A Lightning Review of Dark Matter

R.L. Cooper6-6-2012

Orbital Velocity: A Sample Calculation

• The radial velocity of a probe a distance r from the galactic center

• The mass contained within r is M(r)

Expected Form

• Star light is majority of Baryonic mass• Expectation: radial velocity

fall-off• Similar to Solar System

Keplerian motion

Measured Radial Velocity

• Radial velocity mostly flat•

• There’s a massive halo dictating galactic dynamics

More Evidence for Dark MatterCosmic Microwave Background Large Scale Structure

Dark Matter Properties

• Local galactic velocity• Energy density• • Cross section controls number density

Dark Matter Candidates

Sources• Baryonic Matter (e.g.

MACHOS – MAssive Compact Halo ObjectS)

• Neutrinos• Other exotics (Axions)• Weakly Interacting Massive

Particles (WIMPs - c)

Consequences• Brown dwarfs, neutron

stars, black holes, cold gas clouds, etc.

• Gravitational lensing• Not enough matter

Dark Matter Candidates

Sources• Baryonic Matter (e.g.

MACHOS – MAssive Compact Halo ObjectS)

• Neutrinos• Other exotics (Axions)• Weakly Interacting Massive

Particles (WIMPs - c)

Consequences• Hot dark matter• Low mass implies relativistic

at formation• Large-scale structure is

smoothed• Masses < 1 eV• Steriles?

Dark Matter Candidates

Sources• Baryonic Matter (e.g.

MACHOS – MAssive Compact Halo ObjectS)

• Neutrinos• Other exotics (Axions)• Weakly Interacting Massive

Particles (WIMPs - c)

Consequences• Introduced to address the

strong CP problem of QCD• Low mass – Nambu-

Goldstone boson• Mass << 10-3 eV• Deeper analysis is beyond

the scope of this report

Dark Matter Candidates

Sources• Baryonic Matter (e.g.

MACHOS – MAssive Compact Halo ObjectS)

• Neutrinos• Other exotics (Axions)• Weakly Interacting Massive

Particles (WIMPs - c)

Consequences• 10-1000 GeV, very non-

relativistic at formation• Tiny cross-section• Froze-out in early universe

expansion• Lightest Superparticle in

SUSY (LSP) a top candidate• Kaluza-Klein, extra-dim.

Dark Matter Abundance

• Annhilation rate in early universe

(in equilibrium)• Hubble expansion

freezes out

Dark Matter Collisions

• Non-relativistic 10-3 c• Elastic scattering• • Neutralino LSP can

interact through Higgs, Z, squark with matter

• Interaction on nucleon• • Coherent on nuclei

implies A2 enhancement

(A,Z)

c

Recoil Energy Spectrum

• Recoil energy uniformly distributed from 0 to maximum energy deposit

• Incident WIMP energy Maxwellian

• A given energy deposit• Exponential signal in energy deposit

Detection of Dark Matter

• Other neutral elastic collisions are backgrounds (g, n)

• c and n recoil on nuclei• g recoil on electrons• Recoils have very

different • Can imply different light

output (e.g. quenching)

• Different excitation alter signal time-dependence

• Discrimination possible

Detection Methods

Standard Techniques• And combinations of these

Other Methods

(A,Z), c

n

Ionization

Scintillation

Phonons

Bubble Chamber

Gas / Directional

Axion Cavities

Direct / Indirect searches

Yearly / siderealvariation

a decay

(a, n)reaction

fissions

a

n

n

m

n

nm-inducedn spallation

m-inducedn spallation

Multiplicity?

Uncorrelated Neutron Backgrounds

rock

n

Coherent nscattering

m

n

m-inducedn spallation

Correlated Neutron Backgrounds

rock

radio-impurities

n

Reading Exclusion PlotsFinite detectionthreshold

Flux decrease~ 1 / mc

Excluded

AvailablePhase Space