Water Cherenkov Observatory and find no
statistically significant detection of TeV gamma-ray emission from the Sun.
Using this, we constrain the spin-dependent elastic scattering cross section of
dark matter with protons for dark matter masses above 1 TeV, assuming an
unstable mediator with a favorable lifetime. The results complement constraints
obtained from Fermi-LAT observations of the Sun and together cover WIMP masses
between 4 GeV and $10^6$ GeV. The cross section constraints for mediator decays
to gamma rays can be as strong as $\sim10^{-45}$ cm$^{-2}$, which is more than
four orders of magnitude stronger than current direct-detection experiments for
1 TeV dark matter mass. The cross-section constraints at higher masses are even
better, nearly 7 orders of magnitude better than the current direct-detection
constraints for 100 TeV dark matter mass. This demonstration of sensitivity
encourages detailed development of theoretical models in light of these
powerful new constraints.

High Energy Physics - Phenomenology;High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics