PAMELA, DAMA, INTEGRAL and Signatures of Metastable Excited WIMPs

TL;DR

The paper argues that dark matter models with a GeV-scale mediator naturally host metastable, excited WIMP states with splittings in the MeV–hundreds keV range, offering explanations for PAMELA, DAMA, and INTEGRAL via iDM and XDM mechanisms. It develops the cosmological and phenomenological implications of these states, including relic fractions, cosmological lifetimes, and a range of decay channels, and proposes concrete field-theory realizations with SU(2) multiplets and light mediators. A central prediction is that down-scattering signals at direct-detection experiments can be prominent and often reside at higher recoil energies, motivating expanded search windows and the use of light targets (Ar/Si) to probe MeV-scale excitations; in some scenarios such signals could also be visible in Ge/Xe detectors. The work further outlines how adding light scatterers can modulate excited-state populations, enabling lighter WIMPs to explain INTEGRAL, and emphasizes that future experiments must explore high-energy nuclear and electronic deexcitation channels to fully test these models.

Abstract

Models of dark matter with ~ GeV scale force mediators provide attractive explanations of many high energy anomalies, including PAMELA, ATIC, and the WMAP haze. At the same time, by exploiting the ~ MeV scale excited states that are automatically present in such theories, these models naturally explain the DAMA/LIBRA and INTEGRAL signals through the inelastic dark matter (iDM) and exciting dark matter (XDM) scenarios, respectively. Interestingly, with only weak kinetic mixing to hypercharge to mediate decays, the lifetime of excited states with delta < 2 m_e is longer than the age of the universe. The fractional relic abundance of these excited states depends on the temperature of kinetic decoupling, but can be appreciable. There could easily be other mechanisms for rapid decay, but the consequences of such long-lived states are intriguing. We find that CDMS constrains the fractional relic population of ~100 keV states to be <~ 10^-2, for a 1 TeV WIMP with sigma_n = 10^-40 cm^2. Upcoming searches at CDMS, as well as xenon, silicon, and argon targets, can push this limit significantly lower. We also consider the possibility that the DAMA excitation occurs from a metastable state into the XDM state, which decays via e+e- emission, which allows lighter states to explain the INTEGRAL signal due to the small kinetic energies required. Such models yield dramatic signals from down-scattering, with spectra peaking at high energies, sometimes as high as ~1 MeV, well outside the usual search windows. Such signals would be visible at future Ar and Si experiments, and may be visible at Ge and Xe experiments. We also consider other XDM models involving ~ 500 keV metastable states, and find they can allow lighter WIMPs to explain INTEGRAL as well.

Figures (8)

• Figure 1: Contour plots for the fractional abundance of the excited state after freeze-out, plotted against $n_s/n_{\chi^*} \langle \sigma_{ex} v \rangle$ (assuming the number of scatterers $n_s$ is exponentially suppressed by $e^{-\delta/T}$), and the excitation gap, $\delta$. We consider a WIMP mass of $100$ GeV ($500$ GeV) on the left (right).
• Figure 2: On the left is a contour plot for the fractional abundance of the excited state after freeze-out, plotted against $n_s/n_\chi \langle \sigma_{ex} v \rangle$ (assuming the number of scatterers $n_s$ is not exponentially suppressed by $e^{-\delta/T}$), and the excitation gap, $\delta$. On the right we assume $n_s$ is exponentially suppressed by $e^{-\delta/T}$ as in Fig. \ref{['fig:FractionExp']}, but with kinetic decoupling at $T_{kd} = 1$ GeV. In both cases the WIMP mass is fixed at $500$ GeV.
• Figure 3: On the left pane we show the splittings induced in the $SU(2)$ triplet by radiative corrections, as well as all the relevant couplings. The mixing of the $w_1$ gauge boson to the SM hypercharge is further suppressed compared with the mixing of $w_2$ and $w_3$. This leads to the possibility of deexcitation processes that can be seen in direct detection experiments. The inelastic scattering off nuclei is depicted on the right.
• Figure 4: The spectrum resulting from the Lagrangian in Eq. (\ref{['eqn:up-downLag']}) is shown on the left pane together with the relevant couplings. The up-down scattering associated with INTEGRAL is shown on the right. Notice that dark charge is softly broken by the condensation of $\phi^\prime$, hence allowing the seemingly charge violating transitions.
• Figure 5: In the left pane we depict the recoil energy spectrum expected in CDMS for a deexcitation transition with $|\delta| = 100$ keV. On the right we show the corresponding predicted number of counts observed in CDMS as a function of the deexcitation energy gap, $|\delta|$. The WIMP-nucleon cross-section $\times$ fractional abundance was taken to be $F \sigma = 10^{-42}~\mathrm{cm}^2$. The WIMP mass is 100 GeV, 300 GeV, and 500 GeV, from top to bottom. The horizontal line at 5.3 counts marks the 90% Poisson confidence upper limit on the expected number of signal events.