Ең жеңіл мезон және пионның ашылу тарихы

Lightest meson

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: 134.9768

The pion also plays a crucial role in cosmology, by imposing an upper limit on the energies of cosmic rays surviving collisions with the cosmic microwave background, through the Greisen–Zatsepin–Kuzmin limit.

Theoretical work by Hideki Yukawa in 1935 had predicted the existence of mesons as the carrier particles of the strong nuclear force. From the range of the strong nuclear force (inferred from the radius of the atomic nucleus), Yukawa predicted the existence of a particle having a mass of about 100. Initially after its discovery in 1936, the muon (initially called the "mu meson") was thought to be this particle, since it has a mass of 106. However, later experiments showed that the muon did not participate in the strong nuclear interaction. In modern terminology, this makes the muon a lepton, and not a meson. However, some communities of astrophysicists continue to call the muon a "mu meson". The pions, which turned out to be examples of Yukawa's proposed mesons, were discovered later: the charged pions in 1947, and the neutral pion in 1950. In 1947, the first true mesons, the charged pions, were found by the collaboration led by Cecil Powell at the University of Bristol, in England. The discovery article had four authors: César Lattes, Giuseppe Occhialini, Hugh Muirhead and Powell. Since the advent of particle accelerators had not yet come, high energy subatomic particles were only obtainable from atmospheric cosmic rays. Photographic emulsions based on the gelatin silver process were placed for long periods of time in sites located at high altitude mountains, first at Pic du Midi de Bigorre in the Pyrenees, and later at Chacaltaya in the Andes Mountains, where the plates were struck by cosmic rays. After development, the photographic plates were inspected under a microscope by a team of about a dozen women. Marietta Kurz was the first person to detect the unusual "double meson" tracks, characteristic for a pion decaying into a muon, but they were too close to the edge of the photographic emulsion and deemed incomplete. A few days later, Irene Roberts observed the tracks left by pion decay that appeared in the discovery paper. Both women are credited in the figure captions in the article. In 1948, Lattes, Eugene Gardner, and their team first artificially produced pions at the University of California's cyclotron in Berkeley, California, by bombarding carbon atoms with high speed alpha particles. Further advanced theoretical work was carried out by Riazuddin, who in 1959 used the dispersion relation for Compton scattering of virtual photons on pions to analyze their charge radius. Since the neutral pion is not electrically charged, it is more difficult to detect and observe than the charged pions are. Neutral pions do not leave tracks in photographic emulsions or Wilson cloud chambers. The existence of the neutral pion was inferred from observing its decay products from cosmic rays, a so called "soft component" of slow electrons with photons. The was identified definitively at the University of California's cyclotron in 1950 by observing its decay into two photons. Later in the same year, they were also observed in cosmic ray balloon experiments at Bristol University. Yukawa choose the letter π because of its resemblance to the Kanji character for 介, which means "to mediate". Due to the concept that the meson works as a strong force mediator particle between hadrons.

The use of pions in medical radiation therapy, such as for cancer, was explored at a number of research institutions, including the Los Alamos National Laboratory's Meson Physics Facility, which treated 228 patients between 1974 and 1981 in New Mexico, and the TRIUMF laboratory in Vancouver, British Columbia.