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 ${\ensuremath{\chi}}_{0}$, ${\ensuremath{\chi}}_{1}$, and ${\ensuremath{\chi}}_{2}$, with masses ${M}_{0}\ensuremath{\sim}500\text{ }\text{ }\mathrm{GeV}$, ${M}_{1}\ensuremath{\lesssim}{M}_{0}+2{m}_{e}$, and ${M}_{2}={M}_{1}+\ensuremath{\delta}M\ensuremath{\gtrsim}{M}_{0}+2{m}_{e}$. The small mass splitting $\ensuremath{\delta}M$ should be $\ensuremath{\lesssim}100\text{ }\text{ }\mathrm{keV}$. In addition, we require at least two new gauge bosons (preferably three), with masses $\ensuremath{\sim}100\text{ }\text{ }\mathrm{MeV}$. With this spectrum, ${\ensuremath{\chi}}_{1}$ is stable but can be excited to ${\ensuremath{\chi}}_{2}$ 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 ${\ensuremath{\chi}}_{2}$ decays to ${\ensuremath{\chi}}_{0}$ and nonrelativistic ${e}^{+}{e}^{\ensuremath{-}}$, 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 ${\ensuremath{\chi}}_{1}$ (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 ${Z}^{\ensuremath{'}}$ may also be necessary to get the right relic density and PAMELA/ATIC signals.

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