Transcript dark matter - University of Sheffield

darkThematter
Stuff of the Universe?
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Why do we need it?
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What could it be?
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How could we detect it?
Susan Cartwright
University of Sheffield
dark matter
Why do we need it?
v ≈ constant
The Solar System
vv1/√r
 1/ ¦r
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Rotation of
galaxies indicates
that they contain
matter that we do
not see
M31
(Andromeda)
dark matter
Why do we need it?
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cosmologists get in on the act:
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universal microwave background
radiation “remembers” early universe
detailed structure depends on many
factors
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geometry of universe
Hubble constant
nature of matter
 mapping
the cosmic
background tells us
about the universe
dark matter
Why do we need it?
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In general relativity
spacetime is curved
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universe can have nonEuclidean geometry
but it doesn’t …
The Universe is flat!
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73% dark energy
23% dark matter
4% atoms
Only 0.4% stars!
dark matter
What could it be?
Ordinary Matter
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small, faint stars or
near-stars
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white dwarfs
red dwarfs
brown dwarfs
“Jupiters”
or
Exotic Particles
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neutrinos
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known to exist, but
usually assumed
massless—would need
mass 0.002% of electron
WIMPs (weakly interacting massive
particles)
black holes
gas
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predicted by many
particle physics theories
axions
dark How
matter
could we detect it?
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Ordinary matter:
compact objects (stars
and black holes)
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just look!
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microlensing—look
for light bending round
unseen object 
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Microlensing: light (from star in Large Magellanic Cloud) bending round unseen object
(in halo of our Galaxy)
“focusing” effect causes
background star
to appear brighter
dark How
matter
could we detect it?
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Ordinary matter: gas
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Malin 1
rich clusters of
galaxies contain hot
gas (seen in X-rays)
low surface
brightness galaxies
(lots of gas, few
stars) may be very
common
dark How
matter
could we detect it?
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Exotic particles
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massive neutrinos
 look
for evidence of missing mass in radioactive decays
no evidence found
 look for neutrino oscillations—change of neutrino type
(depends on difference between masses of the two types)
these do happen
but the implied mass is very small indeed
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axions
 can
be converted to photons in magnetic field
no signal found (so far)
dark matter
Mining for WIMPs
WIMPs detected by
observing recoil of
nucleus from (very
rare) direct hit
 Need extremely low
levels of background
(natural radioactivity,
cosmic rays)
Use a deep mine
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Observe scintillation light,
crystal lattice vibrations,
or heat
UKDMC
Boulby Mine
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1 km deep salt mine
near Whitby
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deep  most cosmic
rays are stopped
salt  low
background activity
detectors shielded
by pure copper/lead
or by pure water
UKDMC
Detectors at Boulby
liquid xenon
scintillator
NaI scintillator
DRIFT
directional
detector
UKDMC
Search results
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Most detected events are
background (as expected)
A few looked interesting
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but unfortunately also turned
out to be background (surface
contamination on crystals?)
New detectors using new
technologies
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larger (more mass)
better materials: liquid xenon
better background rejection
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different signals in xenon
directional info in DRIFT
?
8 keV
24 keV
40 keV
56 keV
background
64 keV
72 keV
UKDMC
Search results
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Results from ZEPLIN I compared to other
experiments
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rules out signal
claimed by
DAMA group
world-class
experiment
Journey
from the centre of the Earth
dark
matter
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WIMPs can be captured by the Earth’s gravity
In the Earth’s core they collide with other WIMPs
The resulting reaction produces high energy neutrinos
which escape from the Earth
We could see WIMPs with
a neutrino telescope
The same process
takes place in the
core of the Sun
produced
neutrino
escapes
incoming
WIMP is
captured
Diving for WIMPs
dark matter
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A neutrino telescope detects
the light radiated in water by
muons produced from muontype neutrinos
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need huge detector—use the
sea or Antarctic ice
go deep to reduce cosmic ray
background
AMANDA at South Pole;
ANTARES in Mediterranean
darkThematter
Stuff of the Universe?
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Dark matter certainly exists
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“Ordinary” (baryonic) dark matter certainly
exists
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dark stars, dark galaxies, and hot gas
Massive neutrinos certainly exist
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and most of the universe seems to be dark
but not massive enough to dominate the universe
WIMPs may or may not exist
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and we could know within 5 years
 Watch this space!