The Scientific Reach of Multi-Ton Scale Dark Matter Direct Detection Experiments

The next generation of large scale WIMP direct detection experiments have the potential to go beyond the discovery phase and reveal detailed information about both the particle physics and astrophysics of dark matter. We report here on early results arising from the development of a detailed numerical code modeling the proposed DARWIN detector, involving both liquid argon and xenon targets. We incorporate realistic detector physics, particle physics and astrophysical uncertainties and demonstrate to what extent two targets with similar sensitivities can remove various degeneracies and allow a determination of dark matter cross sections and masses while also probing rough aspects of the dark matter phase space distribution. We find that, even assuming dominance of spin-independent scattering, multi-ton scale experiments still have degeneracies that depend sensitively on the dark matter mass, and on the possibility of isospin violation and inelasticity in interactions. We find that these experiments are best able to discriminate dark matter properties for dark matter masses less than around 200 GeV. In addition, and somewhat surprisingly, the use of two targets gives only a small improvement (aside from the advantage of different systematics associated with any claimed signal) in the ability to pin down dark matter parameters when compared with one target of larger exposure.

Gamma-ray Constraints on Dark Matter Annihilation into Charged Particles

Dark matter annihilation into charged particles is necessarily accompanied by gamma rays, produced via radiative corrections. Internal bremsstrahlung from the final state particles can produce hard gamma rays up to the dark matter mass, with an approximately model-independent spectrum. Focusing on annihilation into electrons, we compute robust upper bounds on the dark matter self annihilation cross section $<sigma_A v >_{e^+e^-}$ using gamma ray data from the Milky Way spanning a wide range of energies, $sim10^{-3} - 10^4$ GeV. We also compute corresponding bounds for the other charged leptons. We make conservative assumptions about the astrophysical inputs, and demonstrate how our derived bounds would be strengthened if stronger assumptions about these inputs are adopted. The fraction of hard gamma rays near the endpoint accompanying annihilation to $e^+e^-$ is only a factor of $alt 10^2$ lower than for annihilation directly to monoenergetic gamma rays. The bound on $<sigma_A v >_{e^+e^-}$ is thus weaker than that for $<sigma_A v >_{gammagamma}$ by this same factor. The upper bounds on the annihilation cross sections to charged leptons are compared with an upper bound on the {it total} annihilation cross section defined by neutrinos.

Electroweak Bremsstrahlung in Dark Matter Annihilation

A conservative upper bound on the total dark matter (DM) annihilation rate can be obtained by constraining the appearance rate of the annihilation products which are hardest to detect. The production of neutrinos, via the process $chi chi to bar u u $, has thus been used to set a strong general bound on the dark matter annihilation rate. However, Standard Model radiative corrections to this process will inevitably produce photons which may be easier to detect. We present an explicit calculation of the branching ratios for the electroweak bremsstrahlung processes $chi chi to bar u u Z$ and $chi chi to bar u e W$. These modes inevitably lead to electromagnetic showers and further constraints on the DM annihilation cross-section. In addition to annihilation, our calculations are also applicable to the case of dark matter decay.

Conservative Constraints on Dark Matter Annihilation into Gamma Rays

Using gamma-ray data from observations of the Milky Way, Andromeda (M31), and the cosmic background, we calculate conservative upper limits on the dark matter self-annihilation cross section to monoenergetic gamma rays, <sigma_A v>_{gamma gamma}, over a wide range of dark matter masses. (In fact, over most of this range, our results are unchanged if one considers just the branching ratio to gamma rays with energies within a factor of a few of the endpoint at the dark matter mass.) If the final-state branching ratio to gamma rays, Br(gamma gamma), were known, then <sigma_A v>_{gamma gamma} / Br(gamma gamma) would define an upper limit on the total cross section; we conservatively assume Br(gamma gamma) > 10^{-4}. An upper limit on the total cross section can also be derived by considering the appearance rates of any Standard Model particles; in practice, this limit is defined by neutrinos, which are the least detectable. For intermediate dark matter masses, gamma-ray-based and neutrino-based upper limits on the total cross section are comparable, while the gamma-ray limit is stronger for small masses and the neutrino limit is stronger for large masses. We comment on how these results depend on the assumptions about astrophysical inputs and annihilation final states, and how GLAST and other gamma-ray experiments can improve upon them.