New ANTARES Dark Matter Annihilation Limit

The ANTARES experiment has a new preprint out setting a limit on the annihilation of dark matter in the center of the Milky Way. ANTARES is an underwater neutrino telescope located in the Mediterranean off the coast of France. It uses photomultiplier tubes placed underwater to look for Cherenkov light from particles created when neutrinos interact in seawater. The basic idea sounds similar to various cosmic ray experiments like IceCube (also looking for neutrinos) and Auger (looking for ultra high energy cosmic rays of more traditional types like protons). Rather than attempt to build an enormous detector, they just place sensors in a fairly uniform natural medium like water. This is a great way to get an enormous fiducial volume, although it is obviously harder to control than a fully man-made detector system.

Since ANTARES looks for neutrinos, the dark matter signal they look for is dark matter annihilation to a neutrino-antineutrino pair. Since WIMP dark matter is assumed to be quite heavy, each neutrino gets a momentum (and energy) equal to that of a dark matter particle. Since the neutrinos are coming out of the center of our own galaxy (about 7 kiloparsecs away), things like redshifting due to the expansion of the universe are irrelevant. The main signal from WIMP annihilation directly to neutrinos would be a nice peak around the WIMP mass. They also look for WIMP annihilation to other states that eventually lead to neutrinos, such as bottom pairs and tau pairs, but the spectra for these are not quite as obvious as for the neutrino case. There are plenty of non-dark matter neutrinos, so the analysis would have to rely on finding a signal on top of a nontrivial background. In the end, they get the best current limit from a neutrino telescope, although not really the best overall.