what mass are the smallest protohalos in thermal wimp dark-matter models? kris sigurdson institute...
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What mass are the smallest protohalos in thermal WIMP dark-matter models?
Kris Sigurdson
Institute for Advanced Study
Space Telescope Science Institute
Hubble Fellows Symposium
April 20, 2006
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Outline
• Thermal WIMPs and Chemical Decoupling• WIMP Kinetic Decoupling• Small-Scale Cutoff in the Matter Power
Spectrum• My Results: The Kinetic Decoupling
Temperature• The Mass of the Smallest Protohalos in
Viable Particle Physics Models
Standard Model of the Universe
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Galaxy Surveys
BBN
Hubble Space Telescope
Supernovae Surveys
CMB
“The Standard Model”
Nonbaryonic Dark Matter
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Thermal WIMPs
1) Cosmologically stable due to a conserved quantum number.
2) In thermal equilibrium at early times (high temperatures) and relic abundance fixed by the total annihilation cross section of pairs to all lighter species.
Weakly Interacting Massive Particles
(particle-antiparticle pairs have no net “charge”)
WIMP Chemical Decoupling
Eventually the annihilation rate drops below the expansion rate of a FRW Universe.
Pairs of WIMPs can no longer find each other efficiently in a Hubble time and the relic abundance is fixed
Expansion Rate
Annihilation Rate
Freeze-Out
WIMP Relic Abundance
Relic Abundance from Freeze-Out Relic Abundance from Cosmology
We now know the total annihilation cross section for thermal WIMP dark matter!
Are the any other astrophysical indicators of the particle physics?
WIMP Chemical Decoupling
Eventually the annihilation rate drops below the expansion rate of a FRW Universe.
Pairs of WIMPs can no longer find each other efficiently in a Hubble time and the relic abundance is fixed
Expansion Rate
Annihilation Rate
Freeze-Out
The End of Dark-Matter Interactions?
NOT The end of:
Same Conserved Quantum Number
The end of:
Elastic Scattering
Inelastic Scattering
Few
Many
Annihilation (Number Changing)
Photons, Neutrinos, Charged Leptons, Quarks
WIMP Kinetic Decoupling
Eventually the scattering rate drops below the expansion rate of a FRW Universe.
WIMPs no longer scatter off the thermal bath efficiently in a Hubble time and start to cool, free-stream, and then gravitationally cluster
Expansion Rate
Scattering Rate
Oscillates with plasma
Evolution of Perturbations
Plasma
Decoupled Dark Matter
WIMP Kinetic DecouplingFrom A. Loeb and M. Zaldarriaga, Phys. Rev. D 71, 103520 (2005) [arXiv:astro-ph/0504112].
WIMP Kinetic DecouplingFrom A. Loeb and M. Zaldarriaga, Phys. Rev. D 71, 103520 (2005) [arXiv:astro-ph/0504112].
Small-Scale Cutoff
The Mass of the Smallest Protohalos
The temperature of kinetic decoupling sets thesmall-scale cutoff in the matter power spectrum.
In turn, the small-scale cutoff sets the mass of the smallest protohalos that form when these scales gononlinear at
What value is Tkd?
What Value is Tkd?
S. Hofmann, D. Schwarz, and H.Stöcker, Phys. Rev. D 64, 083507 (2001) [arXiv:astro-ph/0104173].
A. Green, S. Hofmann, and D. Schwarz, Mon. Not. Roy. Astron. Soc 353, L23 (2004)[arXiv:astro-ph/0309621].
A. Green, S. Hofmann, and D. Schwarz, JCAP 0508, 003 (2005)[arXiv:astro-ph/0503387].
Estimate in “generic” supersymmetric (SUSY) models.
Predict power spectra and that an earth mass cutoff is expected in SUSY models.
An Earth Mass from SUSY?
S. Profumo, KS, M. Kamionkowski,“What mass are the smallest protohalos?”[arXiv:astro-ph/0603373].
Q: Is an earth mass cutoff really generic in WIMP models that produce the cosmological abundance of dark matter?
NO
Our Approach
• Kinetic-decoupling temperature for WIMPs in supersymmetric models of dark-matter.
• Must account for the dark matter abundance from cosmology and be compatible with present constraints from particle physics.
• Include detailed model (with correct cross sections) of the particle physics for both chemical and kinetic decoupling.
S. Profumo, KS, M. Kamionkowski, [arXiv:astro-ph/0603373].
Scattering Cross Section
S. Profumo, KS, M. Kamionkowski, [arXiv:astro-ph/0603373].
Only Z Boson(Heavy Scalars)
Z Boson +Sleptons(Light Scalars)
Resonant ScatteringSame Relic Abundance
Kinetic Decoupling Temperature
S. Profumo, KS, M. Kamionkowski, [arXiv:astro-ph/0603373].
Scan over SUSY Parameter Space.
Maximum
Minimum
Minimum Protohalo Mass
S. Profumo, KS, M. Kamionkowski, [arXiv:astro-ph/0603373].
Scan over SUSY Parameter Space.
Maximum
Minimum
Conclusions of our Paper
• Earth mass protohalos are possible but are NOT a generic prediction of SUSY.
• A wide range of Mc between 10-6 and 100 Mare possible.
• We expect the range derived here to be characteristic of other particle physics models of thermal WIMPS beyond SUSY.
S. Profumo, KS, M. Kamionkowski, [arXiv:astro-ph/0603373].
Observational Signatures?
S. Profumo, KS, M. Kamionkowski, [arXiv:astro-ph/0603373].
Q: Can protohalos survive to the present day and be important in our Galaxy?
A1: For small (Mc < 10-7 M ?), probably No. (Green and Goodwin 2006)
Stellar disruption time less than age of the Galaxy.
A2: For larger Mc Maybe. Could amount to as much as 0.1% of dark matter in the Galaxy by mass (neglecting disruption).
At M: (Diemand et al. 2005) 100% survive at RSun
(Zhao et al. 2005) 0% (Berezinsky 2005) 17%
Must survive mergers, accretion processes, tidal stripping, and tidal disruption by stars. Bright Side: Even if disrupted can produce tidal streams.
Bottom Line: Tough Astrophysical Problem. Simulation of orbits of minihalos in realistic galaxies.
Observational Signatures
• Enhanced Gamma Ray Annihilation Signal?1) Protohalos within our Galaxy could be bright gamma sources.2) Clumpiness may enhance total gamma ray flux from nearby
extragalactic systems.
• Enhanced Direct Detection Rates.1) If a protohalo or the tidal stream of a disrupted protohalo passes
through the Solar neighborhood then direct detection rates will be enhanced by perhaps a factor 10.2) If the smooth dark-matter component is reduced then direct
detection rates at other times will be slightly decreased.
S. Profumo, KS, M. Kamionkowski, [arXiv:astro-ph/0603373].
Particle Physics/Astrophysics Observation
Feedback Loop!
Summary
• Kinetic Decoupling of WIMPs sets the minimum mass of dark-matter protohalos.
• Earth mass protohalos are NOT a generic prediction of SUSY CDM models.
• Perhaps: Another window onto the particle physics of dark matter!
• The Challenge: Relate the physics of dark matter to the astrophysics of dark matter and ultimately to observations of dark matter! Can’t just ignore it.