Dark Matter in the Sun
The minimum DM mass a detector can probe is given by
where is the targets’ mass, is the maximum velocity of DM in the galactic halo, given by the galactic escape velocity plus the Earth’s relative motion in the galaxy, and is the experiment’s recoil threshold. To probe lighter DM one has to decrease the target mass or the recoil threshold. To increase the halo’s cutoff speed is of course not an option without further assumptions, but there is a second DM population in the solar system that can be much faster than halo DM particles.
A DM particle that falls into the gravitational well of the Sun might scatter on one of the hot solar nuclei, and get accelerated, as illustrated on the side. Provided that the particle can escape the Sun, we call it reflected. For sub-GeV DM, solar reflection can provide a flux of DM particles in the solar system, which is not bound by the galactic escape velocity. Given a low-threshold experiment of sufficient exposure, this effect can be used to extend DM searches to masses below the above mentioned minimum.
In our first paper on this topic, we studied the case, where the reflected DM scatters only one single time using analytical methods. However, it will be interesting to see how the reflection flux increases, once multiple scatterings are taken into account as well.