Cosmic-Ray Positrons Strongly Constrain Leptophilic Dark Matter

Observations of antimatter cosmic-rays are powerful probes of dark matter annihilation -- as they are produced copiously by dark matter annihilation but were thought to only be produced through secondary astrophysical processes. Observations by PAMELA and AMS-02 found a significant excess in cosmic-ray positrons, which attracted significant interest from the dark matter community, but has been more successfully explained through the emission of e+e- pairs by high-energy pulsars. Here, we note that -- in scenarios where pulsars dominate the high-energy positron flux -- the smoothness of the positron spectrum can constrain sub-dominant dark matter contributions. This is particularly true for leptophilic dark matter models that produce significant bumps in the cosmic-ray positron spectrum. Using recently released AMS-02 data, we set strong cosntraints on dark matter annihilation to e+e-, mu+mu- and tau+tau- final states - producing limits which fall below the thermal annihilation cross-section in many models.

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Antiheliums from Dark Matter

AMS-02 has tentatively detected approximately 10 anti-Helium 3 nuclei. Such an observation would constitute smoking gun evidence of new physics, because the astrophysical production of antihelium is expected to be negligible. However, most studies of dark matter annihilation have concluded that the dark matter induced antihelium flux should also be small. Here, we carefully analyze previous studies, and discover a antihelium production pathway which had been neglected by previous literature -- the displaced vertex decays of Lambda-bottom antibaryons. The optimal mass (roughly 6 proton masses) and anti-baryon number of Lambda_baryons make them optimal candidates to efficiently produce antihelium nuclei. Intriguingly, standard particle physics codes (e.g, Pythia) predict that this pathway should dominate the production of detectable antihelium, increasing the standard antihelium production rate of dark matter annihilation by nearly a factor of 100 compared to previous computations.

Antihelium from Dark Matter

AMS-02 has reported the tentative detection of approximately a dozen anti-Helium 3 and anti-Helium 4 nuclei. Astrophysical interactions capable of making high-energy anti-nuclei are kinematically suppressed, making such a signal a potential smoking gun for dark matter annihilation. Unfortunately, it is also extremely difficult to explain such a signal with dark matter models either, due to the very small range of coalescence momenta that is capable of producing such particles. We present a new astrophysical method for enhancing the dark matter induced anti-Helium flux, by using Alfven waves to reaccelerate very low-energy anti-Helium particles to higher energies, where they may be more readily detected by AMS-02.

A Robust Detection of an Antiproton Excess

Previous studies by Cuoco et al (1610.03071) and Cui et al. (1610.03840) have uncovered evidence for an excess in cosmic-ray antiprotons in the AMS-02 data, centered at a rigidity of around 5 GeV. The resilience of this excess has been called into question by other analyses (e.g., Reinert & Winkler 1712.00002), which argued that uncertainties in the background modeling could decrease the significance of the signal. We utilize an improved method to constrain solar modulation and antiproton production uncertainties, and test number of diffuse emission models, finding that the statistical significance of the excess remains robust, providing an intriguing signal that remains consistent with dark matter annihilation.

Full Publication List:

7. Cosmic-Ray Positrons Strongly Constrain Leptophilic Dark Matter
Isabelle John, Tim Linden
Journal of Cosmology and Astroparticle Physics 12 2021 007

6. Response to Comment on: "Dark Matter Annihilation Can Produce a Detectable Antihelium Flux through Λb Decays
Martin Winkler, Tim Linden
ArXiv Only

5. Dark Matter Annihilation Can Produce a Detectable Antihelium Flux through Λb Decays
Martin Wolfgang Winkler, Tim Linden
Physical Review Letters 126 101101

4. Anti-Deuterons and Anti-Helium Nuclei from Annihilating Dark Matter
Ilias Cholis, Tim Linden, Dan Hooper
Physical Review D 102 103019

3. A Robust Excess in the Cosmic-Ray Antiproton Spectrum: Implications for Annihilating Dark Matter
Ilias Cholis, Tim Linden, Dan Hooper
Physical Review D 99 103026

2. What Does the PAMELA Antiproton Spectrum Tell Us About Dark Matter?
Dan Hooper, Tim Linden, Philipp Mertsch
Journal of Cosmology and Astroparticle Physics 03 021 (2015)

1. Antihelium from Dark Matter
Eric Carlson, Adam Coogan, Tim Linden, Stefano Profumo, Alejandro Ibarra, Sebastian Wild
Physical Review D, 89 076005



Tim Linden

Assistant Professor, Stockholm University

linden@fysik.su.se