The Nobel Prize in Physics 1990

The Nobel Prize in Physics 1990

The history of particles

1911	Ernest Rutherford* publishes his famous paper The scattering of alpha and beta particles by matter and the structure of atoms, in which measurements made by among others Hans Geiger and Ernest Marsden in 1909 were analysed. Rutherford explains the results of such measurements by the atom having all its mass concentrated to a nucleus of less than 10^-14 m. This discovery marks the birth of nuclear physics.
1919	Ernest Rutherford demonstrates free protons by bombarding nitrogen with alpha particles. He concludes that nuclei have an inner structure.
1932	James Chadwick* discovers the neutron and Werner Heisenberg* proposes that the nucleus consists of protons and neutrons, together termed nucleons.
1933-1934	Otto Stern* and his co-workers discovers that the proton and the neutron have unexpectedly large (anomalous) magnetic moments. This is interpreted to mean that nucleons are not point-like, but occupy a certain volume, and can thus possess an inner structure.
1935	A first model of how nucleons can form stable nuclei (strong interaction) is presented by Hideki Yukawa*.
1950s	It is discovered that the nucleon (like the atom and the nucleus) can be excited to higher energy levels. A large number of new particles, hadrons, related to the nucleon, are discovered. Robert Hofstadter* and his co-workers study the structure of protons and neutrons at the electron accelerator at Stanford. Using electron energies of up to 1 GeV (1 GeV is 10⁹ eV) they measure how charge and magnetism are distributed within the nucleons. It is found that the distributions give a picture of the nucleons as "soft spheres".
1964	Murray Gell-Mann* and Georg Zweig propose a model for the hadrons which, among other things, can theoretically describe the magnetic properties of the nucleon. The model requires three new elementary particles, which Gell-Mann calls quarks. But it is by no means clear that the quarks are true particles – they are perhaps only theoretical tools without experimental reality. Be that as it may, no free quarks are discovered.
1967	The SLAC-MIT experiment starts at the new electron accelerator in Stanford. Jerome I. Friedman, Henry W. Kendall, Richard E. Taylor* and their co-workers obtain in 1968 the first indications that the nucleons have an inner structure with point-like scattering centres. These are later interpreted as being quarks.
Since 1968	Intensive research into the inner structure of the nucleons starts all over the world, and is still continuing.
	* Nobel Laureates

Contents:
Introduction » Breakthrough in our understanding of the inner structure of matter » The history of particles » The SLAC-MIT experiment » The deep inelastic collision » Further Reading »
Based on materials from the 1990 Nobel Poster for Physics. Credits and references for the poster »

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