Elementary Particles, Quarks and Quantum Chromodynamics 247
force must include a study of the meson-nucleon and the meson-meson
interactions as well as the nucleon-nucleon interaction. As is so often the
case, the partial solution of one problem introduces a host of other
problems as well.
The π meson is a fairly long-lived particle on the scale of nuclear
times. A nuclear reaction takes place in 10-23 seconds; therefore, the
10 -16 sec lifetime of the neutral pion, πo and the 2.5 × 10-8 sec lifetimes of
the charged pions, π+ and π- indicate that the pions are fairly stable
particles. The πo decays via the electromagnetic interaction into two
photons, πo → γ + γ, whereas the charged pions decay via the weak
interaction into muons and neutrinos: π+ → μ+ + νμ and π– → μ– + νμ.
The charged pions can also decay into an electron and a neutrino
or a muon, a neutrino and a photon. These two decay modes are much
less frequent, each occurring only once in ten thousand decays. By
comparing the rate of the reaction p + p → π+ + d with the reverse
reaction π+ + d → p + p, it was found that the spin of the pion is zero.
Careful determinations of the masses revealed that the charged pions
have identical masses of 139.6 MeV, but that the mass of the πo is
4.5 MeV less than the mass of the charged pions. The extra mass of
the charged particles is due to the positive electric self-energy due to the
potential energy of the meson’s charge with itself. The differences in the
lifetimes and masses of the different pions are due to these differences in
charge. As far as the nuclear force is concerned, these particles are
completely symmetrical as is the case with the neutron and the proton.
Because of the plentiful supply of pions produced when protons
collide with other protons or nuclei, it was decided to use the pion as a
probe to further explore the nature of the nucleus and the strong
interaction. The very short lifetime of the pion, 2.5 × 10-8 sec might
appear to rule out making a beam of pions. Pions, however, are created
with very high velocities. Even a pion with only 0.7 MeV kinetic energy
has a velocity of 0.1 c and hence can travel on the average 0.1 c × 2.5 ×
10 -8 sec or 75 cm before decaying. This gives the experimentalists
enough room to magnetically collect a beam of charged pions produced
in the collision of a proton with a heavy target such as lead, and then
magnetically direct that beam at a target such as liquid hydrogen. This
enables the experimentalists to study pion-nucleon collisions. The
experimentalists must use charged mesons, however, because they are
unable to control neutral mesons with magnets. The experimentalist uses