Dark Matter Part IV Indirect Detection Methods
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 2
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 3
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 density Boost factor: Local DM density ρ0 = 0.3 GeV/cm3 4
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! 5
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 Not deflected by B field But cannot pass through ionized matter (photosphere) Gamma ray telescopes Space based Look for gammas Ground based Look for showers from gammas in atmosphere 6
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! 7
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 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 Charged particle paths are bent due to galactic magnetic field Impossible to identify source 8
Pamela, AMS02 Space telescopes Measure positron/electron ratio Both see an excess of positrons Could be due to dark matter 9
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 10
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 11
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 12
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 13
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! 14