[Paper Review] Galactic propagation of positrons from particle dark-matter annihilation

I. V. Moskalenko, A. W. Strong|arXiv (Cornell University)|Jun 14, 1999

Dark Matter and Cosmic Phenomena3 citations

TL;DR

This paper models the galactic propagation of positrons produced by dark matter particle annihilation using a 3D numerical code, showing that Green's functions are weakly sensitive to dark matter distribution for a given local energy density. The study finds that detecting a dark matter signal requires favorable conditions and precise measurements, unless dark matter is clumped, which would enhance the signal; results are based on realistic propagation modeling, reducing room for speculation compared to prior work.

ABSTRACT

We have made a calculation of the propagation of positrons from dark-matter particle annihilation in the Galactic halo for different models of the dark matter halo distribution using our 3D code. We show that the Green's functions are not very sensitive to the dark matter distribution for the same local dark matter energy density. We compare our predictions with computed cosmic ray positron spectra ("background") for the "conventional" cosmic-ray nucleon spectrum which matches the local measurements, and a modified spectrum which respects the limits imposed by measurements of diffuse Galactic gamma-rays, antiprotons, and positrons. We conclude that significant detection of a dark matter signal requires favourable conditions and precise measurements unless the dark matter is clumpy which would produce a stronger signal. Although our conclusion qualitatively agrees with that of previous authors, it is based on a more realistic model of particle propagation and thus reduces the scope for future speculations. Reliable background evaluation requires new accurate positron measurements and further developments in modelling production and propagation of cosmic ray species in the Galaxy.

Motivation & Objective

• To model the propagation of positrons generated by particle dark matter annihilation in the Galactic halo.
• To assess how different dark matter halo distributions affect the resulting positron flux.
• To compare predicted positron spectra with observational data, including cosmic ray measurements and constraints from diffuse gamma-rays and antiprotons.
• To evaluate the detectability of a dark matter signal under realistic propagation conditions.
• To reduce speculative interpretations by using a more accurate 3D propagation model.

Proposed method

• A 3D numerical code is used to simulate positron propagation through the Galactic halo, accounting for energy loss, diffusion, and ionization.
• Green's functions are computed for various dark matter halo density profiles, assuming a fixed local dark matter energy density.
• The model incorporates standard cosmic ray propagation physics, including diffusion, convection, and energy losses.
• Positron spectra are calculated using two cosmic ray input spectra: a conventional one matching local measurements and a modified one consistent with diffuse gamma-ray, antiproton, and positron constraints.
• The results are compared to observational limits to assess detectability of dark matter annihilation signals.
• Sensitivity of the results to dark matter distribution is tested by varying halo profiles while keeping local energy density constant.

Experimental results

Research questions

• RQ1 How does the spatial distribution of dark matter affect the predicted flux of positrons in the Galaxy?
• RQ2 To what extent are Green's functions for positron propagation sensitive to different dark matter halo models?
• RQ3 How do different assumptions about the cosmic ray nucleon spectrum influence the predicted positron spectrum?
• RQ4 What level of precision in positron measurements is required to detect a dark matter signal?
• RQ5 Under what conditions can a dark matter annihilation signal be distinguished from the astrophysical background?

Key findings

• The Green's functions for positron propagation are only weakly sensitive to the dark matter halo distribution when the local dark matter energy density is held constant.
• The predicted positron spectra are consistent with observational constraints only when using a modified cosmic ray spectrum that respects limits from diffuse gamma-rays, antiprotons, and positrons.
• A significant dark matter signal is difficult to detect unless the dark matter is clumped, which would enhance the positron flux.
• Detection of a dark matter signal requires favorable astrophysical conditions and highly precise positron measurements.
• The study reduces the scope for speculative interpretations by using a more realistic 3D propagation model compared to previous works.
• Accurate background evaluation demands new, high-precision positron measurements and further advances in modeling cosmic ray propagation.