Title: First Search for Dark Matter Annihilation in the Sun Using the ANTARES Neutrino Telescope
Authors: ANTARES Collaboration: S. Adrián-Martinez, I. Al Samarai, A. Albert, et al.
First Author Institution: Institut d’Investigació per a la Gestió Integrada de les Zones Costaneres (IGIC) – Universitat Politècnica de València.
The background, always the backgroundAs usual in this business, one needs to be careful in distinguishing signal from background. In this particular case, cosmic rays entering the Earth atmosphere can produce downgoing muons and both downgoing and upgoing neutrinos that could mimic the muons from dark matter annihilation. The most significant contribution to the background comes from the downgoing atmospheric muons. To avoid them, the best strategy is to only take into account measurements that were triggered at night, when the muons from dark matter annihilation come from the opposite direction to the background, from below the detector.Moreover, to increase the signal-to-background ratio, one can place a cut on the inferred direction of the muons (not the original neutrino!) with respect to that of the Sun. This cut depends on the energy of the muon: at high energies, the direction of the muon and the neutrino are more similar than for low energy muons. The trade-off is that a more stringent cut on the direction gives a cleaner signal, but at the same time it removes information from the low energy muons. Nevertheless, there is always a remaining background signal that you need to model to see if the data are in excess of (which would be a detection of dark matter annihilation) or consistent with the background.ResultsFrom January 2007 to December 2008, the experiment took an effective total of 294.6 days of data. We reproduce here Figure 4 in the paper, where the main results are presented. This figure shows the distribution of separations between the inferred direction of the muon and the Sun. The number of muons coming from the direction of the Sun is consistent with the background expectation. There is no detection of dark matter annihilation in the Sun by ANTARES. In the next section, we explore what constraints can be placed on the properties and models of dark matter particles using these results. If you are brave enough, carry on reading!
For the brave at heart: Constraints on SUSY parameter space
What do these results tell us about the dark matter particle candidates? Different models for dark matter particles predict different production mechanisms for neutrinos. The Minimal Supersymmetry models (MSSM) considered in this work predict that the neutrino signature comes mostly from the decay of tau particles, W bosons or bottom quarks that are produced when the dark matter particles annihilate.The measurements of the previous section allow the authors to place an upper limit on the neutrino flux from dark matter annihilation in the Sun. This is to say, if there were dark matter annihilation happening in the interior of the Sun, the neutrino flux from this process would have to be below a certain threshold to be consistent with the observations by ANTARES.This information comes in handy. In the interior of the Sun, the rate of dark matter annihilation depends on the rate of capture of dark matter: you need to be capturing dark matter to have anything to annihilate. This is just an equilibrium argument, but a very useful one, since it allows us to set constraints on the cross-section of the interaction dark matter with nucleons in the Sun, most abundantly, protons. The cross-section has a spin-dependent (SD) and a spin-independent (SI) contribution, based on whether the interaction depends or not on the spin of the nucleon. We already have very stringent limits on the SI cross-section of the interactions by means of direct detection experiments. Neutrino telescopes are thus ideal for constraining SD WIMP-proton cross-sections. For a more technical explanation, see this paper.In the left panels of Figure 6 (reproduced below), the ANTARES collaboration presents their bounds on the parameter space defined by the SD cross-section of the interaction and the WIMP mass. Overall, even if we have not had a detection of annihilating dark matter in the Sun, the constraints on the parameter space of SUSY models are getting tighter day by day.