Searching for Dark Matter

Ron-Chou HsiehCYCU

Outline

• It seems to have something there!• What are they?• Link with physics beyond the SM.• DM searches.• Constructing a DM model• Summary

It seems to have something there!

• Galactic rotation curves:– In 1932, Dutch astronomer Jan Oort– In 1933, Swiss astrophysicist Fritz Zwicky– In 1936, Sinclair Smith– In 1970, American astronomer Vera Rubin

The stars move faster than expected!!!

For

3

• Gravitational lensing Spacetime around a massive object (such as a galaxy cluster or a black hole) is curved, and as

a result light rays from a background source (such as a galaxy) propagating through spacetime are bent.

Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter

Bending light around a massive object from a distant source. The orange arrows show the apparent position of the background source. The white arrows show the path of the light from the true position of the source

Collision of two clusters of galaxies (Bullet Cluster, 1E 0657-56)http://chandra.harvard.edu/photo/2006/1e0657/

Collision of two clusters of galaxies (Bullet Cluster, 1E 0657-56)http://chandra.harvard.edu/photo/2006/1e0657/

What are they?• Massive compact halo object (MACHOs)

• A MACHO is a body composed of normal baryonic matter, which emits little or no radiation and drifts through interstellar space unassociated with any planetary system.

• MACHOs may sometimes be black holes or neutron stars as well as brown dwarfs or unassociated planets. White dwarfs and very faint red dwarfs have also been proposed as candidate MACHOs.

• Wilkinson Microwave Anisotropy Probe (WMAP)The Wilkinson Microwave Anisotropy Probe (WMAP) is a NASA Explorer mission that launched June 2001 to make fundamental measurements of cosmology -- the study of the properties of our universe as a whole.

Content of the Universe

WMAP Three year data reveals that its contents include an estimate of 4% atoms, the building blocks of stars and planets. Dark matter comprises 22% of the universe. This matter, different from atoms, does not emit or absorb light. It has only been detected indirectly by its gravity. 74% of the universe, is composed of "dark energy", that acts as a sort of an anti-gravity. This energy, distinct from dark matter, is responsible for the present-day acceleration of the universal expansion.

• Abundances of the Helium

Links with physics beyond the SM

• Basic requirements of DM

 Stability

Charge neutrality

Non negligible mass

to avoid EM interactions and prevent DM to shine

at least on timescales comparable to the age of the universe

a lower limit on the mass of weakly interacting DM candidates to explain the formation of the smallest objects observed in the Universe, which is of a few keV

• Mechanisms to produce the DM relic density

Free out-----DM particles which have been in thermal equilibrium with radiation at some stage of the cosmic evolution should subsequently decouple in two ways: decay and pair annihilation processes.

Free in-----DM particles are created through annihilation into DM particles, A A→DM DM, or through a decay process A→DM B.

The observed DM relic density implies that:

1) The annihilation cross-section of DM particles should be comparable to the weak interaction cross section.

2) Fermionic DM particles should be heavier than a few GeV(Hut-Lee-Weinberg limit).

WIMP!

• The non-baryonic candidate zoo

 Weakly interacting massive particles (WIMPs) AxionsSUSY particlesExtra Dimensions

• WIMPs

Interaction only through the weak nuclear force and gravity, or at least with interaction cross-sections no higher than the weak scale;

Large mass compared to standard particles (WIMPs with sub-GeV masses may be considered to be light dark matter).

o q

o

q~q_

o q, l

o q, l_

Z,

_

0 2o

o

+

W +

DM searches

• Direct detection : elastic WIMP-atom scattering

Three physical consequences of nuclear recoils are used to search for evidence of WIMP scattering.

a) Ionization of target atoms

b) Fluorescent radiation given off by electrons of target atoms

c) Phonon excitations generated in crystals by the nuclear recoils

• Indirect detectionIndirect detection experiments search for the products of WIMP annihilation. If WIMPs are Majorana particles (the particle and antiparticle are the same) then two WIMPs colliding could annihilate to produce gamma rays or particle-antiparticle pairs. This could produce a significant number of gamma rays, antiprotons or positrons in the galactic halo. 

o q

o

q~q_

o q, l

o q, l_

Z,

_

0 2

o

o

+

W +

• The EGRET gamma ray telescope observed more gamma rays than expected from the Milky Way, but scientists concluded that this was most likely due to an error in estimates of the telescope's sensitivity. The Fermi Gamma-ray Space Telescope, launched June 11, 2008, is searching for gamma ray events from dark matter annihilation. At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spherical galaxies and in clusters of galaxies.

• The PAMELA experiment (launched 2006) has detected a larger number of positrons than expected. These extra positrons could be produced by dark matter annihilation, but may also come from pulsars. No excess of anti-protons has been observed.

• A few of the WIMPs passing through the Sun or Earth may scatter off atoms and lose energy. This way a large population of WIMPs may accumulate at the center of these bodies, increasing the chance that two will collide and annihilate. This could produce a distinctive signal in the form of high energy neutrinos originating from the center of the Sun or Earth. It is generally considered that the detection of such a signal would be the strongest indirect proof of WIMP dark matter. High energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this signal.

• WIMP annihilation from the Milky Way Galaxy as a whole may also be detected in the form of various annihilation products. The Galactic center is a particularly good place to look because the density of dark matter may be very high there.

Energy spectrum as detected by EGRET

Diffuse gamma-ray spectrum as calculated with the GALPROP model.

Main sources of background are:

- Decay of o mesons Inelastic pp or p-He collisions- Inverse Compton scattering- Bremsstrahlung from electrons - Extra galactic background

Construction of the Dark Matter model• Basic requirements of DM

 Stability

Charge neutrality

Non negligible mass

to avoid EM interactions and prevent DM to shine

at least on time scales comparable to the age of the universe

a lower limit on the mass of weakly interacting DM candidates to explain the formation of the smallest objects observed in the Universe, which is of a few keV

The observed DM relic density implies that (in WIMP scenario):

1) Interaction only through the weak force and gravity, or at least with interaction cross-sections no higher than the weak scale.

2) Fermionic DM particles should be heavier than a few GeV (Hut-Lee-Weinberg limit).

25

• Minimal DM model

with

• Weakly interactive• Charge neutrality

(Q=0)

26

Elastic scattering of DM with nucleus

We obtain

Here c = 1 for fermionic DM and c = 4 for scalar DM

27Exceed observed data limit!

Must have

28

However, for case , following contributions

29

We obtain

Also exceed observed data limit!

Must consider :

Weak isospin singlet DM

30

Interacting with SM fermion through exotic vector boson

Ansatz: • Annihilating into SM fermion pair• Renormalizable interaction• Fermionic DM

SM Singlet SM Singlet DM

Minimally coupled exotic vector boson

The Lagrangian describing the model is

With

For simplification, we only consider the vector interaction such that

with

31

• Thermal relic abundance

32

The thermal relic abundance in WIMP scenario can be calculated by solving Boltzmann equation through following annihilation process

Interaction rate per particle

Numbers of particle

Boltzmann equation:

with

33

We assume

34

• Elastic scattering with nucleus

Xenon100 2012

DAMA LI 2008

CDMS CDMS II 2011

CoGent 2011 CRESS II 2011

Edelweis s II 2011 KIMS 2011 S IMPLE 2011 Xenon10 2011

ZEPLINIII 2011

Contact Interaction

Ex trapola ted Reg ionThis W ork

1 0 2 0 5 0 1 0 0 2 0 0 5 0 0 1 0 0 0 2 0 0 0

1 0 4 5

1 0 4 4

1 0 4 3

1 0 4 2

1 0 4 1

1 0 4 0

m G e V NZ

cm2

Sp in In d e p e n d e n t c ro ss se c t io n

35

• Annihilation to muon pair

Summary• Astronomical observations and measurements indicate the

existence of Dark Matter• Models in particle physics offer candidates for Dark Matter• We are searching for Dark Matter by

producing new particles at colliders indirect detection of the products of WIMP annihilation direct detection through elastic WIMP-nucleus scattering