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Discovery of the Omega-minus Particle

As the list of "elementary" particles proliferated, it became obvious to physicists that they must look for some underlying structure composed of still smaller and, hopefully, very many fewer "elementary" particles. Progress in understanding the structure of basic matter, first atomic, then nuclear has, in each case, been preceded by a search for order among the seemingly chaotic multiplicity of entities. This same approach has been followed for the subnuclear world.

In the early Sixties theorists found in group theory a ready-made mathematical description that seemed to provide a remarkably successful scheme for classifying the then known hadrons. This scheme was not based on any underlying theory of fundamental structure, nor was it derived from any abstract principle. It simply provided a concise representation that exhibited symmetry and order and, additionally, predictive power.

The bubble chamber picture of the first omega-minus. An incoming K- meson interacts with a proton in the liquid hydrogen of the bubble chamber and produces an omega-minus, a K and a K+ meson which all decay into other particles. Neutral particles which produce no tracks in the chamber are shown by dashed lines. The presence and properties of the neutral particles are established by analysis of the tracks of their charged decay products and application of the laws of conservation of mass and energy.

The baryons and mesons known at the time fell into symmetric families of multiplets (octuplets, decuplets) sharing two identical quantum numbers (spin and parity), but differing in an ordered way in others (mass, charge, baryon number and strangeness). The mathematical group to fit this complex situation-SU3, the symmetric, unitary group of dimension 3- was proposed independently by Professors Gell-Mann at California Institute of Technology and Ne'eman at Imperial College, London, and named by Gell-Mann "The Eightfold Way." This new classification scheme, it was hoped, might lead in time to an understanding of particle structure just as the classification of the line spectra of atoms, following Balmer's discovery of order in the spectrum of the hydrogen atom, provided the first step leading to quantum mechanics and the understanding of the dynamics of atomic structure.

It was thus very important that the validity of SU3 be demonstrated by experiment. A major prediction was that a particle (named by Gell-Mann the omega-minus), an isotopic singlet with spin = 3/2, positive parity, mass of roughly 1,680 MeV, negative charge, baryon number +1, strangeness = -3, and stable to strong decay, should exist to complete the 3/2+ baryon decuplet. It was therefore a major triumph for the scheme when the omega-minus, a baryon with the precise mass, charge and strangeness predicted, was discovered in 1964 by a team of physicists from Brookhaven, the University of Rochester and Syracuse University, led by Nicholas Samios of Brookhaven, using the 80-inch bubble chamber. This crucial experiment also verified the symmetry breaking by medium-strong interactions which accounts for the mass differences within the multiplets.

Since this discovery further developments have led to the concept of "quarks" as the constituents of hadrons and to new higher group symmetry schemes which embody them.