Kati Nikolics

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The DARK MATTER Mystery

• Kati Nikolics

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Contents

• Evidence for the existence of DM
• Properties
• Possible Candidates - Theoretical Models
• Requirements for experimental detection
• Trying to find the cosmic needle in a haystack
  DEAP

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dark traces in the Universe
F. Zwicky 1933 Velocity dispersion curves of
galaxies show a different shape than expected
applying the Virial theorem (v ? 1/r-1)
Huge masses of non-luminous, non-absorbing matter
in the halo
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Wilkinson Microwave Anisotropy Probe (WMAP)
Measurements of the cosmic microwave background
anisotropies suggest matter that interacts with
photons more weakly than the known forces that
couple light interactions to baryonic matter
Cosmological baryon mass fraction baryonic
matter/non-baryonic DM
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Energy/matter distribution in the universe

• Dark matter ? dark energy dark energy increases
  the rate of expansion of the universe
• Only 4 of the content of the universe is
  luminous matter OR ? For every gram of baryonic
  matter there are 5.68 grams of non-baryonic dark
  matter!
• From observations of galaxy clusters ?DM
  0.2-0.3

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Cosmological Significance of DM

• Dark matter dominates internal kinematics and
  motion of galactic systems
• Mass of visible matter not enough for closed
  universe!
• Important factor for structure formation in the
  universe without DM formation of galaxies is
  impossible!

Density parameter ? average density/critical
density
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Properties of DM

• Baryonic (MACHOs Massive Compact Halo Objects
  black holes, etc.) and non-baryonic (WIMPs
  Weakly Interacting Massive Particles exotic
  particles)
• Stable on cosmological time scales
• Right relic density

Structures in universe expected to evolve in
contrast to actual observations (no clustering
possible)
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Relic density

• The relic density is the proportion of dark
  matter to the critical mass density

(?crit is the density that is required for the
universe to be flat)

• Obtained by solving the relativistic Boltzmann
  Equation ? energy and mass of particles produced
  in Big Bang to account for observed structures in
  the Universe?

Yields a DM particle mass of 100 GeV - TeV/c²
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From measurements of the cosmic microwave
background and the spatial distribution of
galaxies

• Cold, non-baryonic matter
• ?nbmh² 0.1110.006
• Baryonic part
• ?bh² 0.0230.001

h Hubble constant in units of 100km/(sMPc)
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Possible CDM Candidates

• Axion A0
• postulated to solve strong interaction CP
  problem why does QCD not break CP symmetry?

Generators of SU(3) group
8 gauge fields
Peccei-Quinn-Theory new term containing axion
field qA added to QCD-Lagrangian which acquires
effective coupling to gluons.
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• Axion searches
• Detection by looking for
• conversion in a strong magnetic field,
  proceeding through the loop-induced
• coupling.
• HOWEVER Extensive searches in the mass region
  between a few µeV and 2 MeV yielded negative
  results so far!

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• Lightest Supersymmetric Particle (LSP)
• MSSM Minimal Supersymmetric Standard Model
  extension to SM, each fermion has a bosonic, and
  each boson a fermionic SS-partner

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R-Parity R (-1)2j3BL
If R-Parity is conserved, then the LSP is stable
and a weakly interacting massive particle (WIMP)
? good CDM candidate
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Neutralinos
There are 4 neutralinos mixtures of Bino, Wino
and two neutral Higgsinos
Indices in order of increasing mass!
In R-parity preserving models, all superymmetric
cascades end up decaying into a neutralino
Chargino mixture of Wino and Higgsino
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Alternative solution?
Instead of introducing new matter
Theory of modified gravity Modificed Gravity
Model (MOG)
Based on covariant generalization of Einsteins
theory with auxiliary (gravitational) fields in
addition to the metric, modifying the Newtonian
1/r² gravitational force law to be valid at small
distances terrestrial scales
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Experimental Requirements for Detection
Expected WIMP interaction rates depend on local
flux and interaction cross section

• Experimental challenge is to detect small number
  of nuclear recoils with low energy threshold
  (order event/1000 kg/year gt 10 keV)
• Local flux fixed by ? mean velocity 220 km/s (
  velocity of our solar system around galactic
  centre), local density 0.3 GeV/cm³, WIMP mass
• Cross section two different couplings for
  non-relativistic WIMPs

spin-dependent
spin-independent
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• Spin-dependent
• Involve axial vector currents ?
• ? must carry spin
• Cross section depends on nuclear spin factor
• Useful target nuclei 19F, 127I

• Spin-indepedent
• Involve scalar and vector WIMP and nucleon
  currents
• Cross section scales m² of target nucleus
• Preferred nuclei
• Ge to Xe

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Detectability
DEAP-1 sensitive to cross sections 10-44 cm²
Most DM experiments are technically very complex
in order to discriminate b/gs from nuclear
recoils
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Cryogenic Dark Matter Search (CDMS)
Measure recoil energy imparted to detector nuclei
through ?-nucleon collisions by employing phonon
detection equipment coupled to arrays of ZIP
detectors.
Exploits difference in deposited charge
versus phonon energy between ?/?s and nuclear
recoils
ZIP detector 250g Ge or 100g silicon crystals,
providing deposited charge and phonon information
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XENON _at_ LNGS
Total Xe mass 1 tonne
Exploits difference in ionization signal
(electrons) versus scintillation signal (photons)
between b/gs and nuclear recoils
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Dark Matter Experiment with Argon Pulse Shape
Discrimination

• Principle Spin-independent WIMP-nucleon
  scattering on liquid 40Ar
• Energy spectrum of recoils is exponential with E
  50 keV
• LAr (DEAP-1 7kg LAr) volume instrumented with
  PMTs to detect scintillation photons
• Mounted in SNOLab very low background rates!

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• Located 2km underground in the Creighton mine
• Home to the SNO detector (Sudbury Neutrino
  Observatory)

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• Other experiments
• Other DM searches
• Low-energy solar neutrinos
• Neutrino-less ??-decay
• Supernova detection using neutrinos
• geophysics

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Direct Detection
Elastic WIMP-nucleus scattering (typical energies
1-100 keV)
Liquid argon scintillates in the far UV. Use
wavelength shifter (TPB) to convert to blue
light. LAr light yield 40 photons/keV electron
energy
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Backgrounds
Discrimination of ?/? backgrounds using only
scintillation time information from PMTs!

• 39 Ar
• Other betas and gammas
• External neutrons
• (?,n) in detector materials
• Surface contamination

ROI (10 to 50 keV) 0.2
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Surface Backgrounds
Acrylic

• Radon in air during handling
• Daughter 218Po plates
• Subsequent ? decays embed nucleus in surface
• Pass through 210Pb (22 year half life)
• 210Po alpha decays
• Recoil nucleus enters LAr

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Scintillation in LAr

• Ionization radiation in LAr leads to formation
  of excited dimers
• Photons are emitted in dimer de-excitation
  processes
• Dimer molecules are in either singlet or triplet
  states, and the lifetimes are well-separated
• 6 ns for singlet state (prompt)
• 1.51 µs for triplet state (delayed)
• Fraction of dimers in singlet or triplet state
  depends on the incident particle type

Discrimination between nuclear recoils and EM
events by pulse shape discrimination net effect
is difference in photon emission vs. time curve
for ?/? events and for nuclear recoils
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PMT pulses from LAr in coincidence with ? in CsI
Background event
g-like
Nuclear Recoil
neutron-like
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7 kg LAr
Neck connects to vacuum and Gas/liquid lines
Quartz windows
11 x 6 (8 CF) tee
8 long acrylic guide
Acrylic vacuum chamber
ET 9390B PMT 5
poly PMT supports
Inner surface 97 diffuse reflector, covered with
TPB wavelength shifter
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Argon chamber PMT
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The DEAP-1 detector
Dark box
Ar liquefying chamber
Water shielding
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Fprompt distribution prompt light/total light
Preliminary
Background mainly ?s
WIMP ROI
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Na-22 data Fprompt distribution
WIMP ROI
Preliminary
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Pulse Shape Discrimination
Preliminary
Fitting analytical function combining
distribution of uncorrelated variables ( singlet
and triplet times) As soon as backgrounds appear
? deviation!
!More and ongoing data taking!
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Conclusion

• There is evidence for the existence of
  non-luminous, so far undetected matter in the
  Universe
• Theoretical models propose SUSY WIMPs as most
  likely DM candidates
• Experiments require very low background for event
  discrimination, since single WIMP event detection
  is not possible
• So far only new limits for cross sections/mass
  have been set The tiny, wimpy particles that
  might make up the Universe's dark matter must be
  even wimpier than some theories suggest (D.
  Harris for CDMS, Nature 2007)
• Actual experimental evidence is yet to be
  provided!

Thanks to Jeff Lidgard Mark Boulay (Queens
University, Canada) on behalf of the DEAP
collaboration. For further information
contactjjeff_at_sno.phy.queensu.ca or
mgb_at_sno.phy.queensu.ca