The Dark Matter Problem astrophysical probe of particle nature of DM

毕效军中国科学院高能物理所

2009/12/16

Outline

What we have learned from astrophysics evidence of DM and its abundance DM is not baryonic DM is not hot

“problems” of LCDM model cuspy halos and missing satellites alternative models of DM astrophysical answers

What we learned from particle physics WIMP: the classic CDM

direct detection indirect detection: excesses of electrons and positrons non-standard CDM

Evidences — galaxy scale

From the Kepler’s law, for r much larger than the luminous terms, you should have v∝r-1/2 However, it is flat or rises slightly.

r

rGMvcirc

)(

The most direct evidence of the existence of dark matter.

Corbelli & Salucci (2000); Bergstrom (2000)

dynamics of galaxy cluster

Virial theorem

U=2K K = mi vi

2

U ~ GM2/R

mass to light ratio (B) typical cluster: 100/h-300/h Sun

stellar pop: 1-10 Sun

critical: 1390 h +- 35%

Coma cluster

X-ray cluster

hydrostatic equilibrium

beta model:

However, X-ray emission measures the temperature and M/Mvisible=20

Strong Gravitational Lensing

Weak Lensing mass reconstruction

RXJ1347.5-1145 (Bradac et al 2005)

Image ellipticity -> shear->

invert the equation

Cosmological scalethe WMAP result

Spergel et al 2003

mh2=0.135+-0.009

m=0.27+-0.04

WMAP Combined fit:

Results depend on Supernovae and Hubble constant data.

Non-baryonic

From BBN and CMB, it has Bh2=0.02+-0.002. Therefore, most dark matter should be non-baryonic. DMh2=0.113+-0.009

Nature of the dark matter—Hot or cold

Hot dark matter is relativistic at the collapse epoch and free-streaming out the galaxy-sized over density. Larger structure forms early and fragments to smaller ones.

Cold DM is non-relativistic at de-coupling, forms structure in a hierarchical, bottom-up scenario. HDM is tightly bound from

observation and LSS forma-tion theory

What we learned

In the universe there exists non-baryonic, non-hot, dark matter

Problems at small scale of CDM

Galactic satellite problem and cusp at GC

Nature of dark matter or astrophysics process?

Predicted number

Observed number of luminous satellite galaxies

• The predicted number of substructures exceeds the luminous satellite galaxies: dark substructures?

• Satellite galaxies are seen in Milky Way, e.g. Saggittarius, MCs

20km/s 100km/s10km/s

The first dark halos

Due to collisional damping and free-streaming, the smallest halo (no sub-structure) is 10-6 solar mass (earth mass) for neutralino. Detection of such halo may probe the nature of DM.

Diemand, Moore, Stadel 2005

Universal Density ProfileNFW profile

Navarro, Frenk, White 1997

CuspDark matter distribution—Density profile

Observation of rotation curve favors cored profile strongly

Dark matter halo profile

simulation (Navarro, Frenk, white 1996): cusp

observation: core

NFW96, rotation curve

Nature of dark matter or astrophysics process?

missing satellites: CDM solution

• satellites do exist, but star formation suppressed (after reionization?)

• satellites orbit do not bring them to close interaction with disk, so they will not heat up the disk.

• Local Group dwarf velocity dispersion underestimated

• halo substructure may be probed by lensing (still controversial)

• galaxy may not follow dwarf

Alternatives to CDM

WDM: reduce the small scale powerSelf-Interacting Dark Matter (Spergel & Steinhardt 2000) Strongly Interacting Massive Particle Annihilating DMDecaying DMFuzzy DM

WDM

From Jing 2000

SIDM

DM strongly interact with itself, but no EM

interaction can create an core in hierachical scenario (eventually core collapse -> isothermal profile)

Interaction strength: comparable to neutron-neutron

Difficulty: make spherical clusters: against lensing

SIMP

Motivation:• SIDM may have QCD interaction but not EM• Not detectable in WIMP search, blocked.

CMB & LSS constraint:Before decoupling, photons and baryons are tightly coupled, interaction with baryon will cause additional damping of perturbation

From particle physics

Thermal history of the WIMP (thermal production)

ff

ff

At T >> m,

At T < m,

At T ~ m/22, ， decoupled, relic density is inversely proportional to the interaction strength

For the weak scale interaction and mass scale (non-relativistic dark matter particles) , if and

fTv

scmh

13272 103

1326103~ scmv210～ GeVM 100weak～ 22/22 cv

WIMP is a natural dark matter candidate giving correct relic density (proposed trying to solve hierarchy problem).

Thermal equilibrium abundance

Hvn ~

Collisional Damping and Free Streaming

Initial density perturbation is damped by the free streaming of the particles before radiation-matter equality

perturbations on scales smaller than rFS is smoothed out.

Kinetic decoupling at T ~ 1 MeV (Chen, Kamionkowski, Zhang 2001)

This is why we introduce hot, warm, and cold dark matter.

Detection of WIMP Indirect detection DM increases in Galaxies,

annihilation restarts(∝ρ2); ID looks for the annihilation products of WIMPs, such as the neutrinos, gamma rays, positrons at the ground/space-based experiments

Direct detection of WIMP at terrestrial detectors via scattering of WIMP of the detector material.

Direct detection

p

e+

_

indirect detection

llll

Summary of the present limits

PAMELA results of antiparticles in cosmic rays

Nature 458, 607 (2009)

Positron fraction Antiproton fraction

Phys.Rev.Lett.102:051101,2009

400+ citations after submitted on 28th Oct. 2008, 1paper per day

The total electron+positron spectrum

Chang et al. Nature456, 362 2008

ATIC bump Fermi excess

Phys.Rev.Lett.102:181101,2009

Primary positron/electrons from dark matter – implication from new data

DM annihilation/decay produce leptons mainly in order not to produce too much antiprotons.

Very hard electron spectrum -> dark matter annihilates/decay into leptons.

Very large annihilation cross section, much larger (~1000) than the requirement by relic density. 1) nonthermal production, 2) Sommerfeld enhancement 3) Breit-Wigner enhancement 4) dark matter decay.

J. Zavala, M. Vogelsberger, and S. White, Astro-ph/0910.5221

Astro-ph/0911.0422

Emission from the GC

Constraint on the central density of DM

TensionExist for the annihilating DM scenario,but consistent with decay scenario

Bi et al., 0905.1253

Liu, Yuan, Bi, Li, Zhang, 0906.3858

Constraints on the minimal subhalos by observations of clusters

Standard CDM predicts the minimal subhalos

Observation constrains

Fermi limit to

DM is warm

A. Pinzke et al., 0905.1948

Nonthermal production of dark matter 暗物质可以通过早期宇宙

产物的衰变产生，这样的暗物质可以有很大的湮灭截面，同时产生的速度大，压低小尺度的结构。这样银心的伽马射线没有超出，因此受到的限制会减弱。

银心的伽马射线、河外星系团、河外弥散伽马的限制可以满足

fTv

scmh

13272 103

Lin, Huang, Zhang, Brandenberger, PRL86,954 (2001)

Bi, Brandenberger, Gondolo, Li, Yuan, Zhang, 0905.1253