1. Matter vs. Antimatter The Question of Symmetry
Joseph C. Brossett

2. Origin of Antimatter In 1928, the Dirac equation is introduced.
Equation calls for the existence of negative-energy states.
“Holes” in these negative-energy states called for positively charged electrons.

3. Origin of Antimatter
Dirac suggests that the “holes” could correspond to new particles with opposite charge of known particles.
The positron is observed a year later.

4. The Question of Symmetry
These developments led to a question that is the focus of this talk.
“Why is our universe dominated by matter particles and not antimatter particles?”

7. Definitions
Parity invariance – states that the probability of a particle process occurring is exactly the same as the process occurring with the position vectors and the directions of the particles reversed
Baryon – three quarks bound together (proton)
Lepton – particle that has spin ½ and experience no strong forces (electrons)

8. Defintions
Meson – a particle made up of a quark and a anti-quark

9. Ideas to Answer the Symmetry Question
Initial conditions idea considered unacceptably by Pauli and many others.
Conservation of matter replaced by conservation of baryon number and lepton number.
CP violation found in weak interactions.
Pauli suggests that CP violation could explain the asymmetry observed today.

10. Ideas to Answer the Symmetry Question
An experiment in 1964 shows that CP violation does in fact occur. (K0 –meson)

11. “Baryogenesis and Leptogenesis”
A change in net baryon number is allowed by CP non-conservation and these processes could have occurred at times when the universe was out of equilibrium (high temperature > 100 GeV).
Also, leptogenesis (change in net lepton number) resulting from massive neutrinos could lead to CP violation.

12. Question Unanswered
The observed asymmetry is as yet not able to be explained.
Some “new physics” and further study of CP violation is needed to understand why the matter and antimatter asymmetry is as it is.