Early Cosmic Ray Results

Earlier we indicated that particles interacted with each other via the exchange of a virtual intermediary particle which interchanges energy, momentum, and other physical properties between the interacting particles. This idea originated with the Japanese physicist Hideki Yukawa in 1935 in an effort to understand the forces between nucleons. Yukawa hypothesized that the force which holds nucleons together is associated with the exchange of a boson, i. e., a particle with integer spin, with rest energy $m c^2 \approx 100 \mbox{ MeV}$. The range of this force at low momentum transfers is $I \approx \hbar / (mc) \approx 2 \times 10^{-15} \mbox{ m}$, or comparable to the observed size of an atomic nucleus.

In 1947 two new particles were discovered in cosmic rays, the negatively charged muon with a rest energy of $106 \mbox{ MeV}$, and the pion, which comes in three varieties, the $\pi^+$, the $\pi^-$, and the $\pi^0$, which respectively have positive, negative, and zero charge. The rest energies of the $\pi^+$ and $\pi^-$ are $140 \mbox{ MeV}$ while that of the $\pi^0$ is $135 \mbox{ MeV}$. All of these particles are unstable in that they decay into other, more stable particles in a tiny fraction of a second. In particular, the negative pion decays into a muon plus an antineutrino, while the neutral pion decays into two gamma rays, or high energy photons. The antineutrino which results from pion decay is actually distinct from the antineutrino emitted in nuclear beta decay; it is called the mu antineutrino since it is associated with the muon in the same way that the antineutrino in beta decay is associated with the electron. To further distinguish between the two, the latter is called the electron antineutrino. The muon itself decays into an electron, a mu neutrino, and an electron antineutrino.

The muon and its associated neutrino are rather peculiar. In all respects except mass the muon appears to be identical to the electron. The physicist I. I. Rabi is reputed to have responded ``Who ordered that?'' upon learning of the properties of the muon. Furthermore, the electron neutrino only interacts with the electron and the muon neutrino only interacts with the muon. This is the first hint that elementary particles occur in families which appear to be replicated at higher energies.

Since the muon is a fermion with spin $1/2$, it can't be Yukawa's intermediary particle since all intermediary particles are bosons with integral spin. Furthermore, as with the electron, it is not subject to the nuclear force. The pions are more promising candidates for being intermediary particles of the nuclear force, since they are bosons with spin $0$. However, as we shall see, the situation is more complex than Yukawa imagined, and the force between nucleons cannot be so simply treated. However, Yukawa's idea of intermediary particle exchange lives on in today's theories of sub-nuclear particles.