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.

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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.

Searching for Dark Matter with Neutron Star Mergers and Fast Radio Bursts

We propose several new search strategies for asymmetric dark matter models, which are notoriously difficult to detect due to their lack of annihilation radiation (or potentially decay). This expands on a long-running research platform aimed at determining the conditions under which the asymmetric dark matter density inside compact objects like neutron stars could reach a critical value that causes the entire system to collapse into a black hole. We propose to search for suhc a scenario utilizing LIGO detections of Neutron star mergers, and CHIME detections of fast radio bursts - both of which would have altered features if the neutron stars in such objects had previously collapsed into black holes. We find, for example, that searches which verified an astrophysical distribution of neutron star mergers on the sky could set constraints on the asymmetric dark matter cross-section several orders of magnitude better than from terestrial experiments.

Lukewarm Neutron Stars Probe Dark Matter

We investigate a novel method of searching for dark matter that interacts with baryons. Dark matter particles that encounter neutron stars should scatter, injecting their kinetic energy at the time of encounter into the bulk of the neutron star material. This energy will be reprocessed and emitted as heating, setting a minimum neutron star temperature that does not depend on neutron star age, but does depend on the ambient dark matter density. We find that for typical dark matter models near Earth, the minimum temperature is approximately 2000K, producing potentially observable infrared emission. The observation of a single, much colder neutron star could set generic constraints on dark matter nuclear cross-sections much stronger than accessible to current terrestrial detectors.

Full Publication List:

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