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Section 3: The Particle Zoo in Cosmic Rays

The satisfyingly simple view that all matter consisted of three subatomic particles—electrons, protons, and neutrons—did not last long. A veritable zoo of new subatomic particles began to emerge in the 1930s, when physicists started to study cosmic rays. These are particles produced by nature's accelerators: energetic protons from the Sun, neutron stars, supernovae, and extra-galactic sources. The particles impinge on our upper atmosphere, collide with the nuclei of oxygen or nitrogen, and produce showers of newly created particles. Although most cosmic rays have relatively short lifetimes, the effects of special relativity allow many of them traveling at extremely high speeds to reach the Earth before they decay. This effect, which physicists call "time dilation," increases with the particle's speed and is described by the Lorentz factor equation: . math icon See the math

Carl Anderson, Paul Dirac, and a positron track observed in a cloud chamber.

Figure 5: Carl Anderson, Paul Dirac, and a positron track observed in a cloud chamber.

Source: © Positron Track: Lawrence Berkeley National Laboratory. Portraits: Wikimedia Commons, Public Domain. More info

To detect cosmic rays, physicists relied on cloud chambers—sealed compartments filled with vapor that is cooled and kept very near the dew point. Charged particles passing through the vapor create tracks of ionization and cause tiny droplets to condense. The vapor in the cloud chamber reveals the particles' track, much as the contrails behind a jet show the path of an airplane. By applying an external magnetic field to bend the tracks, physicists gleaned more clues about the particles' momentum and charge.

California Institute of Technology physicist Carl Anderson started the riot of discovery in 1932. He identified a stable, positively charged particle, called the positron, in a cloud chamber. The find came four years after English theorist Paul Dirac had predicted the existence of antiparticles. Working on the relativistic equation of motion for the electron, Dirac found a mysterious second solution with negative energy. The correct interpretation, he postulated, was a particle with the same mass as the electron but the opposite charge. In other words, the positron is the electron's antiparticle. When a positron and an electron meet, they annihilate each other with a flash of energy in the form of radiation—another demonstration of Einstein's equation, .

Dirac later speculated about the existence of other worlds made of antimatter that ought to exist if the laws of physics were completely symmetric with respect to matter and antimatter. As we shall see later in this unit, this was a prescient speculation. It has spurred experiments that still continue today.

An astonishing new particle

The existence of antimatter was a shocking development that many scientists and nonscientists found difficult to accept, even though theorists could readily accommodate the positron. But the next particle to be discovered, the muon, really came out of left field. Discovered in 1936, also in a cloud chamber experiment, it behaved like an electron but had about 200 times more mass. "Who ordered that?" asked the Nobel Prize-winning Columbia University physicist I.I. Rabi.

The muon's most common decay mode.

Figure 6: The muon's most common decay mode.

Source: © Wikimedia Commons, Public Domain. Author: Thymo, 6 April 2009. More info

Studies showed that the muon is long-lived, decaying in about a microsecond. That makes it one of the most common particles from cosmic ray showers that survive all the way to the Earth before decaying. The particle was actually the first member of a second generation of Standard Model particles to be discovered, although it would take decades for physicists to appreciate that fact. A "generation" is a family of related subatomic particles; the first generation consists of particles that do not decay, such as the electron. We shall meet more of the second and further generations later in this unit.

About ten years after the discovery of the muon, photographic emulsions taken of cosmic rays revealed the particles called pions and kaons. Experimentalists had eagerly sought the pion, to fulfill the prediction of Japanese physicist Hideki Yukawa. It stemmed from his effort to understand why the electrical repulsion of all the protons packed into a tiny space did not tear apart atomic nuclei. Yukawa postulated the existence of a short-range strong nuclear force, attractive between two protons, which could overcome their electrostatic repulsion. As the carrier of that force, he proposed the pion, with a mass about one-sixth that of the proton. The discovery of the pion confirmed the existence of this new force, as we shall see in Unit 2.

Pions play an important role in explaining why atomic nuclei do not split apart.

Figure 7: Pions play an important role in explaining why atomic nuclei do not split apart.

Source: © Wikimedia Commons, Public Domain. Author: JabberWok2, 30 November 2007. More info

On the other hand, nobody predicted the kaon, whose unusual behavior quickly earned it the nickname "the strange particle." (Theorists later formalized the concept of strangeness; it applies to particles such as the kaon that decay more slowly than expected.) Since pions and kaons have masses intermediate between the electron's and the proton's, scientists called them mesons, from the Greek mesos, for "medium." The electron and muon were named leptons, from the Greek leptos, or "thin."