Introduction to

QEDQuantum Electrodynamics

Part IV

Applications of QED

• Through the use of atom smashers, over 400 particles have been discovered.

• This multitude of particles has created a need to explain their number and the nature of their interactions.

• QED deals specifically with electrons and photons, but its form and function can be applied to other particle interactions.

QCD

• A great number of these newly-discovered particles are simply combinations of quarks.

• The quantum theory of the interactions of quarks via the strong force is called Quantum Chromodynamics (QCD).

• 3 make a baryon and 2 make a meson

Elementary Particles

Baryons

Gluons

• Quarks interact via the gluon.• The gluon functions in many ways like a

photon.• The probability of a coupling occurring is

the constant “g” (similar in function to “j” for electrons)

• Diagrams of interactions will look very similar to that of the electron and photon.

Elementary Particles

Quark-Gluon Coupling

Color

• Quarks and gluons have a property called “color”.

• Quarks can change color by coupling with a gluon.

• Colors are red, green, and blue.

Change of Color

Gluon Coupling

Rules of Color

• All particles created by quarks must be colorless.

• Baryons have one of each color and mesons have a quark-anti-quark pair.

• Impossible to have a single quark.

Change of Flavor

• A down quark can change into an up quark.

• This is done by emitting a W particle, which then decays into an electron-anti-neutrino pair.

• This process is called beta decay.

Beta Decay

Elementary Particles

Neutral Currents

• There is a particle Z0 that is a neutral W boson.

• Z0 has no charge.

• Couplings with Z0 result in no change in a particle’s charge. (Neutral Currents)

Z0 Couplings

W-Anti-W Coupling

Electro-Weak Force

• The observed coupling constant is almost identical as that of the photon.

• The three W’s and the photon would then appear to be somehow interconnected.

• Electrodynamics and the weak force were successfully combined by Stephen Weinberg and Abdus Salam

“Redundant” Particles

• As nuclei have been bombarded by protons of higher and higher energy, new particles have appeared.

• These particles seem to mimic lower-energy versions and differ only by their higher mass.

• E.g. The Muon is identical to an electron, except it’s about 200 times heavier.

Muon Interactions

Beta Decay with a Muon

More Elementary Particles

Even More Elementary Particles

Diagrams:

Feynman, Richard P. QED: The Strange Theory of Light and Matter. Princeton University Press. Princeton, NJ, 1988.

Questions?