Collisions between high-energy protons and antiprotons produce a
variety of elementary particles whose properties and decay schemes can
be studied with the giant UAI detector at CERN.A large body of experimental evidence also points to the existence of many hadrons
whose lifetimes may be only about 10^23 s. What can be meant by the idea of a par-
ticle that is in being for so brief an interval? Indeed, how can a time of 10^23 s be
measured?
Ultra-short-lived particles cannot be detected by recording their creation and
subsequent decay because the distance they cover in 10 ^23 s is only 3 10 ^15 m
even if they move at nearly the velocity of light—a length characteristic of hadron
dimensions. Instead, such particles appear as resonant states in the interactions of
longer-lived (and hence more readily observable) particles. Resonant states occur in
atoms as energy levels; in Sec. 4.8 we reviewed the Franck-Hertz experiment, which
demonstrated the existence of atomic energy levels by showing that inelastic electron
scattering from atoms occurs only at certain energies.
An atom in a certain excited state is not the same as that atom in its ground state
or in another excited state. However, such an excited atom is not spoken of as though
it is a member of a special species only because the electromagnetic interaction that
gives rise to the excited state is well understood. The situation is somewhat different
for elementary particles because the weak and strong interactions that also govern them
are more complicated and were not really understood until relatively recently.Elementary Particles 483
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