Publications

Galactic Gas Models Systematically Affect Diffusive Halo Height Models

Observations of radioactive secondary ratios (e.g., 10Be/9Be) which are produced when two secondary isotopes are formed in known abundance ratios, provide excellent cosmic clocks, due to the fact that the abundance ratio predictably changes as a function of time. This is potentially of great interest, because it allows for detailed measurements of cosmic-ray diffusion throughout the Milky Way halo. We point out a critical, but previous unappreciated systematic, the very local distribution of galactic gas, which signifcantly inhibits our ability to use isotopic ratios to study galactic diffusion. Because the best isotopic ratio measurements are produced at low energies (where magnetic fields can best descrminate between particles of different masses), the radioactive component necessarily propagates only a short distance in the galaxy, and our observations are highly biased towards very local gas structures. We show that utilizing detailed spiral arms models can change our best-fit constraints on the galactic halo height by a factor of 4, indicating that more detailed understanding of local gas is necessary in order to take advantage of upcoming isotopic measurements.

Prospects for Antinuclei Detection in Light of Antiproton Limits

AMS-02 has tentatively observed a handful of heavy antinuclei events most consistent with a flux of antihelium nuclei that exceeds the expected flux from cosmic-ray propagation models. We pproduce updated models of cosmic-ray propagation that correctly take into account recently updated nuclear cross-section measurements and observational constraints from recently released AMS-02 data. We find that standard astrophysical production models should produce no more than ~0.1 antihelium events, and approximately 1 antideutron event. Dark Matter annihilation can produce a larger antinuclei flux, with an expectation of approximately 1 event for standard annihilation models, but potentially many more events if optimistic models of Lambda-b driven antihelium production are accurate. If any antihelium-4 is discovered, novel physics is required.

Antiprotons Constrain Dark Matter Annihilation

Numerous studies over the last 8 years have argued about the potential evidence for dark matter annihilation in the spectral features of AMS-02 antiproton data. We perform a highly advanced analysis using the DRAGON2 cosmic-ray propagation code, taking into account systematic uncertainties in both AMS-02 data and the antiproton production cross-sections. We constrain our cosmic-ray propagation models via a detailed fit to multiple cosmic-ray primary-to-secondary ratios. We find no statistically significant evidence for an excess that can be attributed to dark matter annihilation, and set strong and robust constraints on the dark matter annihilation cross-section. We do find slight, but in this case not statistically significant, evidence for a spectral feature around 66 GeV, in agreement with previous works.

Dark Matter signals become brighter righter when inverse-Compton is modeled more accurately

Searches for cosmic-ray positrons are one of the most sensitive probes of dark matter annihilation. This is particularly true for leptophilic dark matter models that can be difficult to detect via other channels. The principle impediment to detecting the sharply peaked positron spectrum from leptophilic dark matter is the cooling of the positrons via synchrotron and inverse-Compton scattering, which has normally been treated via a simplified model that assumes these losses are continuous. We show that when the stochasticity of inverse-Compton losses is taken into account, the expected signal becomes twice as bright, making it easier to detect heavy dark matter.

Strongly Coupled Dark Matter Models Can Produce Antihelium 4

AMS-02 has tentatively detected a flux of antihelium nuclei (around 10 events in total). This observation, if confirmed, would be very exciting because antihelium is not supposed to be produced by standard astrophysics -- and would point towards new physics. More confusingly, a handful of these events may be consistent with antihelium 4 -- which is difficult to produce even in dark matter models. We propose the first realistic dark matter model capable of producing an observable antihelium 4 flux. If the WIMP is very heavy (near unitarity scale), and annihilates through a strongly coupled dark sector, then each annihilation will produce a shower of dark pions with very high multiplicity. If these particles quickly decay to standard model quarks through a portal interaction -- the result is a high multiplicity of baryons produced promptly at the same vertex. This significantly enhances the formation of heavy antinuclei, and makes observations of antihelium-4 possible.

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.

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.

Dark Matter Searches with Cosmic Rays

Galactic Gas Models Systematically Affect Diffusive Halo Height Models

Research Topics

Prospects for Antinuclei Detection in Light of Antiproton Limits

Antiprotons Constrain Dark Matter Annihilation

Dark Matter signals become brighter righter when inverse-Compton is modeled more accurately

Strongly Coupled Dark Matter Models Can Produce Antihelium 4

Cosmic-Ray Positrons Strongly Constrain Leptophilic Dark Matter

Antiheliums from Dark Matter

Full Publication List:

White Papers

Snowmass2021 Cosmic Frontier White Paper: Puzzling Excesses in Dark Matter Searches and How to Resolve Them

Positron Annihilation in the Galaxy

Tim Linden