Recall from last lecture:

(igm d/dxm - m)y = 0
where y is four-component spinor, each component being a wave function.

Lepton Flavors, Quark Flavors, and Interactions

Not all particles interact via all the forces. It is useful to state explicitly which forces act on which particles.

We do not see free quarks in our experiments -- we call this phenomenon confinement. An individually produced quark will dress into a hadron on a time scale of 10-25 sec, as either a 3-quark combination called a baryon, or a quark-antiquark combination called a meson. For now, I list some of the hadrons and their constituent quark combinations. Also listed for each is its mass, and a quantity called strangeness that is discussed in the section on conservation laws.

MesonCompositionMass (MeV)Strangeness
p+u dbar139.570180
K0d sbar497.672+1
K-ubar s493.677-1
r-ubar d771.10
w0u ubar782.570

All of these particles have antiparticles, even the neutral particles, K0, L, and X0

Conservation Laws

Conserved quantities play an important role in physics, especially particle physics. I want to remind you of quantities that are conserved generally as well as the particular ones introduced for leptons and quarks. The table below lists the quantities with some examples of their application and some comments. Note that some quantities are conserved only in certain types of interactions.

QuantityForbidden ReactionComment
Chargepp –> p‾pCharge conservation rules out reactions such as the above.

Energy / Momentum

g –> e+e-This is called pair creation and can only happen in the presence of other matter; energy and momentum cannot both be conserved in this reaction.
Angular Momentumr0 –> gg
Lepton Numberm- –> e-g

Lepton number conservation forbids this decay; neutrino oscillation indicates that lepton number can be violated in weak interactions.

Baryon Numberp –> p0e+

Baryon number conservation means that the lightest baryon, the proton, is absolutely stable; some GUT's predict proton decay, but with a lifetime of >1031 years.

Quark FlavorL –> pp-

Strangeness is conserved in strong and EM interactions; this is weak decay, explaining the long lifetime of the lambda.

The Cosmic Connection

I encourage you to read this section. Particle physics is important in the theory of the big bang, the processes in stars, and the collapse of stars into white dwarfs, neutron stars, and black holes. I will discuss these topics later as time permits.

Copyright © Robert Harr 2003