# 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: . See the math

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

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.

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