Solar Gamma Rays Powered by Secluded Dark Matter

arXiv:0910.1567v1 [hep-ph] 8 Oct 2009 Solar Gamma Rays Powered by Secluded Dark Matter Brian Batell (a), Maxim Pospelov (a,b), Adam Ritz (b), and Yanwen Shang (a) (a)Perimeter Institute for Theoretical Physics, Waterloo, ON, N2J 2W9, Canada (b)Department of Physics and Astronomy, University of Victoria, Victoria, BC, V8P 1A1 Canada Abstract Secluded dark matter models, in which WIMPs annihilate first into metastable mediators, can present novel indirect detection signatures in the form of gamma rays and fluxes of charged particles arriving from directions correlated with the centers of large astrophysical bodies within the solar system, such as the Sun and larger planets. This naturally occurs if the mean free path of the mediator is in excess of the solar (or planetary) radius. We show that existing constraints from water Cerenkov detectors already provide a novel probe of the parameter space of these models, complementary to other sources, with significant scope for future improvement from high angular resolution gamma-ray telescopes such as Fermi-LAT. Fluxes of charged particles produced in mediator decays are also capable of contributing a significant solar system component to the spectrum of energetic electrons and positrons, a possibility which can be tested with the directional and timing information of PAMELA and Fermi. October 2009 1. Introduction The search for weakly interacting massive particles (WIMPs) as a component of non-baryonic dark matter has become a focal point of modern particle physics [1]. There are several complementary experimental and observational approaches to WIMP detection [2]. Direct detection experiments probe the terrestrial scattering of WIMPs with nuclei and typically require low radiation environments to keep backgrounds under control. High energy colliders such as the Tevatron and the LHC offer the possibility of producing WIMPs and measuring their properties in the laboratory, provided the challenging missing energy signatures can be disentangled. Indirect searches for dark matter annihilating into gamma and cosmic rays in the galactic halo are also promising, although susceptible to various, often uncertain, astrophysical backgrounds. Finally, neutrino telescopes such as Super-Kamiokande and Ice Cube can search for indirect evidence of the annihilation of WIMPs captured in the core of the Sun and the Earth, in the form of an observable muon signature arising from neutrino charged current scattering in the detector. While the latter two examples are well-known indirect signatures for any thermal relic WIMP dark matter candidate, more generic WIMPs forming part of a larger dark sector can lead to further novel signatures. In this paper, we demonstrate that models of secluded dark matter [3] present an additional observational possibility: high-energy gamma rays and charged particles arriving from a direction tightly correlated with the centers of the Sun, Earth and other planets. Such novel signatures can be effectively probed with the powerful new generation of gamma ray telescopes. The primary feature of secluded models of dark matter [3] is a two-stage dark matter annihilation process: WIMPs annihilate first into metastable mediators, which subsequently decay into Standard Model (SM) states. This breaks the more-or-less rigid link between the size of the WIMP annihilation and WIMP-nucleus scattering cross sections. It has been shown that a small mass for the mediator allows for new phenomenological possibilities in the form of enhanced WIMP annihilation at small velocities [4, 5] that may help to explain various astrophysical anomalies, e.g. the positron excess observed by PAMELA above 10 GeV [6] and perhaps the unexpectedly hard electron spectrum observed by Fermi above a few hundred GeV [7]. Furthermore, a relatively small mediator mass kinematically removes heavy SM particles from the final state [4, 5], reconciling these effects with the absence of any enhancement in the cosmic ray anti-proton signal [8]. The lifetime of the mediator is essentially a free parameter, limited only by the Big Bang Nucleosynthesis bounds of τ <∼1 s. If this lifetime is rather long, the decay of the mediator will occur a long distance away from the point of the original WIMP annihilation. Denoting the WIMP particle χ and the mediator particle V , assuming χχ →2V as the main annihilation channel, and taking mV ≪mχ, we arrive at the following estimate for the mediator travel distance: L = cτV γV = 3 × 106 km × τV 0.01 s × γV 103. (1) Such large boosts γV = mχ/mV are easily achieved if the the dark matter mass is near the electroweak scale and the mediator mass is below a GeV. With regard to the annihilation of WIMPs captured within the Sun, one can see that this distance may very well exceed the solar radius (R⊙= 6.96 × 105 km) in which case, unlike conventional WIMPs, most of the decay products will not be absorbed. For somewhat shorter lifetimes, the interesting 1 mediator ν γ, e, µ ... γ, e

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