Dan Hooper

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In Search of Particle Dark Matter

• Dan Hooper
• Particle Astrophysics Center
• Fermi National Laboratory
• dhooper_at_fnal.gov

Massachusetts Institute of Technology March 13,
2006
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What do we know about dark matter?
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What do we know about dark matter?
Ask An Astrophysicist
? A Great Deal!
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The Existence of Dark Matter

• Galaxy and cluster rotation curves have pointed
  to the presence of large quantities of
  non-luminous matter for many decades (compelling
  since the 1970s)
• White dwarfs, brown dwarfs, Jupiter-like planets,
  neutron stars, black holes, etc?

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The Dark Matter Density

• WMAP best-fit LCDM model (for a flat Universe)
• 73 Dark energy (WL? 0.73)
• 27 Matter (?Mh2 0.27)
• ??h2 ? 0.0076

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Baryonic Abundance

• Big Bang nucleosynthesis combined with cosmic
  microwave background determine WBh2 ? 0.024
• But, we also know WM 0.3, so most of the matter
  in the Universe is non-baryonic dark matter!

Fields and Sarkar, 2004
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A Cosmological Concordance Model
LCDM
D. Spergel et al., ApJ, 2003
J. Tonry et al, 2003
SDSS (and 2dF), 2005
M. Tegmark et al, 2004
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The world is full of obvious thing which nobody
by any chance ever observes. -Sherlock Holmes

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What do we know about dark matter?
Ask An Astrophysicist
? A Great Deal!
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What do we know about dark matter?
Ask An Astrophysicist
? A Great Deal!
Ask A Particle Physicist
?Next to Nothing (but we have some good guesses)
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The Particle Nature of Dark Matter
Axions, Neutralinos, Gravitinos, Axinos,
Kaluza-Klein States, Heavy Fourth Generation
Neutrinos, Mirror Particles, Stable States in
Little Higgs Theories, WIMPzillas, Cryptons,
Sterile Neutrinos, Sneutrinos, Light Scalars,
Q-Balls, D-Matter, SuperWIMPS, Brane World Dark
Matter,
A virtual zoo of dark matter candidates have been
proposed over the years. 100s of viable
candidates. Weakly Interacting Massive Particles
(WIMPs) are a particularly attractive class of
dark matter candidates.
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The Thermal Abundance of a WIMP

• Freeze-out process Stable particle X in thermal
  equilibrium in early Universe later annihilation
  suppressed by Hubble expansion
• Freeze-out occurs at a temperature

Automatically generates observed relic density!!!
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The Thermal Abundance of a WIMP

• Relic abundance of a WIMP is naturally in ? h2
  0.01-1 range
• Strong motivation for WIMP dark matter
• Other types of dark matter only generate observed
  dark matter density if fixed by hand (axions,
  thermal gravitinos, WIMPzillas, etc.)
• Exception SuperWIMP scenario
• ? Focus on WIMP candidates for dark matter

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Supersymmetry

• Introduces new bosons for fermions and vice versa
• Elegant extension of the Standard Model
• Natural solution to hierarchy problem (stabilizes
  quadradic divergences to Higgs mass)
• Restores unification of couplings
• Necessary in most string theories
• Likely to be discoverd at Tevatron and/or LHC

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Supersymmetry

• To obtain sufficent proton stability,
  R-parity must be introduced
• R-parity ensures that the Lightest Supersymmetric
  Particle (LSP) is stable
• Possible supersymmetric dark matter candidate if
  LSP is not electrically charged or strongly
  interacting
• The identity of the LSP depends of the mechanism
  supersymmetry breaking

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The Minimal Supersymmetric Standard Model (MSSM)

• Introduce minimal new particle content
• New bosons squarks, sleptons
• New fermions photino, zino,
• neutral higgsinos (neutralinos)
• charged wino and higgsino (charginos)
• and gluino
• Of these, only the lightest neutralino and
  sneutrino are electrically neutral and
  non-strongly interacting (gravitino, axino are
  also possibilities in models beyond the MSSM)
• Only the lightest neutralino naturally generates
  the observed dark matter density

The lightest neutralino the most natural SUSY
dark matter candidate
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The Lightest Neutralino
Higgsinos
Bino
Wino

• Neutralinos are Majorana particles (their own
  antiparticles)
• Properties of the lightest neutralino can vary
  wildly depending on its composition
• Tree-level annihilation to heavy fermions, higgs
  or gauge bosons, any of which can dominate
• Magnitudes of the low velocity annihilation cross
  section and the elastic scattering cross section
  with nucleons can vary by many orders of
  magnitude
• Very rich phenomenology

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The Particle Dark Matter Candidate Zoo
Axions, Neutralinos, Gravitinos, Axinos,
Kaluza-Klein States, Heavy Fourth Generation
Neutrinos, Mirror Particles, Stable States in
Little Higgs Theories, WIMPzillas, Cryptons,
Sterile Neutrinos, Sneutrinos, Light Scalars,
Q-Balls, D-Matter, SuperWIMPS, Brane World Dark
Matter, etc
Some old saying about eggs and baskets comes to
mind
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Universal Extra Dimensions

• All Standard Model Particles Propagate in Bulk
• Kaluza-Klein (KK) Towers Appear, M R-1 TeV
• Extra-Dimensional Momentum Conservation ?
  KK-Number Conservation
• Realistic Models Must Be Orbifolded, KK-Number
  Violated, but KK-Parity
  Conserved ? Lightest KK Particle (LKP) Stable
• B(1) Most Natural Choice For LKP
• Excellent Dark Matter Candidate

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Kaluza Klein Dark Matter

• t-channel KK fermion exchange diagrams dominate
• Unlike neutralinos, annihilations are NOT
  helicity suppressed ? light fermions final
  states frequent
• Annihilations mostly to (large hypercharge)
  fermion pairs
• 20 taus, 20 muons, 20 electrons, 35 quarks,
  3 neutrinos, 2 higgs bosons
• Yields observed dark matter density for m 900
  GeV
• Coannihilations could accomodate 550-3000 GeV
• EWPO constrain m 700 GeV

(Flacke, Hooper, March-Russell, 2005)
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How To Search For A WIMP Colliders

• If mDM mEW (along with associated particles),
• discovery likely at LHC
  and/or Tevatron
• Strong constraints from LEP data

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How To Search For A WIMP Astrophysics

• Direct Detection
• - Momentum transfer to detector
• through elastic scattering
• Indirect Detection
• - Observation of annihilation
• products (?, e, p, ?, etc.)

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Indirect Detection Anti-Matter

• Matter and anti-matter generated equally in dark
  matter annihilations (unlike other processes)
• Cosmic positron, anti-proton and anti-deuteron
  spectrum may contain signatures of particle dark
  matter
• Upcoming experiments (PAMELA, AMS-02) will
  measure the cosmic anti-matter spectrum with much
  greater precision, and at much higher energies

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Indirect Detection Anti-Matter

• Anti-protons/anti-deuterons
• -Energy loss length of tens of kpc (samples
  entire dark matter halo)
• -Depends critically on understanding of halo
  profile, galactic magnetic fields, and radiation
  (diffusion parameters and background difficult)
• -For anti-deuterons, very low background and
  very low rate (single event discovery?)
• Positrons
• -Few kpc (or less) energy loss length
  (samples only local volume)
• -Less dependent on understanding of halo
  profile, diffusion parameters
• -Possible hints for dark matter present in
  existing data(?)

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Indirect Detection Positrons

• Positrons produced through a range of dark matter
  annihilation channels
• (decays of heavy quarks, heavy leptons, gauge
  bosons, etc.)
• Positrons move under influence of galactic
  magnetic fields
• Energy losses through inverse compton and
  synchotron scattering with starlight, CMB

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Indirect Detection Positrons

• Determine positron spectrum at Earth by solving
  diffusion equation

Energy Loss Rate
Diffusion Constant
Source Term

• Inputs
• Diffusion constant
• Energy loss rate
• Annihilation cross section/modes
• Halo profile (inhomogeneities?)
• Boundary conditions
• Dark matter mass

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Indirect Detection Positrons

• Observed spectrum depends on dark matter particle
  properties

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Indirect Detection Positrons

• Supersymmetric (neutralino) origin of positron
  excess?
• Spectrum can fit HEAT data
• Large annihilation rate required (non-thermal
  cross section and/or large degree of
  inhomogeneities)

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Indirect Detection Positrons

• Kaluza-Klein Dark Matter
• -Hard annihilation modes preferred (20 to each
  of ee-, ??-, ??-)
• ? still good fit to data
• -As with neutralinos, ultimately requires
• a large quantity of local inhomogeneities to
• normalize to HEAT data

Hooper and G. Kribs PRD (hep-ph/0406026)
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Indirect Detection Positrons

• The Annihilation Rate (Normalization)
• -If a thermal relic is considered, a large degree
  of local
• inhomogeneity (boost factor) is
  required in dark matter halo
• -Might local clumps of dark matter accommodate
  this?
• Two mass scales
• -Sum of small mass (10-1 - 10-6 M?) clumps
• ? Small boost (2-10, whereas 50 or
  more is required)
• -A single large mass clump (104 - 108 M?)
• ? Unlikely at 10-4 level

Hooper, J. Taylor and J. Silk, PRD
(hep-ph/0312076) H. Zhao, J. Taylor, J. Silk and
Hooper (hep-ph/0508215)
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Indirect Detection Positrons
Where does this leave us?

• Future cosmic positron experiments hold great
  promise
• PAMELA satellite, planned to be launched in 2006
• AMS-02, planned for deployment
• onboard the ISS (???)

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Indirect Detection Positrons
With a HEAT sized signal

• Dramatic signal for either PAMELA or AMS-02
• Clear, easily identifiable signature of dark
  matter

Hooper and J. Silk, PRD (hep-ph/0409104)
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Indirect Detection Positrons
With a smaller signal

• More difficult for PAMELA or AMS-02
• Still one of the most promising dark matter
  search techniques

Hooper and J. Silk, PRD (hep-ph/0409104)
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Indirect Detection Positrons
Prospects for Neutralino Dark Matter

• AMS-02 can detect a thermal (s-wave) relic up
  to 200 GeV, for any boost factor, and all likely
  annihilation modes
• For modest boost factor of 5, AMS-02 can detect
  dark matter as heavy as 1 TeV
• PAMELA, with modest boost factors, can reach
  masses of 250 GeV
• Non-thermal scenarios (AMSB, etc), can be easily
  tested

Value for thermal abundance
Hooper and J. Silk, PRD (hep-ph/0409104)
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Indirect Detection Positrons
Prospects For Kaluza-Klein Dark Matter

• For KKDM, AMS-02 can exclude thermal mass range
  for modest boost factors
• Coannihilation scenarios with large masses (m 1
  TeV) may remain untested
• Other DM models with annihilations to charged
  leptons will be highly constrained, especially
  for lower masses

Lower limit from EWPO
Value for thermal abundance
Value for thermal abundance
Cross section for KKDM
Hooper and J. Silk, PRD (hep-ph/0409104)
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Indirect Detection Neutrinos

• WIMPs elastically scatter with massive bodies
  (Sun)
• Captured at a rate 1018 s-1 (??p/10-8 pb) (100
  GeV/m?)2
• Over billions of years, annihilation/capture
  rates equilibrate
• Annihilation products absorbed, except for
  neutrinos

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Indirect Detection Neutrinos

• The IceCube Neutrino Telescope
• Full cubic kilometer instrumented volume
• Technology proven with predecessor, AMANDA
• First string of detectors deployed in 2004/2005,
• 8 more strings deployed in 2005/2006 (80 in
  total)
• Sensitive to muon neutrinos above 100 GeV
• Similar physics reach to KM3 in
• Mediterranean Sea

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Indirect Detection Neutrinos

• Neutrino flux depends on the capture rate, which
  is in turn tied to the elastic scattering cross
  section
• Direct detection limits impact rates anticipated
  in neutrino telescopes

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Indirect Detection Neutrinos

• WIMPs become captured in the Sun through
  spin-independent and spin-dependent scattering
• Direct detection constraints on spin-dependent
  scattering are still very weak

Spin-Dependent
Spin-Independent
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Indirect Detection Neutrinos
What Kind of Neutralino Has a Large
Spin-Dependent Coupling?
?
Z
q
q
q
q
Always Small
? fH12 - fH22
Substantial Higgsino Component Needed
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Indirect Detection Neutrinos
What Kind of Neutralino Has a Large
Spin-Dependent Couplings?
Large Rate At IceCube/KM3
Large Rate in IceCube/KM3


F. Halzen and Hooper (hep-ph/0510048)
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Indirect Detection Neutrinos
B(1)
B(1)
Kaluza-Klein Dark Matter
q(1)

• Spin-dependent scattering naturally dominates
• Annihilations to ??- (20), ?? (3) lead to
  large number of neutrinos
• Spin-independent cross section well beyond reach
  of direct detection

q
q
? SD 2 x 10-6 pb (TeV/m)4 (0.1/rq)2


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Indirect Detection Neutrinos

Kaluza-Klein Dark Matter

• Low masses, quasi-degenerate KK quarks, lead to
  large rates
• Relic density and EWPO
• considerations lead to prediction of
• 0.5-30 events/yr in km-scale telescopes
• (IceCube, KM3)

Hooper and G. Kribs, PRD (hep-ph/0208261), F.
Halzen and Hooper (hep-ph/0510048)
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Indirect Detection Gamma-Rays

Advantages of Gamma-Rays

• Propagate undeflected (point sources possible)
• Propagate without energy loss (spectral
  information)
• Distinctive spectral features (lines), provide
  potential smoking gun
• Wide range of experimental technology (ACTs,
  satellite-based)

Disadvantages of Gamma-Rays

• Flux depends critically on poorly known inner
  halo profiles
• ? predictions dramatically vary from model
  to model
• Astrophysical backgrounds

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Indirect Detection Gamma-Rays
The Galactic Center Region

• Likely to be the brightest source of dark matter
  annihilation radiation
• Detected in TeV gamma-rays by three
• ACTs Cangaroo-II, Whipple and HESS
• Possible evidence for dark matter?

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Indirect Detection Gamma-Rays

The Cangaroo-II Observation

• Consistent with WIMP in 1-4 TeV mass range
• Roughly consistent with Whipple/Veritas

Hooper, Perez, Silk, Ferrer and Sarkar, JCAP,
astro-ph/0404205
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Indirect Detection Gamma-Rays

The Cangaroo-II Observation

• Consistent with WIMP in 1-4 TeV mass range
• Roughly consistent with Whipple/Veritas

The HESS Obsevation

• Superior telescope
• Inconsistent with Cangaroo-II
• Extends at least to 10 TeV
• WIMP of 10-40 TeV mass needed

D. Horns, PLB, astro-ph/0408192
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Indirect Detection Gamma-Rays

Can A Neutralino Be As Heavy As 10-40 TeV?

• Very heavy neutralinos tend to overclose the
  Universe
• Largest annihilation cross sections (lowest relic
  abundance) are found for Wino-like or
  Higgsino-like neutralinos
• ?h20.1 for 1 TeV Higgsino, or 3 TeV Wino
• Significantly larger masses are possible only if
  coannihilations are carefully arranged (for
  example, S. Profumo, hep-ph/0508628)

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Indirect Detection Gamma-Rays
Can A Neutralino Be As Heavy As 10-40 TeV?

• Electroweak precision observables indicate the
  presence of a light higgs boson (near the EW
  scale)
• Large contributions to the higgs mass come from
  particle loops

• In unbroken SUSY, boson and fermion loops exactly
  cancel
• If mSUSY mHiggs , extreme fine tuning
  required
• mSUSY below 1 TeV is strongly preferred

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Indirect Detection Gamma-Rays
Messenger Sector Dark Matter

• In Gauge Mediated SUSY Breaking (GMSB) models,
  SUSY is broken in 100 TeV sector
• LSP is a light gravitino (1-10 eV), poor DM
  candidate
• Lightest messenger particle is naturally stable,
  multi-TeV scalar neutrino is a viable dark matter
  candidate

Dimopolous, Giudice and Pomarol, PLB
(hep-ph/9607225) Han and Hemfling, PLB
(hep-ph/9708264) Han, Marfatia, Zhang, PRD
(hep-ph/9906508) Hooper and J. March-Russell,
PLB (hep-ph/0412048)
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Indirect Detection Gamma-Rays
Messenger Sector Dark Matter

• Gamma-ray spectrum (marginally) consistent with
  HESS data
• Normalization requires highly cuspy,
• compressed, or spiked halo profile
• With further HESS observation of
• region, dark matter hypothesis should
• be conclusively tested
• Source appears increasingly likely to
• be of an astrophysical origin

Hooper and J. March-Russell, PLB (hep-ph/0412048)
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Indirect Detection Gamma-Rays
Astrophysical Origin of Galactic Center Source?

• A region rich in extreme astrophysical objects
• Particle acceleration associated with
  supermassive black hole?
• Aharonian and Neronov (astro-ph/0408303),
• Atoyan and Dermer (astro-ph/0410243)
• Nearby Supernova Remnant to close
• to rule out
• If this source is of an astrophysical
• nature, it would represent a extremely
• challenging background for future
• dark matter searches to overcome
• (GLAST, AMS, etc.)

Hooper, Perez, Silk, Ferrer and Sarkar, JCAP,
astro-ph/0404205
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Indirect Detection Gamma-Rays
Dwarf Spheriodal Galaxies

• Several very high mass-to-light dwarf galaxies in
  Milky Way
• (Draco, Sagittarius, etc.)
• Little is known for certain about the halo
  profiles of such objects
• For example, draco mass estimates range from 107
  to 1010 solar masses
• ? broad range of predictions for
  annihilation rate/gamma-ray flux
• May provide several very bright sources of dark
  matter annihilation radiation or very, very
  little
• Detection of Draco by CACTUS experiment???
• (Bergstrom Hooper, hep-ph/0512317 Profumo
  Kamionkowski, astro-ph/0601249)

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Summary

• Very exciting prospects exist for direct,
  indirect and collider searches for dark matter
• Cosmic anti-matter searches will be sensitive to
  thermally produced (s-wave) WIMPs up to
  hundreds of GeV (PAMELA) or 1 TeV (AMS-02)
• Kilometer scale neutrino telescopes (IceCube,
  KM3) will be capable of detecting mixed
  gaugino-higgsino neutralinos, and many KKDM
  models
• Gamma-ray astronomy is improving rapidly, but it
  is difficult to predict the prospects for dark
  matter detection given the astrophysical
  uncertainties Dwarf spheriodals are among the
  most promising sources

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The Cork Is Still In the Champagne Bottle

• In addition to indirect searches
• Direct detection experiments (CDMS) have
• reached 10-7 pb level, with 1-2 orders of
• magnitude expected in near future (many
• of the most attractive SUSY models)
• Collider searches (LHC, Tevatron) are
• exceedingly likely to discover Supersymmetry
• or whatever other new physics is associated
• with the electroweak scale

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But Maybe Not For Long