Searching for Dark Matter
Ron-Chou Hsieh
CYCU
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It seems to have something there!
What are they?
Link with physics beyond the SM.
DM searches.
Constructing a DM model
Summary
• Galactic rotation curves:
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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
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• 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.
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
Strong gravitational lensing as observed by
the Hubble Space Telescope in Abell 1689
indicates the presence of dark matter
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/
• Massive compact halo object (MACHOs)
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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 antigravity. This energy, distinct from dark
matter, is responsible for the presentday acceleration of the universal
expansion.
• Abundances of the Helium
• Basic requirements of DM
 Stability
at least on timescales comparable to the age of the universe
 Charge neutrality
to avoid EM interactions and prevent DM to shine
 Non negligible mass
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.
WIMP!
The observed DM relic density implies that:
1)
2)
The annihilation cross-section of DM particles should be comparable to the weak interaction
cross section.
Fermionic DM particles should be heavier than a few GeV(Hut-Lee-Weinberg limit).
• The non-baryonic candidate zoo
 Weakly interacting massive particles (WIMPs)
Axions
SUSY particles
Extra 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
o
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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 detection
Indirect 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.
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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
Main sources of background are:
- Decay of o mesons
Inelastic pp or p-He collisions
- Inverse Compton scattering
- Bremsstrahlung from electrons
- Extra galactic background
Diffuse gamma-ray spectrum
as calculated with the
GALPROP model.
• Basic requirements of DM
 Stability
 Charge neutrality
 Non negligible mass
at least on time scales comparable to the age of the universe
to avoid EM interactions and prevent DM to shine
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)
2)
Interaction only through the weak force and gravity, or at least with interaction cross-sections no
higher than the weak scale.
Fermionic DM particles should be heavier than a few GeV (Hut-Lee-Weinberg limit).
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• Minimal DM model
•Weakly interactive
•Charge neutrality (Q=0)
with
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Elastic scattering of DM with nucleus
We obtain
Here c = 1 for fermionic DM and c = 4 for scalar DM
Exceed observed data limit!
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Must have
However, for case
, following contributions
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We obtain
Also exceed observed data limit!
Must consider :
Weak isospin singlet DM
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Ansatz:
•Annihilating into SM fermion pair
•Renormalizable interaction
•Fermionic DM
SM Singlet DM
SM Singlet
Interacting with SM fermion through exotic vector boson
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Minimally coupled exotic vector boson
The Lagrangian describing the model is
With
For simplification, we only consider the vector interaction such
that
with
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•Thermal relic abundance
The thermal relic abundance in WIMP scenario can be calculated by solving Boltzmann
equation through following annihilation process
Boltzmann equation:
with
Interaction rate per particle
Numbers of particle
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We assume
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•Elastic scattering with nucleus
Spin Independent cross section
10
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CoGent 2011
Contact Interaction
DAMA LI 2008
10
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10
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Xenon10 2011
KIMS 2011
Z
N
cm2
SIMPLE 2011
CRESSII 2011
ZEPLINIII
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10
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EdelweissII 2011
CDMS CDMSII 2011
Xenon100 2012
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2011
This Work
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10
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100
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m GeV
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Extrapolated
Region
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2000
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•Annihilation to muon pair
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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