Popular history of particle accelerators experiments (part 1)

Large Hadron Collider (LHC) will start operation this year, so there are a lot of fuss in press and blogs. Some people try to obtain public attention with legal claims against CERN. Nonetheless, general public is not aware of particle physics and accelerator experiments, and conceive LHC as final ultimate device for discovery of theory of everything or even as a doomsday machine. I want to review the history of particle physics experiments to show that LHC is next step in research. It shouldn’t be viewed as final or outstanding.

My posts are based on lectures of I.N. Meshkov at sixth winter school "Introduction to theory of fundamental interactions" in Dubna.

Ancient Greeks proposed two opposite ideas of structure of matter. One conception was that everything consists of several kinds of indivisible particles while the other told that everything could be divided into infinitely small parts.

So the target of particle physics is to find out which point of view is closer to real world and to study properties of particles for better understanding of our world which consists of particles.

Today we know that there are several kinds of elementary particles, which are united into so called “Standard model”.

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Рис. 1: Standard model of elementary particles

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String theory tell us that all these particles are modes of oscillation of more elementary objects, which are called "strings but now it is just a hypothesis which could not be checked with experiment.

History of particle physics started at 1895 from discovery of photons as X-rays particles by Konrad Rentgen and discovery of electron by Joseph John Thomson (1897). First experiments didn’t use particle accelerators.

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Рис. 2: Konrad Rentgen Рис. 3: Joseph John Thomson

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First electrostatic accelerators were created in 1931-1932 by R. Van-der-Graaf and J.D. Cockroft with E.T.S. Walton, and allowed the discovery of neutron by James Chadwick. He used reaction

These early accelerators were not very powerful and sophisticated, so many discoveries in particle physics were made from observations of cosmic rays, which started at 1912 by, Victor Hess. He has found in experiment with 3 electrometers carried out in a free balloon flight up to an altitude of 5300 meters that the ionization rate increased approximately fourfold over the rate at ground level. He concluded :"The results of my observation are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above".

In 1932, Carl D. Anderson discovered positron by passing cosmic rays through a "Wilson chamber"with lead plate surrounded by a magnet to distinguish the particles charge. It was the beginning of the competition of accelerators with cosmic rays.

Cosmic rays consist of particles produced in outer space in thermonuclear reactions in stars and during supernova bursts. Some of these particles have energy which greatly exceeds energy of LHC and all the accelerators possible in near future. So it is one of the reasons why LHC wouldn’t destroy the Earth. But in spite of such enormous energy cosmic rays are rather inconvenient for particle physics experiments, because the density of such high-energy particles is very low.

In 1929-1932 Ernest O. Lawrence invented the resonance acceleration method which allowed construction of first cyclotron and first resonance linear accelerator, but next advance in particle physics was achieved in cosmic rays observations at 1936. Carl D. Anderson discovered of "mesotron which was supposed to be hypothetical Yukawa particle carrying interaction between protons and neutrons in nuclei. Later it was found out that mesotron was not Yukawa particle, but other electron-alike fundamental particle, so it was renamed to muon.

Break-through in accelerator technics was made by Vladimir I. Veksler (USSR, 1944) and Edwin McMillan (USA, 1945) with the discovery of the “Phase stability principle” and construction of the first electron synchrotron (Veksler) and the first synchrocyclotron (E.McMillan).

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Рис. 4: Vladimir I. Veksler Рис. 5: Edwin McMillan Рис. 6: Vladimir I. Veksler and Edwin McMillan at JINR (Dubna) 1958

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Cosmic rays observations also made big progress, in 1947 pions, -hyperon and kaons were discovered. But cosmic rays observations were not very accurate, so there were some false “discoveries”, such as “Varitrons”. So first synchrocyclotrons allowed to improve results of cosmic rays observation.

In 1947 Berkeley Synchrocyclotron was put into operation. It had 184 inches (4.7 m) in diameter, and energy of accelerated -particles of 400 MeV. On this accelerator Lattes and E. Gardner achieved first artificial production of pions in reaction

Similar synchrocyclotron “Phasotron” was built in Dubna (USSR) at 1949. It is still in operation now and is used for cancer therapy, muon-catalysis, pion and muon physics experiments and nuclear physics experiments.

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Рис. 7: Phasotron, JINR (Dubna)

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Since the end of 1940-th accelerators are the main tools of particle physics and majority of experimental results of fundamental character has been obtained both in particle, and nuclear physics at accelerators.