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Constraints on dark matter and the shape of the Milky Way dark halo from the 511 keV line

Y. Ascasibar, P. Jean, C. Boehm, J. Knoedlseder

TL;DR

The authors test whether Light Dark Matter can account for the Galactic 511 keV emission and use its morphology to constrain the Milky Way's dark halo. They model DM annihilation/decay with velocity-dependent cross-sections and compare predicted 511 keV maps to INTEGRAL/SPI data, finding that annihilating scalar LDM with a cuspy halo best fits the observations, with an inner slope of $\gamma=1.03\pm0.04$, while decaying DM is disfavoured unless $\gamma>1.5$. They translate the fit into particle-physics constraints, deriving $a_{26}=(2.6\pm0.12)\times10^{-4}\ m_{MeV}^2$ for the $a$-term and arguing that heavy fermions or a pure $Z'$ cannot alone explain the data, thereby favoring a scalar LDM scenario with $F$-exchange. The work emphasizes that precise knowledge of the Milky Way's halo profile critically informs the origin of the 511 keV line and that upcoming particle-physics measurements (e.g., precision determination of $\alpha$, electron $g-2$, and collider searches for $F$-particles) could strongly test the LDM hypothesis.

Abstract

About one year ago, it was speculated that decaying or annihilating Light Dark Matter (LDM) particles could explain the flux and extension of the 511 keV line emission in the galactic centre. Here we present a thorough comparison between theoretical expectations of the galactic positron distribution within the LDM scenario and observational data from INTEGRAL/SPI. Unlike previous analyses, there is now enough statistical evidence to put tight constraints on the shape of the dark matter halo of our galaxy, if the galactic positrons originate from dark matter. For annihilating candidates, the best fit to the observed 511 keV emission is provided by a radial density profile with inner logarithmic slope gamma=1.03+-0.04. In contrast, decaying dark matter requires a much steeper density profile, gamma>1.5, rather disfavoured by both observations and numerical simulations. Within the annihilating LDM scenario, a velocity-independent cross-section would be consistent with the observational data while a cross-section purely proportional to v^2 can be rejected at a high confidence level. Assuming the most simplistic model where the galactic positrons are produced as primaries, we show that the LDM candidate should be a scalar rather than a spin-1/2 particle and obtain a very stringent constraint on the value of the positron production cross-section to explain the 511 keV emission. One consequence is that the value of the fine structure constant should differ from that recommended in the CODATA. This is a very strong test for the LDM scenario and an additional motivation in favour of experiments measuring alpha directly. Our results finally indicate that an accurate measurement of the shape of the dark halo profile could have a tremendous impact on the determination of the origin of the 511 keV line and vice versa.

Constraints on dark matter and the shape of the Milky Way dark halo from the 511 keV line

TL;DR

The authors test whether Light Dark Matter can account for the Galactic 511 keV emission and use its morphology to constrain the Milky Way's dark halo. They model DM annihilation/decay with velocity-dependent cross-sections and compare predicted 511 keV maps to INTEGRAL/SPI data, finding that annihilating scalar LDM with a cuspy halo best fits the observations, with an inner slope of , while decaying DM is disfavoured unless . They translate the fit into particle-physics constraints, deriving for the -term and arguing that heavy fermions or a pure cannot alone explain the data, thereby favoring a scalar LDM scenario with -exchange. The work emphasizes that precise knowledge of the Milky Way's halo profile critically informs the origin of the 511 keV line and that upcoming particle-physics measurements (e.g., precision determination of , electron , and collider searches for -particles) could strongly test the LDM hypothesis.

Abstract

About one year ago, it was speculated that decaying or annihilating Light Dark Matter (LDM) particles could explain the flux and extension of the 511 keV line emission in the galactic centre. Here we present a thorough comparison between theoretical expectations of the galactic positron distribution within the LDM scenario and observational data from INTEGRAL/SPI. Unlike previous analyses, there is now enough statistical evidence to put tight constraints on the shape of the dark matter halo of our galaxy, if the galactic positrons originate from dark matter. For annihilating candidates, the best fit to the observed 511 keV emission is provided by a radial density profile with inner logarithmic slope gamma=1.03+-0.04. In contrast, decaying dark matter requires a much steeper density profile, gamma>1.5, rather disfavoured by both observations and numerical simulations. Within the annihilating LDM scenario, a velocity-independent cross-section would be consistent with the observational data while a cross-section purely proportional to v^2 can be rejected at a high confidence level. Assuming the most simplistic model where the galactic positrons are produced as primaries, we show that the LDM candidate should be a scalar rather than a spin-1/2 particle and obtain a very stringent constraint on the value of the positron production cross-section to explain the 511 keV emission. One consequence is that the value of the fine structure constant should differ from that recommended in the CODATA. This is a very strong test for the LDM scenario and an additional motivation in favour of experiments measuring alpha directly. Our results finally indicate that an accurate measurement of the shape of the dark halo profile could have a tremendous impact on the determination of the origin of the 511 keV line and vice versa.

Paper Structure

This paper contains 12 sections, 31 equations, 5 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Dark matter characteristics
  3. The Milky Way dark halo
  4. Comparison with SPI data
  5. Results
  6. The nature of dark matter
  7. The Milky Way density profile
  8. Discussion
  9. Systematic effects
  10. Compatibiliy with low-energy gamma rays
  11. Tests of the LDM model in particle physics experiments
  12. Conclusions

Figures (5)

  • Figure 1: Dark matter annihilation through the exchange of a charged heavy fermion $F$ (left) or a neutral light gauge boson $Z'$ (right).
  • Figure 2: Top panel: Densty profiles considered in the present work. All of them are described by equation (\ref{['eqrho']}), with values of the parameters given in Table \ref{['tabProf']}. Bottom panel: The corresponding cumulative mass profiles.
  • Figure 3: Velocity dispersion profile obtained from the Jeans equation for each of the models in Table \ref{['tabProf']}. Grey line on the top panel shows expression (\ref{['eqSigma']}).
  • Figure 4: $\Delta_{\rm MLR}=\rm{MLR}(\gamma)-\rm{MLR}(1)$ for different central asymptotic slopes $\gamma$. Continuous line plots the best-fitting ninth-order polynomial.
  • Figure 5: Comparison with COMPTEL (circles) and EGRET (diamonds) data Strong00. Lines plot the gamma ray flux expected for $m_{\rm dm}=30$ and 100 MeV, according to expression \ref{['eqGam0']}. Solid and dashed lines correspond to the regions $|b|<5\degr$, $|l|<30\degr$ (\ref{['eqGam1']}) and $|b|<5\degr$, $|l|<5\degr$ (\ref{['eqGam2']}).