3 From Protons and Neutrons to a Zoo of Particles
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Fig. 3.2 Cosmic ray showers. Source: Wikipedia.org
Several particles were discovered in the cosmic rays before the accelerators
became the major players in particle searches.
Particle collision experiments using accelerators such as cyclotrons and synchrotrons were preceded by experiments studying the collisions involving the
cosmic rays. There were also advances in technologies for detecting particles.
The cloud chamber discovered by Charles Wilson, a Scottish physicist, was
a primary instrument for detecting particles that result from the collision of
high energy particles, both in the laboratory as well as those in cosmic rays.
It led to the discovery of new particles beyond the protons, electrons, and
neutrons, as mentioned. For example, the positron, the anti-matter partner of
the electron was discovered in 1932 in the cosmic rays by Carl Anderson, which
provided a spectacular confirmation of Dirac’s prediction of anti-matter. This
was the first new particle discovered. Anderson also discovered a new particle
called the muon in 1936. We will discuss more about the muon later. A few
years later, George Rochester and Clifford Butler working at the University of
Manchester used the cloud chamber in 1947 to discover another new kind of
matter in cosmic rays, the K-meson. The cloud chamber was superseded by
the bubble chamber invented by Donald Glaser in 1952. Glaser was awarded
the Nobel Prize for this discovery in 1960.