dark matter

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dark matter

• The Stuff of the Universe?

• Why do we need it?
• What could it be?
• How could we detect it?

Susan Cartwright University of Sheffield
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dark matter
Why do we need it?
v constant
The Solar System
v?1/vr

• Rotation of galaxies indicates that they contain
  matter that we do not see

M31 (Andromeda)
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dark matter
Why do we need it?

• cosmologists get in on the act
• universal microwave background radiation
  remembers early universe
• detailed structure depends on many factors
• geometry of universe
• Hubble constant
• nature of matter
• mapping the cosmic background tells us about
  the universe

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dark matter
Why do we need it?

• In general relativity spacetime is curved
• universe can have non-Euclidean geometry
• but it doesnt
• The Universe is flat!
• 73 dark energy
• 23 dark matter
• 4 atoms

Only 0.4 stars!
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dark matter
What could it be?
Ordinary Matter or Exotic Particles

• small, faint stars or near-stars
• white dwarfs
• red dwarfs
• brown dwarfs
• Jupiters
• black holes
• gas

• neutrinos
• known to exist, but usually assumed
  masslesswould need mass 0.002 of electron
• WIMPs (weakly inter-acting massive particles)
• predicted by many particle physics theories
• axions

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dark matter
How could we detect it?

• Ordinary matter compact objects (stars and black
  holes)
• just look!
• microlensinglook for light bending round unseen
  object

?
?
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Microlensing light (from star in Large Magel-
lanic Cloud) bending round unseen object
(in halo of our Galaxy)
focusing effect causes background star to
appear brighter
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dark matter
How could we detect it?

• Ordinary matter gas
• rich clusters of galaxies contain hot gas (seen
  in X-rays)
• low surface brightness galaxies (lots of gas, few
  stars) may be very common

Malin 1
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dark matter
How could we detect it?

• Exotic particles
• massive neutrinos
• look for evidence of missing mass in radioactive
  decays
• no evidence found
• look for neutrino oscillationschange of neutrino
  type (depends on difference between masses of the
  two types)
• these do happen
• but the implied mass is very small indeed
• axions
• can be converted to photons in magnetic field
• no signal found (so far)

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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

Observe scintillation light, crystal lattice
vibrations, or heat
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UKDMC
Boulby Mine

• 1 km deep salt mine near Whitby
• deep ? most cosmic rays are stopped
• salt ? low background activity
• detectors shielded by pure copper/lead or by pure
  water

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UKDMC
Detectors at Boulby
liquid xenon scintillator
DRIFT directional detector
NaI scintillator
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UKDMC
Search results

• Most detected events are background (as expected)
• A few looked interesting
• but unfortunately also turned out to be
  background (surface contamination on crystals?)
• New detectors using new technologies
• larger (more mass)
• better materials liquid xenon
• better background rejection
• different signals in xenon
• directional info in DRIFT

background
?
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UKDMC
Search results

• Results from ZEPLIN I compared to other
  experiments
• rules out signal claimed by DAMA group
• world-classexperiment

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dark matter
Journey from the centre of the Earth

• WIMPs can be captured by the Earths gravity
• In the Earths core they collide with other WIMPs
• The resulting reaction produces high energy
  neutrinos which escape from the Earth
• We could see WIMPs witha neutrino telescope

incomingWIMP iscaptured
producedneutrinoescapes
The same process takes place in the core of the
Sun
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dark matter
Diving for WIMPs

• A neutrino telescope detects the light radiated
  in water by muons produced from muon-type
  neutrinos
• need huge detectoruse the sea or Antarctic ice
• go deep to reduce cosmic ray background
• AMANDA at South PoleANTARES in Mediterranean

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dark matter
The Stuff of the Universe?

• Dark matter certainly exists
• and most of the universe seems to be dark
• Ordinary (baryonic) dark matter certainly
  exists
• dark stars, dark galaxies, and hot gas
• Massive neutrinos certainly exist
• but not massive enough to dominate the universe
• WIMPs may or may not exist
• and we could know within 5 years
• Watch this space!