Dark Matter:A particle physics approach

4/23/2018

Lecture 23

Extra Credit:Thursday April 26th 2:00 PM

Room 190 Physics and Astronomy

• Probing the Cause of Lagging Gaseous Halos with Radio Continuum Observations • Presented by Tim Braun (UNM)

Fundamental Forces

• Strong nuclear force: • Holds nuclei, protons and neutrons, of atoms together. • Strongest force but works on small distances.

• Electromagnetic force:• Binds electrons to protons• Two charges, +/-, most things are electrically neutral, has infinite range• 1000 times weaker than strong force

• Weak force:• Changes protons to neutrons• Range 10-18 meters.• 10-16 weaker than strong force

• Gravity• Attractive force between objects with mass• Infinite range• 10-41 weaker than strong force

The Standard Model of Particle Physics

Fundamental Electromagnetic interaction

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Light that radiates from a star

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Light being converted into matter anti matter

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Gravity - General Relativity

Primary Evidence for Dark Matter

Dark matter predicted not to interact with ordinary matter, or itself,

except through gravity. Gas clouds, by contrast, can run into each other.

A collision of two clusters provides dramatic evidence for dark matter:

cluster

trajectory

cluster

trajectory

red shows hot gas from

two clusters, seen with

Chandra X-ray

observatory.

The gas clouds have run

into each other, slowing

each one down

blue shows inferred distribution

of cluster mass from gravitational

lensing of background galaxies.

The dark matter has gone straight

through with no interaction, like

the galaxies have.

The Bullet Cluster

Dark Matter

• Dark matter is matter that has been observed to only interact via the gravitational force.

• We have definitive proof that dark matter exists when we look at large scale objects (galaxies, globular clusters, etc…).

• Dark matter exists as bent space time ( seen from gravitational lensing and rotation curves).

• Global scientific effort to find dark matter as a particle.

Lambda Cold Dark Matter

• ΛCDM is the Standard Model for the Big Bang. It predicts:• the existence and structure of the cosmic microwave background• the large-scale structure in the distribution of galaxies• the abundances of hydrogen and helium• the accelerating expansion of the universe

• Cold Dark Matter is postulated in order to explain:• Large-scale structure of galaxies• Flat rotation curves • Gravitational lensing by galaxy clusters

• CDM is slow and collision-less:• Axions• Weakly Interacting Massive Particles (WIMPs)

Axion

• Named after Axion, a brand of laundry detergent, for the notion that the new particle could "clean up" a problem in physics.

• 1-100 billion times lighter than a proton, copiously produced in the early universe.

• Just like an electron/positron pair, axion/anti-axion pair could be produced from photon.

• Why don’t we see this in the lab?• Incredibly rare process

ADMX- Axion Dark Matter Experiment

• ADMX is an axion antenna, convert dark matter axions to detectable to microwave photons.

• Uses a strong magnetic field to “capture” axions and turn there mass into a radio signal.

• Prior experiments used the “Shine Light at a Wall” method.

• Measuring the cooling rate of white dwarf stars also sets a limit on the axion.

WIMPs• Weakly interacting massive particles (WIMPs) best candidate for dark matter • Interact with normal matter through gravity and the weak force.

• For every theorist, there are two theories of WIMPs

• Incredibly massive:• Clumps around normal matter• 1-1000 times more massive than the proton

• Low cross section:• Does not collide with itself or normal matter.

• Only will collide with anything in extreme scenarios (black hole accretion disk, supernova)

• Detection methods include:• WIMP annihilation• Direct detection• Direct production

WIMP Annihilation

• What happens when two dark matter particles collide?• Do they produce high energy

particles we know of? If so, we can look for this.

• If two dark matter particles annihilate, they will produce an emission line at a specific energy.

• Mechanisms that could create ‘false’ dark matter annihilation signals are not well know.

Direct Detection

• If WIMPs are weakly interacting, eventually they will collide with normal matter.

• If they collide with normal matter, what does that signal look like?

• Best experiments use scintillating noble liquids (Xenon, Argon…)

• Every experiment is deep underground away from cosmic photons.

XENON Dark Matter

Experiment

Direct Production

• Large Hadron Collider (LHC) at CERN used E = mc2

to look for incredibly high mass particles.

• By colliding protons together at very high speeds, they try to replicate energies similar to the Big Bang.

• The LHC has an incredible ability to track every particle produced from their collisions.

• Dark matter particles can not be tracked.

• By looking for an absence of particles, we can search for dark matter.

Dark Energy

• The expanse of the universe is accelerating.

• The only way to explain this is to give all of space some energy density (vacuum energy).

• Vacuum energy is like the density of the nothing (a vacuum).

• Has a negative (repulsive pressure) to explain the acceleration of the universe.

• Some models of dark energy predict that the force of dark energy will continue to grow until it dominates all other forces in the universe.

Chameleon particle

• The chameleon is a hypothetical scalar particle that couples to matter more weakly than gravity, postulated as a dark energy candidate.

• It has a variable effective mass which is an increasing function of the ambient energy density• Incredibly massive near other massive objects (like the earth)

• Incredibly light in intergalactic space

• On Earth it would be so massive that the LHC could not create it.

• In intergalactic space, it would be so light, we would not see it with our telescopes.

WIMP Results

•No definitive results for WIMP detection. Only limits.

•What is the point of looking for a dark matter?• If no dark matter is found, the community has developed

novel new detectors that can be used for industrial and commercial applications.

• Science for science sake is always beneficial.