Slightly Non-Minimal Dark Matter in PAMELA and ATIC

TL;DR

This work introduces a nonminimal dark-matter framework in which a heavy stable particle $X$ annihilates to a light unstable mediator $Y$ that decays to Standard Model leptons, potentially explaining the PAMELA and ATIC lepton excesses without an antiproton surplus. The authors present a concrete scalar mediator model with a decoupled hidden sector, derive the relic abundance accounting for differences between hidden and visible sector temperatures via a factor $F$, and compute the galactic $e^\pm$ flux by solving a diffusion equation with a calibrated injection spectrum. They find that with $M \sim 700$–$800$ GeV and reasonable boost factors, the model can fit PAMELA/ATIC data and be compatible with Fermi within uncertainties, while predicting gamma-ray signals that distinguish scalar mediators from vector ones. The framework can be embedded in hidden-valley or unparticle theories, offering attractive indirect-detection prospects while keeping direct detection and collider signatures suppressed in the minimal realization.

Abstract

We present a simple model in which dark matter couples to the standard model through a light scalar intermediary that is itself unstable. We find this model has several notable features, and allows a natural explanation for a surplus of positrons, but no surplus of anti-protons, as has been suggested by early data from PAMELA and ATIC. Moreover, this model yields a very small nucleon coupling, well below the direct detection limits. In this paper we explore the effect of this model in both the early universe and in the galaxy.

Figures (6)

• Figure 1: The ratio of dark matter mass $M$ to its temperature at decoupling $T_{\mathrm{hid}, XY}$ for $F=46.6/i$ where $i=${1,2,4} from top to bottom.
• Figure 2: The energy spectrum dN/dE of the electrons as a function of the lab frame energy for M=800GeV, m=200MeV
• Figure 3: Required values of $B\lambda^2$ to generate the positron excess described in the text. The top dashed curve is the "min" model, and the solid curve is the "med" models.
• Figure 4: Positron excess below 120GeV. The dash-dot curve is $M=100{\rm ~GeV }$. The dashed curve is $M=800{\rm ~GeV }$. The bottom dotted line is the background level.
• Figure 5: The ratio of the ATIC signal to background along with the model fit for the parameters described in the text.