The Dark Side of the Universe

What is dark matter?

Who cares?

What holds Galaxies together?

• What stops galaxy from flying apart?

Gravity holds stars, galaxies together

Object 2Object 1

Gravity pulls objects together

Strength of gravitational pull depends on masses of object 1 and object 2

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MMGF N

Fritz Zwicky

• 1932 computed the mass of the Coma cluster

Many pieces of evidence

• Mass of the Milky Way determined from the orbital velocities of globular clusters

Distance Mass (billion solar masses)

20 kpc 200

50 kpc 440

50-100 kpc 500

100 kpc 1000

1 kpc = 1 kilo par sec = 3 x 1019 m

• Observe:

• Compute:

• Analysis required 400 times more mass than the mass of the stars in the galaxies

Much of this mass is hot gas

• Hot gas radiates X-rays

Kepler’s 3rd Law:

• Measured velocities:

Mercury 46 km/sec

Earth 30 km/sec

Jupiter 13 km/sec

Neptune 5 km/sec

rv

1

What Actually Happens

To understand this, there must be mass that we don’t see

The Evidence

• Look at galaxies and measure their speed• Estimate how heavy they are from what we see • Look at nearby objects and see if there is

enough mass to hold galaxy together– ESTIMATE IS WAY OFF……– ASSUME THERE IS HEAVY MATTER THAT HOLDS

UNIVERSE TOGETHER THAT WE DON’T SEE– We call this dark matter

The Picture

• Galaxies are surrounded by heavy dark matter which we don’t see: Galactic Halo

What is Dark Matter?

Dark Matter Not Dark Matter

Dark matter is matter which cannot be observed through the light or radio waves it emits

How do we know dark matter is really there?

• A large concentration of mass such as a cluster of galaxies will bend light– Prediction of general relativity

• It acts as a gravitational lens

• If there is a bright galaxy or a quasar behind a cluster, the bending measures the mass of the cluster

Gravitational Lens

• Orange lines show apparent source of light

• White line is actual light path

We are interested in the things we don’t see

As we look into the universe, we see stars, galaxies, clusters…

Space Telescope Image Where the mass is

We live in special times

We have a census of the universe

Colors show distribution of temperature

• Differences in WMAP picture result from gravitational clumping of matter

The Universe has an Energy Budget Crisis

• Stars and galaxies are only ~0.5%

• Neutrinos are ~0.3–10%

• Rest of ordinary matter (electrons and protons) are ~5%

• Dark Matter ~30%

• Dark Energy ~65%

The Cosmic Questions

• What is Dark Matter?

• What is Dark Energy?

• How much is the neutrino component?

• What is the evidence for dark matter and dark energy? How do we know there is missing mass?

These questions connect particle physics and cosmology in an inescapable way

Searching for Dark Matter

• If dark matter particles are concentrated at the center of the galaxy, they might annihilate

• Maybe we can see the gamma rays

• We can also look for dark matter particles hitting the earth

• Imagine that in the early universe, there were additional particles, including one that is

NEUTRAL, HEAVY, AND ABSOLUTELY STABLE

Created and annihilated in pairs

• We call this a WIMP (weakly interacting massive particle)

The Early Universe

The Universe Expands and Cools

Today

The left-over density is density mN2

We get the observed density for mN2 200 GeV

• The density of WIMPS depends on how quickly they annihilate

• To get the right density today, the effective size is 1/1000 of the atomic nucleus

• WIMPs must be particles with weak interactions and masses 100-1000 times heavier than a proton.

Particle Dark Matter

• We’d like to detect dark matter in the lab– To show they’re in the galactic halo …

• And to produce them at an accelerator– To measure their properties …

Why put detector underground?

If we saw something, how would we know what it was?

• We need to know the probability that a dark matter particle will hit another particle and interact

What is the Dark Matter?

• WIMPs are likely candidates: stable, electrically neutral, weakly interacting particles

• There are no WIMPs in the Standard Model of particle physics

• But most theories of unification have candidates– Lightest supersymmetric particle …– Lightest Kaluza-Klein particle in theories with extra dimensions

Dark matter is evidence for new physics in the Standard Model of

particle physics!

Supersymmetry

• Supersymmetry assumes that every particle has a partner particle

• We haven’t seen any of them

• The partner of the photon could be the dark matter

• What kind of theory naturally gives the WIMPS needed to be dark matter?

SUPERSYMMETRY

Supersymmetry predicts many particles, not just WIMPS

Supersymmetric Theories• Predict many new undiscovered particles (>29!)• Very predictive models

– Can calculate particle masses, interactions, everything you want in terms of a few parameters

• Any Supersymmetric particle eventually decays to the lightest supersymmetric particle (LSP) which is stable and neutral!!!

Dark Matter Candidate

Supersymmetric explanation of Dark Matter can be tested!

• Supersymmetric models have natural dark matter candidate

• Lightest supersymmetric particle (LSP) is neutral and weakly interacting

• On general grounds, LSP contributes correct amount of dark matter if its mass is 300 GeV-1 TeV

• Supersymmetric particles within reach of LHC and a Linear Collider

Dark matter inescapably links particle physics and

astrophysics

A major goal of LHC and Linear Collider is to produce the LSP (supersymmetric dark matter

candidate) and measure everything about it

• Two kinds of particles seen in nature

– Fermions: Matter is made of fermions (quarks and leptons)

– Bosons: Particles which mediate forces

– Is there a theory which treats these types of particles in an identical fashion? (we call this unification)

• We say that fermions have spin ½ and bosons have spin 1 or 0.

• So a symmetry relating fermions and bosons would change spin by ½

• This symmetry is supersymmetry

New particles in Supersymmetric Theory

• Spin ½ quarks spin 0 squarks• Spin ½ leptons spin 0 sleptons• Spin 1 gauge bosons spin ½ gauginos• Spin 0 Higgs spin ½ Higgsino

Experimentalists dream….many particles to search for!

What mass scale?

We haven’t seen any of these particles!

At the LHC

• Gluon –gluon collisions create pairs of new particles.

• These decay to lighter particles plus WIMPS

• An event might look like:

We can look for WIMPS at the LHC

• WIMPS produced at the LHC wouldn’t make a signal in the detector

• Particles produced with the WIMPS would interact however

• The rates are expected to be large

Supersymmetry

• Can we find it?

• Can we tell what it is?

• Masses of new particles depend on mechanism for breaking Supersymmetry

• Couplings of new particles predicted in terms of few parameters

• Simplest version has 105 new parameters

Do the forces unify?

• Coupling constants change with energy

• Coupling constants unify in supersymmetric models

Hint for new physics?