Elementary Particles, Quarks and Quantum Chromodynamics 241
exchanged particle, the photon, is zero. The first to attempt an
explanation of the nuclear force due to the exchange of virtual particles
was Heisenberg in 1934, who thought that a neutron and a proton could
have a strong interaction by the exchange of an electron and a neutrino.
But the range R = h/mec of this interaction would be 4 × 10-11 cm, which
is about 400 times the actual range of the nuclear force. This hypothesis
had to be abandoned.
A year later, in 1935, the Japanese physicist H. Yukawa proposed a
theory for the strong interaction incorporating Heisenberg’s idea of
exchange. He postulated that nucleons interact by the virtual exchange of
a new particle which he called a mesotron and which later was shortened
simply to meson. Yukawa assumed that this particle was strongly
coupled to nucleons and that it had a rest mass somewhere between 100
and 200 MeV. The choice of the meson’s mass was made in order to
explain the range of the nuclear force. The name meson comes from the
Greek word for middle and refers to the fact that the meson has a mass
between that of the electron and the proton.
The particle was assumed to have an electrically neutral state as well
as two charged states with charge ±e. This enables one to explain both
the direct force and the exchange force. The neutron and proton exert a
direct force on each other by the exchange of a neutral meson and an
exchange force by the exchange of a charged meson.
Within a year of Yukawa’s predictions, Anderson, the discoverer of
the positron, observed a new particle among the cosmic ray tracks he was
studying. The new particle had a mass of 105 MeV, which was right
within the range predicted by Yukawa. This discovery generated a great
deal of excitement since it seemed to confirm Yukawa’s hypothesis.
Then in 1947 the bubble burst when Conversi, Pancini, and Piccioni
through their cosmic ray work discovered that this new particle had a
very weak coupling to the proton and neutron and hence could not be the
intermediate meson predicted by Yukawa. This new particle, which was
called a muon and is denoted by μ, is interesting in itself. It behaves to all
extents and purposes exactly like an electron except that it has a mass
210 times as large as the electron’s mass. It has the same charge and spin
as the electron. It also has an antiparticle, the positive muon. The muon is
unstable and decays into the electron an antielectron neutrino and a mu
neutrino via the weak interaction μ– → e– + νμ +νe.
The muon together with the electron, the neutrino and their
antiparticles for the class of particles called leptons after the Greek word