A new twist on excited dark matter: implications for INTEGRAL, PAMELA/ATIC/PPB-BETS, DAMA

TL;DR

This work targets the INTEGRAL 511 keV line by proposing excited dark matter with three nearly degenerate states (chi_0, chi_1, chi_2) and a small mass splitting DeltaM ≲ 100 keV. The mechanism relies on Sommerfeld-enhanced, off-diagonal scatterings chi_1 chi_1 → chi_2 chi_2 mediated by light ~100 MeV gauge bosons, followed by chi_2 → chi_0 e^+ e^− via a third mediator mixing with SM gauge bosons; a triplet SU(2) dark sector with Higgs triplet and 5-plet VEVs yields the required spectrum and couplings. While chi_1 stability rules out DAMA explanations, the framework can accommodate PAMELA/ATIC signals and potentially sharpen the ATIC feature, especially if a leptophilic Z' is invoked to adjust relic density and annihilation channels. The paper also discusses cosmological implications, including possible nonthermal production mechanisms to avoid early-universe depletion of chi_1, and outlines concrete model-building steps to realize the spectrum and couplings in a natural, gauge-theoretic setting.

Abstract

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 chi_0, chi_1 and chi_2, with masses M_0 ~ 500 GeV, M_1 <~ M_0 + 2 m_e, and M_2 = M_1 + delta M >~ M_0 + 2 m_e. The small mass splitting delta M should be <~ 100 keV. In addition, we require at least two new gauge bosons (preferably three), with masses ~100 MeV. With this spectrum, chi_1 is stable, but can be excited to 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 chi_2 decays to chi_0 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 chi_1 (and hence seqestering 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 VEV's, 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.

Figures (9)

• Figure 1: Left: DM spectrum needed in present work to account for the INTEGRAL/SPI observations; right: spectrum suggested by ref. AH.
• Figure 2: Left: Sommerfeld-enhanced scattering $\chi_1\chi_1\to\chi_2\chi_2$. Right: decay $\chi_2\to\chi_0 e^+ e_-$.
• Figure 3: Left: Enhancement factor $I'$, eq. (\ref{['Ieq']}), as a function of $\Gamma \equiv M_1\alpha^2/2\delta M$, in the disfavored case $\delta M = m_e$, where the new gauge boson is taken to be massless and $M$ is optimized. The maximum enhancement is too small to match observations. Right: integrands for successive partial waves contributing to $I'$, versus $u/a$, where $u = v^2/v_0^2$ and $a = 2\delta M/M v_0^2$. One would need significant contributions from many more partial waves to make $I'$ large enough.
• Figure 4: Enhancement factor $I\equiv e^{-u_t}I' = e^{-a} I'$ versus $a \equiv 2 \delta M / (M v_0^2)$ for the small $\delta M$ model, with $\Gamma=10$, $\eta=1.2$. Successive contributions from partial waves $l=0-10$ are shown. $I=1.3$ (dotted line) is the value indicated by the INTEGRAL observations.
• Figure 5: Energy spectrum of $e^+e^-$ observed by ATIC (solid circles), taken from ref. atic.