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The galactic 511 keV line from electroweak scale WIMPs

Maxim Pospelov, Adam Ritz

TL;DR

The paper investigates whether electroweak-scale WIMPs with near-degenerate states can explain the galactic 511 keV line observed by INTEGRAL/SPI. It systematically tests three production channels for galactic positrons: collisional excitation of WIMPs, WIMP-nucleus recombination, and delayed decays of a metastable excited state, deriving stringent constraints for each. It shows that collisional excitation fails the unitarity bound, recombination is ruled out by terrestrial heavy-isotope limits, and only metastable-state decays with MeV-scale splitting remain viable, for which a Higgs-portal two-singlet realization can yield the required flux. The results highlight a concrete Higgs-mediated portal as a mechanism linking TeV-scale dark matter to MeV positrons and open avenues for further phenomenology and astrophysical tests.

Abstract

We consider possible mechanisms via which electroweak scale WIMPs χ^0 could provide the source of the INTEGRAL/SPI 511 keV photon flux from the galactic centre. We consider scenarios where the WIMP spectrum contains near-degeneracies, with MeV-scale splitting, and focus on three possible production mechanisms for galactic positrons: (i) collisional excitation of the WIMP to a nearby charged state, χ^0 + χ^0 -> χ^+ + χ^-, with the subsequent decay producing positrons; (ii) capture of the WIMP by nuclei in the galactic interstellar medium, χ^0 + N -> e^+ + (χ^- N); and (iii) the decay of a nearby long-lived state surviving from the Big Bang, χ^0_2 -> χ_1^0 + e^+ + e^-. We find that process (i) requires a cross-section which is significantly larger than the unitarity bound, process (ii) is allowed by unitarity, but is impractical due to terrestrial bounds on the χ-N cross-section, while process (iii) is viable and we construct a simple model realization with singlet dark matter fields interacting with the Standard Model via the Higgs sector.

The galactic 511 keV line from electroweak scale WIMPs

TL;DR

The paper investigates whether electroweak-scale WIMPs with near-degenerate states can explain the galactic 511 keV line observed by INTEGRAL/SPI. It systematically tests three production channels for galactic positrons: collisional excitation of WIMPs, WIMP-nucleus recombination, and delayed decays of a metastable excited state, deriving stringent constraints for each. It shows that collisional excitation fails the unitarity bound, recombination is ruled out by terrestrial heavy-isotope limits, and only metastable-state decays with MeV-scale splitting remain viable, for which a Higgs-portal two-singlet realization can yield the required flux. The results highlight a concrete Higgs-mediated portal as a mechanism linking TeV-scale dark matter to MeV positrons and open avenues for further phenomenology and astrophysical tests.

Abstract

We consider possible mechanisms via which electroweak scale WIMPs χ^0 could provide the source of the INTEGRAL/SPI 511 keV photon flux from the galactic centre. We consider scenarios where the WIMP spectrum contains near-degeneracies, with MeV-scale splitting, and focus on three possible production mechanisms for galactic positrons: (i) collisional excitation of the WIMP to a nearby charged state, χ^0 + χ^0 -> χ^+ + χ^-, with the subsequent decay producing positrons; (ii) capture of the WIMP by nuclei in the galactic interstellar medium, χ^0 + N -> e^+ + (χ^- N); and (iii) the decay of a nearby long-lived state surviving from the Big Bang, χ^0_2 -> χ_1^0 + e^+ + e^-. We find that process (i) requires a cross-section which is significantly larger than the unitarity bound, process (ii) is allowed by unitarity, but is impractical due to terrestrial bounds on the χ-N cross-section, while process (iii) is viable and we construct a simple model realization with singlet dark matter fields interacting with the Standard Model via the Higgs sector.

Paper Structure

This paper contains 5 sections, 28 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Collisional excitation in WIMP-WIMP scattering
  3. Positron emission in WIMP-nucleus recombination
  4. Delayed decays of excited WIMP states
  5. Concluding Remarks

Figures (3)

  • Figure 1: Collisional excitation producing two charged states $\chi_2^{\pm}$ and their subsequent decay back to $\chi_1^0$ producing positrons.
  • Figure 2: One the left, the rapid decay of the excited charged state $\chi_2^-$, and on the right, the capture process of $\chi_1^0$ forming the bound state $(N\chi_2^-)$ and producing a positron.
  • Figure 3: Higgs-mediated decay of a long-lived metastable state $\chi^0_2$ to the WIMP $\chi^0_1$ producing positrons.