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Phenomenology of Dark Matter annihilation into a long-lived intermediate state

Ira Z. Rothstein, Thomas Schwetz, Jure Zupan

TL;DR

This work introduces a dark matter annihilation scenario where χχ→φφ, followed by φ’s long lifetime decay into SM particles, distributes SM production over galactic distances and mitigates gamma-ray constraints that challenge standard annihilation explanations of the PAMELA/ATIC/FERMI e± excesses. The authors formulate the LLP framework, derive a photon flux that interpolates between annihilation and decay via a decay length λ, and define an effective density ρ_eff that governs the SM source term. Applying this to cosmic-ray data, they demonstrate that for λ on the order of a few to ~10 kpc the electron-positron signals can be fit without overproducing gamma rays observed by HESS in the GC/GR regions, with best-fit DM masses around the TeV scale and ⟨σv⟩ near 10^{-22} cm^3 s^{-1}. The study also discusses neutrino constraints, relic-density considerations, and potential LHC signatures, suggesting a link between high-scale physics and the LLP decay and offering distinguishing observational features such as smoothed angular distributions and clump suppression. The proposed Λ ~ 10^{13} GeV scale for the LLP decay aligns intriguingly with neutrino mass seesaw scales, hinting at a broader connection between DM phenomenology and leptonic mass generation.

Abstract

We propose a scenario where Dark Matter (DM) annihilates into an intermediate state which travels a distance $λ\equiv v/Γ$ on the order of galactic scales and then decays to Standard Model (SM) particles. The long lifetime disperses the production zone of the SM particles away from the galactic center and hence, relaxes constraints from gamma ray observations on canonical annihilation scenarios. We utilize this set up to explain the electron and positron excesses observed recently by PAMELA, ATIC, and FERMI. While an explanation in terms of usual DM annihilations seems to conflict with gamma ray observations, we show that within the proposed scenario, the PAMELA/ATIC/FERMI results are consistent with the gamma ray data. The distinction from decay scenarios is discsussed and we comment on the prospects for DM production at LHC. The typical decay length $λ\gtrsim 10$ kpc of the intermediate state can have its origin from a dimension six operator suppressed by a scale $Λ\sim 10^{13}$ GeV, which is roughly the seesaw scale for neutrino masses.

Phenomenology of Dark Matter annihilation into a long-lived intermediate state

TL;DR

This work introduces a dark matter annihilation scenario where χχ→φφ, followed by φ’s long lifetime decay into SM particles, distributes SM production over galactic distances and mitigates gamma-ray constraints that challenge standard annihilation explanations of the PAMELA/ATIC/FERMI e± excesses. The authors formulate the LLP framework, derive a photon flux that interpolates between annihilation and decay via a decay length λ, and define an effective density ρ_eff that governs the SM source term. Applying this to cosmic-ray data, they demonstrate that for λ on the order of a few to ~10 kpc the electron-positron signals can be fit without overproducing gamma rays observed by HESS in the GC/GR regions, with best-fit DM masses around the TeV scale and ⟨σv⟩ near 10^{-22} cm^3 s^{-1}. The study also discusses neutrino constraints, relic-density considerations, and potential LHC signatures, suggesting a link between high-scale physics and the LLP decay and offering distinguishing observational features such as smoothed angular distributions and clump suppression. The proposed Λ ~ 10^{13} GeV scale for the LLP decay aligns intriguingly with neutrino mass seesaw scales, hinting at a broader connection between DM phenomenology and leptonic mass generation.

Abstract

We propose a scenario where Dark Matter (DM) annihilates into an intermediate state which travels a distance on the order of galactic scales and then decays to Standard Model (SM) particles. The long lifetime disperses the production zone of the SM particles away from the galactic center and hence, relaxes constraints from gamma ray observations on canonical annihilation scenarios. We utilize this set up to explain the electron and positron excesses observed recently by PAMELA, ATIC, and FERMI. While an explanation in terms of usual DM annihilations seems to conflict with gamma ray observations, we show that within the proposed scenario, the PAMELA/ATIC/FERMI results are consistent with the gamma ray data. The distinction from decay scenarios is discsussed and we comment on the prospects for DM production at LHC. The typical decay length kpc of the intermediate state can have its origin from a dimension six operator suppressed by a scale GeV, which is roughly the seesaw scale for neutrino masses.

Paper Structure

This paper contains 12 sections, 47 equations, 7 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Gamma rays from DM annihilations via a long-lived state
  3. General discussion
  4. The effective DM profile for non-relativistic intermediate state $\phi$
  5. Application to recent cosmic ray data
  6. The electron-positron signals
  7. Gamma ray constraints from HESS GC and GR observations
  8. Neutrinos from the galactic center
  9. Can $\chi$ be a thermal relic?
  10. Discussion and conclusions
  11. INJECTION SPECTRA
  12. Differential equation for $I(\lambda_D)$

Figures (7)

  • Figure 1: Upper panel: effective DM density profiles as defined in Eq. \ref{['eq:rho_eff']} for various values of $\lambda$. Lower panel, effective DM profiles relative to the NFW profile (corresponding to $\lambda = 0$).
  • Figure 2: Interpolation between the SM particle source terms for DM annihilation and decay. The dashed curves show $\rho^2_\mathrm{NFW}(r)/\rho_\odot^2$ for annihilation and $\rho_\mathrm{NFW}(r)/\rho_\odot$ for decay, whereas the solid curve corresponds to $\rho_\mathrm{eff}^2(r)/\rho_\odot^2$ for DM annihilation into a long lived intermediate state with a decay length of $\lambda = 10$ kpc.
  • Figure 3: $J$-factors for GC and GR (background subtracted as in Aharonian:2006au) as a function of $\lambda$. Dashed lines correspond to the $J$-factors for $\lambda=0$.
  • Figure 4: Allowed regions at $3\sigma$ for PAMELA (gray), PAMELA+HESS+FERMI (red), and PAMELA+HESS+FERMI (dark red) and the constraints from HESS photon observations of the galactic center (GC) and galactic ridge (GR) for $\lambda =0, 1, 10, 100$ kpc. The solid (dashed) curves show HESS photon constraints at 90% CL with (at $3\sigma$ without) including a power law background in the fit, see text for details. The regions above the curves are excluded.
  • Figure 5: PAMELA data on the positron fraction (left) and the electron-positron data (right) compared to the predicted spectra for $\lambda=0$ (blue curves) and $\lambda=10$ kpc (black curves) at the best fit values given in Tab. \ref{['tab:best-fit']}. Solid curves correspond to signal + background, whereas with the dashed curves we show background and signal (right panels only) components separately. Upper panels are for PAMELA+FERMI+HESS, lower panels for PAMELA+ATIC+HESS. The green curves in the left plots show the spectrum at the best fit to only PAMELA data for $\lambda = 10$ kpc.
  • ...and 2 more figures