Elementary Particles

D

Discovery of Antimatter

In 1931 British physicist Paul Dirac produced the precursor of modern particle theories. Dirac’s equations described the known electromagnetic properties of particles well, but to make his theory work more comprehensively, Dirac had to introduce the idea of antiparticles, antimatter counterparts of existing particles. The existence of these particles was confirmed in 1933, when American physicist Carl Anderson saw something peculiar while looking at tracks made by cosmic rays in a type of particle detector called a cloud chamber. A particle passing through the cloud chamber seemed to have the mass of an electron, but it had a positive rather than a negative charge—he had discovered the positron. Anderson shared the 1936 Nobel Prize in physics for this confirmation of Dirac’s theory.

E

Search for Carriers of the Strong Force

In 1934 Japanese physicist Yukawa Hideki predicted the existence of a force carrier holding neutrons and protons together in the nucleus of an atom. He believed this particle should have a mass between the mass of the electron and that of the proton. Yukawa’s theory attempted to describe how the strong force affects particle interactions, but it was not complete because it did not describe the fundamental interactions between quarks and gluons. It was, however, highly successful at describing the way protons and neutrons bond inside the nucleus. The theory predicted the existence of the pion, the meson that holds the particles in an atomic nucleus together.

When Carl Anderson and American physicist Seth Neddermeyer detected a new particle in cosmic ray experiments two years later, many thought this new particle was Yukawa’s meson. But some properties of the new particle did not match Yukawa’s theory. This dilemma appeared to be solved in 1947 when yet another particle, the pion, was found in cosmic rays. The pion’s behavior was consistent with predictions in Yukawa’s theory. The particle that Anderson and Neddermeyer discovered was later found to be the muon, but in the beginning, no one could tell the purpose of this particle. Anderson and Neddermeyer’s muon turned out to be the first indication of a new type of lepton. Scientists detected the muon neutrino in 1962 and thereafter regarded the muon and its neutrino partner as a second generation of leptons.

In the same year that the pion was discovered, physicists detected another particle in cosmic ray experiments. This particle, now called the lambda, behaved differently than known particles. Starting in 1953, scientists found many more such unexpected particles. Because these particles were different, physicists called them “strange.” These particles were eventually shown to include strange quarks, which received their name from the description of the particles they compose.

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F

Invention of the Cyclotron

While cosmic ray experiments revealed a myriad of particles, scientists also sought ways to create unusual and unstable particles in laboratories. American physicist Ernest Lawrence invented the cyclotron, a type of circular accelerator, in 1932. The cyclotron, however, could not achieve very high energies. Lawrence’s model was improved (independently) by American physicist Edwin McMillan and Soviet physicist Vladimir Veksler in the 1940s, resulting in the synchrocyclotron. The high energies available using the synchrocyclotron led to many important particle discoveries.

G

Separating Leptons and Quarks

By the 1960s hundreds of different “elementary” particles had been seen. Physicists found they could separate these particles into two main groups: those that interacted by the strong force and those that did not. They called the strongly interacting particles hadrons, and the particles without strong interactions leptons. American physicist Murray Gell-Mann proposed in 1964 that many of these observed particles might not be elementary after all. He showed that all of the properties of hadrons could be explained if they were various combinations of three quarks. Normal matter, such as protons, neutrons, and pions, contains only up and down quarks, and strange matter (such as the lambda particles) contains one or more strange quarks along with up and down quarks. Gell-Mann was honored for his contributions in 1969 with the Nobel Prize in physics. Gell-Mann’s quark theory was confirmed experimentally by American physicists Jerome Friedman and Henry Kendall and Canadian physicist Richard Taylor in 1969. Their experiment demonstrated that protons have internal structure. This experiment earned them the 1990 Nobel Prize in physics.

In 1964, the same year Gell-Mann introduced his quark theory, British physicist Peter Higgs proposed the existence of the Higgs boson, building on the work others had done in the early 1960s. Some scientists also predicted that same year that a fourth quark—the charm quark—should exist. Hadrons containing the charm quark were finally detected in 1976, leaving the number of quarks and the number of leptons equal at four apiece. Scientists divided the leptons and quarks into two generations, with the up and down quarks and the electron and electron neutrino in the first, and the strange and charm quarks and muon and muon neutrino in the second.

H

A Third Generation of Particles

A third generation of particles entered the scene in 1975, just a year before the charm quark was discovered. American physicist Martin Perl and his collaborators detected a third charged lepton, the tau. Scientists assumed immediately that a third neutrino accompanied the tau, but it has not yet been directly detected. Perl shared the 1995 Nobel Prize in physics with American physicist Frederick Reines for his part in discovering the tau lepton.

Physicists discovered a third generation of quarks in 1977. American physicist Leon Lederman and his collaborators discovered mesons that contained a fifth quark: the bottom quark. Scientists assumed the bottom quark should have a partner, called the top quark, and so the hunt for this particle was on. This hunt finally ended in 1995, when evidence of the top quark was detected at the Fermi National Accelerator Laboratory in Batavia, Illinois. While the existence of the top quark was no surprise, the mass of it was. The top quark is over 40 times heavier than the bottom quark, and 174 times heavier than the proton, which contains three first generation quarks (two up quarks and one down quark).

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