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The PAMELA Positron Excess from Annihilations into a Light Boson

Ilias Cholis, Douglas P. Finkbeiner, Lisa Goodenough, Neal Weiner

TL;DR

The paper tackles the PAMELA positron excess and its lack of antiproton signal by proposing dark matter annihilation into a new light mediator φ that decays predominantly to leptons. Using GALPROP with an Einasto halo, it demonstrates that χχ→φφ followed by φ decays such as φ→e⁺e⁻ or φ→μ⁺μ⁻ can provide excellent fits to the observed spectrum, especially when Sommerfeld enhancement is invoked to boost the annihilation cross section. It finds that different φ decay modes correspond to different preferred WIMP masses (≈100 GeV for e⁺e⁻, ≳400 GeV for μ⁺μ⁻) but all are viable within the framework, and predicts potential continuation of the rise at higher energies with implications for related observations (WMAP haze, Fermi/GLAST, HESS) and possible connections to other cosmic-ray anomalies. The scenario thus offers a simple, leptophilic dark matter explanation consistent with current constraints and testable with forthcoming data.

Abstract

Recently published results from the PAMELA experiment have shown conclusive evidence for an excess of positrons at high (~ 10 - 100 GeV) energies, confirming earlier indications from HEAT and AMS-01. Such a signal is generally expected from dark matter annihilations. However, the hard positron spectrum and large amplitude are difficult to achieve in most conventional WIMP models. The absence of any associated excess in anti-protons is highly constraining on any model with hadronic annihilation modes. We revisit an earlier proposal, whereby the dark matter annihilates into a new light (<~GeV) boson phi, which is kinematically constrained to go to hard leptonic states, without anti-protons or pi0's. We find this provides a very good fit to the data. The light boson naturally provides a mechanism by which large cross sections can be achieved through the Sommerfeld enhancement, as was recently proposed. Depending on the mass of the WIMP, the rise may continue above 300 GeV, the extent of PAMELA's ability to discriminate electrons and positrons.

The PAMELA Positron Excess from Annihilations into a Light Boson

TL;DR

The paper tackles the PAMELA positron excess and its lack of antiproton signal by proposing dark matter annihilation into a new light mediator φ that decays predominantly to leptons. Using GALPROP with an Einasto halo, it demonstrates that χχ→φφ followed by φ decays such as φ→e⁺e⁻ or φ→μ⁺μ⁻ can provide excellent fits to the observed spectrum, especially when Sommerfeld enhancement is invoked to boost the annihilation cross section. It finds that different φ decay modes correspond to different preferred WIMP masses (≈100 GeV for e⁺e⁻, ≳400 GeV for μ⁺μ⁻) but all are viable within the framework, and predicts potential continuation of the rise at higher energies with implications for related observations (WMAP haze, Fermi/GLAST, HESS) and possible connections to other cosmic-ray anomalies. The scenario thus offers a simple, leptophilic dark matter explanation consistent with current constraints and testable with forthcoming data.

Abstract

Recently published results from the PAMELA experiment have shown conclusive evidence for an excess of positrons at high (~ 10 - 100 GeV) energies, confirming earlier indications from HEAT and AMS-01. Such a signal is generally expected from dark matter annihilations. However, the hard positron spectrum and large amplitude are difficult to achieve in most conventional WIMP models. The absence of any associated excess in anti-protons is highly constraining on any model with hadronic annihilation modes. We revisit an earlier proposal, whereby the dark matter annihilates into a new light (<~GeV) boson phi, which is kinematically constrained to go to hard leptonic states, without anti-protons or pi0's. We find this provides a very good fit to the data. The light boson naturally provides a mechanism by which large cross sections can be achieved through the Sommerfeld enhancement, as was recently proposed. Depending on the mass of the WIMP, the rise may continue above 300 GeV, the extent of PAMELA's ability to discriminate electrons and positrons.

Paper Structure

This paper contains 3 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Annihilations to a New Light Boson
  3. Discussion

Figures (2)

  • Figure 1: Dark matter annihilation mode, $\chi \chi \rightarrow \phi\phi$. Here $\phi$ can be a scalar or a vector. The grey circle includes all diagrams, including possible non-perturbative contributions. The subsequent decays of $\phi$, for instance to $\phi \rightarrow e^+e^-$ or $\phi \rightarrow \mu^+ \mu^-$ produce the high energy positrons observed at PAMELA.
  • Figure 2: The positron fraction as a function of energy for the four annihilation modes considered here: $\chi \chi \rightarrow \phi \phi$, followed by, (a) $\phi \rightarrow e^+e^-$, (b) $\phi \rightarrow \mu^+ \mu^-$, (c) $\phi \rightarrow e^+e^- ,\, \mu^+ \mu^-$ (1:1), (d) $\phi \rightarrow \pi^+ \pi^-$ . The boost factor is defined relative to a cross section $\langle \sigma v \rangle = 3 \times 10^{-26} {\rm cm^3 s^{-1}}$ and $\rho_0 = 0.3 {\, \rm GeV cm^{-3}}$. Such a boost reasonably can arise from a Sommerfeld enhancement, without appeals to substructure.