Cosmic Rays from Leptophilic Dark Matter Decay via Kinetic Mixing

TL;DR

The paper investigates leptophilic dark matter decay arising from a hidden U(1)_X kinetically mixing with the Standard Model, within a supersymmetric framework. By analyzing decays of either the visible-sector neutralino or the hidden gaugino and computing the resulting gamma-ray, positron, and antiproton fluxes through a two-zone diffusion propagation model, it identifies viable regions (notably light sleptons in mSUGRA-like setups) where the PAMELA positron excess can be explained without violating antiproton and gamma-ray constraints. The study predicts correlated signatures in the extragalactic gamma-ray background and highlights cascade decays as a potential source of multi-peak structures, while noting that reproducing the ATIC double-peak feature is challenging under standard propagation assumptions. Overall, the work highlights a concrete, testable mechanism linking hidden-sector physics to cosmic-ray observations and points to Fermi as a key probe of the predicted gamma-ray signal.

Abstract

If interpreted in terms of decaying dark matter, the steep rise in the positron fraction of cosmic rays above 10 GeV, as observed by the PAMELA experiment, suggests an underlying production mechanism that favors leptonic channels. We consider a scenario where a portion of the dark matter is made of the gauginos of an unbroken hidden-sector U(1), which interact with the visible sector only through a tiny kinetic mixing. The second component of the dark matter is made of neutralinos, and depending on the mass spectrum, the lightest neutralino or the hidden gaugino becomes unstable and subject to decay. We analyze the cosmic rays, namely the contributions to the positron, the extragalactic gamma-ray and the antiproton flux, which potentially result from these decays and demonstrate that the production of antiprotons can be naturally suppressed. Furthermore, we briefly discuss the apparent double-peak structure of the ATIC data in light of cascade-decaying hidden gauginos, as well as possible signatures at Fermi.

Figures (5)

• Figure 1: Positron fraction, total electron+positron, extragalactic gamma-ray and antiproton flux of a decaying neutralino $\chi_1^0$ as predicted for our exemplary mSUGRA scenario. The used branching ratios are shown in Tab. \ref{['tab:DNmSUGRA']}. The mass of the decaying neutralino is $301\,{\rm GeV}$, the hidden gaugino mass varies between $1\,{\rm GeV}$ (solid), $50\,{\rm GeV}$ (dotted), $100\,{\rm GeV}$ (dashed) and $150\,{\rm GeV}$ (dot-dashed). We used the MED propagation model. In the lower left plot, the grey lines indicate the flux without background. In the lower right plot, we only show the flux without background.
• Figure 2: Positron fraction, total electron+positron flux and extragalactic gamma-ray flux for an idealized, three-body decaying bino-like neutralino. We neglect effects from $h^0$ and $Z^0$ bosons and assume pure democratic three-body decay into charged lepton pairs. The masses of the neutralino and the hidden gaugino are $500\,{\rm GeV}$ and $150\,{\rm GeV}$ (thick solid lines) or $1850\,{\rm GeV}$ and $300\,{\rm GeV}$ (thick dotted lines), respectively. The thin lines show the predictions when the decay into the tau-channel is neglected. The mass of the right-handed sleptons is assumed to be by a factor $1.1$ larger than the neutralino mass.
• Figure 3: Positron fraction, extragalactic gamma-ray flux, antiproton flux and total electron + positron flux from the decay of a hidden gaugino as predicted by our mSUGRA scenario. The branching ratios are shown in Tab. \ref{['tab:DHmSUGRA']}. The mass of the hidden gaugino varies between $600\,{\rm GeV}$ (solid), $800\,{\rm GeV}$ (dotted), $1000\,{\rm GeV}$ (dashed) and $1200\,{\rm GeV}$ (dot-dashed).
• Figure 4: Energy spectrum of positrons from an idealized cascade-decaying hidden gaugino. Only two-body decay into right-handed slepton/lepton pairs is taken into account. The slepton subsequently decays into the lightest neutralino. The spectrum exhibits two pronounced peaks, which we denote by $E_h$ and $E_l$. We show plots for a lightest neutralino with mass $150\,{\rm GeV}$ (solid) and with $1\,{\rm TeV}$ (dashed). The position of the peaks is fixed to $E_h=700\,{\rm GeV}$ and $E_l=200\,{\rm GeV}$, as suggested by the ATIC data. The masses of the right-handed sleptons follow then from Eq. \ref{['eqn:NLSPmass']}. We also indicate the part of the positrons that comes solely from the tau/stau decay channel (blue area).
• Figure 5: Positron fraction and total electron+positron flux for an idealized cascade-decaying hidden gaugino. Like in Fig. \ref{['fig:DHspectrum']}, only leptonic decay modes are taken into account. We assume democratic decay into the three right-handed charged sleptons (thick lines), or into only the selectron and smuon (thin lines). The mass of the lightest neutralino varies between $150\,{\rm GeV}$ (solid) and $1\,{\rm TeV}$ (dotted). We show the plots for the propagation models MED (upper plots) and M2 (lower plots) of Ref. DLD+08 (see Tab. \ref{['tab:param-positron']}).