christopher rogan- wimp dark matter
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WIMP Dark Matter
Christopher Rogan
Ph135c - 04/27/07
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What is a WIMP?
A. B.
C. Weakly interacting massive particle
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What is a WIMP?
A. B.
C. Weakly interacting massive particle
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Evidence for CDM• See Matt’s talk from 04/24/07
• In a nutshell, estimated that ~25% of critical density must be nonrelativisticnonbaryonic matter
• => Cold Dark Matter
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WIMP candidates
• Many theories for physics beyond the
Standard Model feature stable lightest
particles (KK neutrinos, gravitinos,
neutralinos)
• Often potential WIMP CDM candidates
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Example: MSSM
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WIMPs in early universeEarly hot universe, WIMPs
effectively mass-less, in
thermal equilibrium with
SM particles
WIMPs go to
SM particles
and vice-versa
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WIMPs in early universeSatisfy Boltzman equation:
Expansion rate
Actual number density
Thermal equilibrium
number density @ T
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WIMPs in early universeIn hot universe
As the universe expands, temperature drops, and eventually
(about when ) , or:
Freeze-out! Thermal bath of particles becomes nearly
transparent to WIMPs. Ratio of number density to entropy
density remains constant.
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WIMPs in early universeWhat we find is:
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WIMPs in early universeHence very interesting results:
• WIMPs are well within non-relativistic regime
• A value of the relic density corresponding tothat expected for CDM implies an annihilationcross section on the order of the weakinteraction
• New physics at electroweak scale with stableneutral particle => CDM WIMP candidate
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CMB
measurements
can be used to
constrain
theories with
CDM
candidates
mSUGRA with
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WIMP CDM Direct Detection
• Look for nuclear recoils in terrestrial
detectors
• Rates depend on local halo density,
velocity distribution in Milky Way, the
WIMP mass, and the cross section onthe target nuclei
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CDMS• Z(depth)-sensitive Ionization and
Phonon detectors (ZIPs) measure the
ionization and athermal phonon
• Six Ge (250 g each) and six Si (100 g
each) ZIPs @ 50 mK
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Use time delay,pulse rise time and
energy distribution
‘partition’ to
discriminate
between electron
and nuclear recoils
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Spin dependent cross-sections
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WIMPs @ accelerators
Large Hadron
Collider
proton-proton
collisions at CM
energy of 14
Tev
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Compact Muon Solenoid
(CMS)General Purpose
detector designed to
discover new physics
at TeV scale
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LSP’s @ CMS• Must identify SUSY events from
Standard Model background
• CMD candidates are stable - escapedetector
• Determining LSP properties difficult
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Sample CMS Event
~20 ‘pile-up’
interactions per
bunch crossing
LSP’s identified by
identifying missing
transverse energy
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LSP @ CMS
R-parity conservationimplies LSP’s are produced
in pairs
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R-parity conserving
SUSY events feature
a particle cascade to
the stable LSP
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SUSY events must be distinguished from SM background
For example, W/Z + jets
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Other backgrounds
more difficult to
distinguish fromSUSY
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Neural networks,
with kinematic
variables as
inputs, can be
trained to
distinguish
between signal
and background
events
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Cuts on neural network output selects SUSY events
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Outlook• WIMPs(potenitally the lightest
neutralino) are appealing CDM
candidates
• Through direct and accelerator
searches we continue to look for WIMPs