Lecture 14

Dark Matter

Part IVIndirect Detection Methods

PHYS 2961 Lecture 14 2

WIMP Miracle Again

● Weak scale cross section● Produces the correct relic

abundance● Three interactions possible

with DM and normal matter● DM Production● DM Annihilation● DM – normal matter

scattering

● We already discussed scattering (direct detection)

● Production: create DM in collider

● Annihilation: Freeze out process, and indirect detection

PHYS 2961 Lecture 14 3

Annihilation

● After freeze out, WIMPs stopped annihilating

● So how can we detect this today?

● There should still be rare collisions of DM today

● The mass energy of the DM pair gives high energy SM particles

● We look for these signatures

Indirect DM detection● Look for these annihilation

products

PHYS 2961 Lecture 14 4

Where to look for DM signals

If DM is all around us, where should we look?● Annihilation rate depends on cross section● Also depends on two DM particles finding one

another● Proportional to ρ2

● Detection depends on likelihood for a given set of annihilation products● Called the branching ratio BR

i

Since rate depends on ρ2

Look for high DM densityBoost factor:

Local DM density ρ0 = 0.3 GeV/cm3

PHYS 2961 Lecture 14 5

Origin of Boost

● How does DM accumulate?● It's supposed to be non-collisional!● It occasionally scatters off of normal

matter● Loses energy in the process, can

become gravitationally bound● Frequency of collisions proportional to

amount of matter

Where is there a lot of matter?● Stars (e.g. our sun)● Galactic center● Dwarf Galaxies● Galaxy clusters

What annihilation products do we look for?Depends on where we look!

PHYS 2961 Lecture 14 6

Gamma Rays

Gamma rays:● Two classes● DM annihilates directly to 2

gammas● Gamma energy = DM mass● Smoking gun of DM

● DM annihilates to quarks and gauge bosons (W,Z,h)● Subsequent decay gives

spectrum of gammas

Gamma ray telescopes● Space based

● Look for gammas● Ground based

● Look for showers from gammas in atmosphere

● Not deflected by B field● But cannot pass through ionized

matter (photosphere)

PHYS 2961 Lecture 14 7

Gamma ray telescopes

Fermi-LAT● Space telescope● Sees a line at 3.5 keV

● Coming from galactic center● Inconsistent with background● Could be warm DM

● Generating interest in warm DM● Mass and KE around keV● Doesn't fit as well with structure

formation● But not ruled out like hot DM● Also not likely from SUSY

Time will tell if this is DM or something else!

PHYS 2961 Lecture 14 8

Antimatter

● Very few known sources● DM annihilations can create matter antimatter

pairs● Simple to complex:

● Positrons● Antiprotons● Antideuterons

● Carry only some of DM mass energy● Spectrum

● Charged particle paths are bent due to galactic magnetic field

● Impossible to identify source

● Space based telescopes● Measure antimatter/matter ratio

● e+/e-, p/p● Ratio cancels many systematics● Very clean measurement

● Look for antimatter here at Earth● From “local” DM annihilations

PHYS 2961 Lecture 14 9

Pamela, AMS02

● Space telescopes● Measure positron/electron ratio● Both see an excess of positrons● Could be due to dark matter

PHYS 2961 Lecture 14 10

Neutrinos

● Direct neutrino production, Eν = DM

mass● Suppressed in SUSY, allowed in other

theories● Indirectly produced in SM decays

● Carry only some of DM mass energy

● Not deflected by B fields● Pass through ionized matter● Excellent for directionality

● Local source of neutrinos from DM● Our Sun!● We can see the neutrinos from

inside the photosphere

PHYS 2961 Lecture 14 11

Ice Cube

● Look for neutrinos with the Antarctic ice sheet

● Look for a “sonic boom” of light● Cherenkov light● Comes from particle traveling

faster than light in matter

PHYS 2961 Lecture 14 12

DM Production

● Collide two SM particles (like at the LHC)● Enough center of mass energy to pair

produce DM● DM will fly right through the detector● No signal● Shows up as missing energy

● LHC is a hadron collider● Proton – proton collisions● But protons are not fundamental particles● Incoming momentum not known● So bow do we look for missing energy?

● Incoming momentum directed on beam axis● Transverse momentum is zero● Outgoing transverse momentum must also

be zero● Look for collisions with net transverse

momentum

PHYS 2961 Lecture 14 13

Tri Lepton Search

● Known physics gives missing transverse momentum

● Neutrinos● Need a way to tag events that would

come from dark matter

● One example: Tri lepton search● Create heavy SUSY particles● These decay into leptons and DM● Three leptons can be tagged in

detector● Accompanied with missing transverse

momentum

● Other searches look for similar types of events

● Something easy to tag, with missing energy due to DM

● No detection of DM or new physics yet● There is a hint of a new boson● Both CMS and ATLAS experiments see it● Too few statistics to be sure yet● Could be related to SUSY or another

model with DM● We should find out about this later this

year

PHYS 2961 Lecture 14 14

How do we find DM?

To solve the DM mystery, we must:● See DM in multiple direct detection

experiments● Measure its mass, cross section, spin

● See DM annihilation products in indirect detection experiments● Measure mass, cross section, density

● Produce DM in colliders● Measure its mass, cross section, spin

● We have to understand everything about the WIMP miracle● Origin and density of DM

● Also understand the larger theory that contains DM

● Paradigm changing science!