Introduction to Particles
Radioactivity and high-energy particles are with us in our daily lives.
However, they are studied in laboratories with sophisticated detectors.
Millikan coined the term cosmic rays for radioactivity
measured at high-altitude. This term is now used for high-energy
particles from the space.
The Earth is constantly bombarded with a stream of high-energy particles
arriving not only from the Sun, but also from interstellar and galactic
For more information on cosmic rays, see
Advanced Composition Explorer (ACE) Mission
Primary galactic cosmic rays consist of
protons (83%, energy up to 2 GeV common, some 1000 GeV)
- helium nuclei (16%)
- other nuclei (1%)
Secondary cosmic rays consist of high-energy subatomic particles.
Electrons are stable, but muons and taus are not.
m+ ® e+ +
ve + vm
m- ® e- +
anti-ve + anti-vm
t+ ® e+ +
ve + vt
t- ® e- +
anti-ve + anti-vt
C.F. Powell studied cosmic rays using photographic emulsions as detectors.
He recorded some tracks similar to muon but slightly heavier, 1/7th (as opposed
to 1/9th) of the proton mass. These tracks are due to pions.
Mesons consist of a quark and antiquark. The early known mesons are
pions and kaons.
Some of the pions decays are given here:
p+ ® m+
p- ® m-
p+ ® e+
p- ® e-
po ® g + g
The decays of kaons are more complicated:
Ko ® g + g
or p+ + p-
K+ ® m+ + nm
or p+ + po or ...
K- ® m- + nm
or p- + po or ...
(note error in lecture notes please)
(see table of Properties of Some Baryons)
Baryons consist of three quarks.
Protons and neutrons are well known.
Lambda and sigma particles contain a strange quark.
Charmed lambda contains a charm quark.
Symmetry and Particles: A discussion of symmetry, group theory, and
the fundamental properties of particles.