New twist on excited dark matter: Implications for INTEGRAL, PAMELA/ATIC/PPB-BETS, DAMA

We show that the 511 keV gamma ray excess observed by INTEGRAL/SPI can be more robustly explained by exciting dark matter (DM) at the center of the galaxy, if there is a peculiar spectrum of DM states χ₀, χ₁, and χ₂, with masses M₀∼500GeV, M₁≲M₀+2m_e, and M₂=M₁+δM≳M₀+2m_e. The small mass splitting δM should be ≲100keV. In addition, we require at least two new gauge bosons (preferably three), with masses ∼100MeV. With this spectrum, χ₁ is stable but can be excited to χ₂ by low-velocity DM scatterings near the Galactic center, which are Sommerfeld-enhanced by two of the 100 MeV gauge boson exchanges. The excited state χ₂ decays to χ₀ and nonrelativistic e⁺e^-, mediated by the third gauge boson, which mixes with the photon and Z. Although such a small 100 keV splitting has been independently proposed for explaining the DAMA annual modulation through the inelastic DM mechanism, the need for stability of χ₁ (and hence sequestering it from the standard model) implies that our scenario cannot account for the DAMA signal. It can, however, address the PAMELA/ATIC positron excess via DM annihilation in the galaxy, and it offers the possibility of a sharper feature in the ATIC spectrum relative to previously proposed models. The data are consistent with three new gauge bosons, whose couplings fit naturally into a broken SU(2) gauge theory where the DM is a triplet of the SU(2). We propose a simple model in which the SU(2) is broken by new Higgs triplet and 5-plet vacuum expectation values, giving rise to the right spectrum of DM and mixing of one of the new gauge bosons with the photon and Z boson. A coupling of the DM to a heavy Z^' may also be necessary to get the right relic density and PAMELA/ATIC signals.