Free Beyond the God Particle by Leon M. Lederman, Christopher T. Hill Page A

muon (pronounced mew-on), or µ , was discovered in 1937. It seemed to be the thing predicted by Yukawa, but people soon realized that the muon was a case of false identity—the cops had arrested the wrong guy. The muon was discovered by using cosmic rays that (somehow) produced it 10 miles up in the sky. The muons then traveled to the surface of the earth where they could be detected. The reason the muon was initially thought to be the particle Yukawa had predicted (called the pion [pie-on], or π ) was because it had Yukawa's predicted mass. But the muon did not interact strongly enough with protons and neutrons to be a pion since it could travel all the way to the earth's surface (muons only interact electromagnetically, or through the weak force). This new particle definitely was not the agent of the strong force, as predicted by Yukawa. In fact, the muon seemed to be a mere carbon copy of the electron but 200 times heavier, with a lifetime of about 2 millionths of a second (whereby the muon “decays” into an electron and a pair of neutrinos). 14
    Almost all pure physics research was interrupted by World War II, as the world's scientists were redirected to serve military needs. The quest for Yukawa's pions could resume only after the war. In the meantime humans had conquered the atomic nucleus, with its strong force—and unleashed its fury. 15

In 1947, the pions, predicted by Hideki Yukawa to explain the strong nuclear force, were finally discovered by using cosmic rays. This vindicated Yukawa's ingenious theory, for which he later won the Nobel Prize in 1949.
    The pions arrived in three types, distinguished by their electric charges, π + , π – , and π 0 , where the superscript refers to the particle's electric charge. We often refer to π + and π – as the “charged pions” and π 0 as the “neutral pion.” 1 The strong force, which welds the protons and neutrons together to build the atomic nucleus, arises as Yukawa had theorized through the “exchange of pions,” hopping back and forth between protons and neutrons as quantum fluctuations. The picture of the atom and its nucleus was now complete.
    Soon there would be particle accelerators, and numerous “elementary particles” emerged from experiments. Most of the multitude of new objects were strongly interacting, that is, they “felt” the strong force, and they interacted strongly with the pions, protons, and neutrons. It was also discovered that the proton, the neutron, the pions, and the long list of new strongly interacting particles, were not point-like objects but actually had finite sizes of about a hundredth of a trillionth of a centimeter. In the extreme cases some new particles were discovered that had lifetimes as short as the time it took light to transit their finite diameters. The nascent world of particle physics was never more confusing and chaotic than in the 1950s as the first higher-energy particle accelerators came online.
    Throughout this time, the poor muon seemed to be an oddball, an almost uninvited guest at the dinner table. 2 The muon has a mass about 200 times that of the electron. It “decays” (through the weak interaction) into an electron and two very difficult to observe particles called neutrinos, living a mere two millionths of a second when at rest in the laboratory. Otherwise, the muon seemed to play no particular role in anything else, pointless in the fabric of nature. Its serendipitous and seemingly random appearance had elicited the famous quip by I. I. Rabi, “Who ordered that?” 3

    Figure 3.3. The Atom, Pion Exchange, and the Atomic Nucleus. An atom consists of the cloudlike motion of electrons about a dense nucleus containing protons and neutrons. The nucleus is held together by the exchange of pions that hop back and forth between the neutrons and protons.

    We could retrace the long and winding road taken by particle physics from 1947 onward. There followed the era of the 1950s and 1960s when