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.

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Using Unusual Supernovae to Probe Dark Matter Interactions

Observations have uncovered a strange sub-category of supernovae, Ca-Rich Gap Transients, which are dim Type Ia supernovae that have spectral lines indicating an unusual abundence of Ca (indicative of an origin as low-mass white dwarfs), and a morphological distribution that makes them overabundent in the outskirts of galaxies. The fact that these systems are consistent with the explosion of low-mass (~0.5 solar mass) white dwarfs makes it difficult to utilize standard stellar or binary synthesis models to explain their evolution. The fact that they are found primarily in the outskirts of galaxies also suggests physics that does not correlate with the total star-formation rate. We propose a model where asteroid-mass primordial black holes collide with, and precipitate the explosions of these white dwarfs. We show that the morphology of these events is dominated by dwarf spheroidal galaxies orbiting around a host galaxy, and naturally explains the morphology of these systems.

Dark Matter Microhalos in the Solar Neighborhood: Pulsar Timing Signatures of Early Matter Domination

The impressive regularity of pulsating neutron stars allow them to be used as extremely acurate "clocks" that operate on Myr or even Gyr timescales. Pulsar timing arrays have taken repeated observations of nearby MSPs over 20 Myr, looking for small deviations which may be due to local gravitational effects affecting either the neutron star environment or local solar neighborhood. We note that these arrays are quickly becoming sensitive to the enhanced dark matter substructure that is expected when the universe goes through a period of Early Matter Domination before the onset of radiation domination during big-bang nucleosynthesis. Current, or near-future, observations (20 years with approximately 70 pulsars), could begin to constrain novel EMDE parameter space -- while future studies including 200 pulsars over 40 years could raise the minimum energy floor for early matter domination as high as 150 MeV.

Celestial-Body Focused Dark Matter Annihilation

Dark Matter searches traditionally proceed through either direct methods (searching for dark matter scattering with standard model particles), or through indirect methods (through dark matter annihilation into standard model particles). We propose a new search that proceeds through a combination of scattering and annihilation. Dark Matter particles in the galaxy scatter with celestial bodies (most importantly brown dwarfs and neutron stars). They become trapped inside and their density increases until they annihilate. If this annihilation proceeds through a light-mediator capable of escaping the compact object, its subsequent decay into standard model particles can produce a detectable signature similar to standard annihilation. However, the morphology and amplitude of the signal have detectable differences because the annihilation rate depends on the rate of dark matter capture. We show that current gamma-ray constraints can place strong limits on the dark matter scattering cross-section in these models, outperforming both standard direct detection experiments and previous Solar gamma-ray searches.

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.

Breaking a Dark Degeneracy

The standard picture of the "thermal WIMP annihilation cross-section" assumes a standard cosmology where radiation dominates the energy-density of the universe between the end of inflation and thermal freeze-out. However, this area is not probed by current cosmological constraints. If the universe instead includes a temporary period of matter domination between the end of inflation and the start of BBN, the standard thermal-freeze out picture changes considerably -- often producing much smaller annihialtion cross-sections that are difficult to probe. Fortunately, such models simultaneously lead to a significant increase in the dark matter substructure, allowing current gamma-ray experiments to constrain much smaller annihilation cross-sections. We compute current indirect detection constraints from gamma-ray observations of the isotropic gamma-ray background and the Draco dwarf, finding cross-sections which can lie as low as 1e-32 cm^3 s^-1.

HAWC Constrains Annihilation to Long-Lived Mediators

Dark Matter Annihilation directly to neutrino final states has long been the worst-case scenario for indirect detection searches. One promising direction involves IceCube searches for dark matter annihilation in the Sun, as the neutrinos could easily escape through the dense solar material. A related search, utilizing gamma-ray telescopes, involves dark matter annihilation to meta-stable final states, which can propagate out of the solar interior before decaying to more common particles (e.g., gamma-rays, bottom quarks, etc.). We utilize HAWC observations of the Sun to examine on this unique channel, setting constraints that lie several orders of magnitude below previous experiments.

Full Publication List:

15. Cosmic Ray Antihelium from a Strongly Coupled Dark Sector
Martin Winkler, Pedro De la Torre Luque, Tim Linden
To Be Submitted to PRD

14. White Dwarfs in Dwarf Spheroidal Galaxies: A New Class of Compact-Dark-Matter Detectors
Juri Smirnov, Ariel Goobar, Tim Linden, Edvard Mörtsell
To Be Submitted to PRL

13. Dark Matter Microhalos in the Solar Neighborhood: Pulsar Timing Signatures of Early Matter Domination
Sten Delos, Tim Linden
Physical Review D 105 123514

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

11. Celestial-Body Focused Dark Matter Annihilation Throughout the Galaxy
Rebecca Leane, Tim Linden, Payel Mukhopadhyay, Natalia Toro
Physical Review D, 103 (2021) 7

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

9. Breaking a Dark Degeneracy: The gamma-ray signature of early matter domination
M. Sten Delos, Tim Linden, Adrienne Erickcek
Physical Review D 100 123546

8. Constraints on Spin-Dependent Dark Matter Scattering with Long-Lived Mediators from TeV Observations of the Sun with HAWC
HAWC Collaboration
Physical Review D 98 123012

7. Searching for Dark Matter with Neutron Star Mergers and Quiet Kilonovae
Joe Bramante, Tim Linden, Yu-Dai Tsai
Physical Review D 97 055016

6. Dark Kinetic Heating of Neutron Stars: An Infrared Window On WIMPs, SIMPs, and Higgsinos
Masha Baryakhtar, Joe Bramante, Shirley Li, Tim Linden, Nirmal Raj
Physical Review Letters 119 131801

5. Indirect Detection Constraints on s and t Channel Simplified Models of Dark Matter
Linda Carpenter, Russell Colburn, Jessica Goodman, Tim Linden
Physical Review D 94 055027

4. On the R-Process Enrichment of Dwarf Spheroidal Galaxies
Joseph Bramante, Tim Linden
The Astrophysical Journal 826 1 57

3. The Galactic Center Excess in Gamma-Rays from Annihilation of Self-Interacting Dark Matter
Manoj Kaplinghat, Tim Linden, Haibo Yu
Physical Review Letters, 114 211303

2. Detecting Dark Matter with Imploding Pulsars in the Galactic Center
Joseph Bramante, Tim Linden
Physical Review Letters, 113 191301

1. Tying Dark Matter to Baryons with Self-Interactions
Manoj Kaplinghat, Ryan Keeley, Tim Linden, Haibo Yu
Physical Review Letters, 113 021302 (2014)



Tim Linden

Assistant Professor, Stockholm University

linden@fysik.su.se