*Presentation Speech by Professor Ivar Waller, member of the Nobel Committee for Physics*

Your Majesty, Your Royal Highnesses, Ladies and Gentlemen.

Elementary particle physics which is now so vigorous was still in its infancy when Murray
Gell-Mann in 1953 published the first of the papers which have been honoured with this
years Nobel Prize in physics.

The physicists were, however, already then aquainted with a rather large number of
particles which apparently were indivisible and therefore elementary building stones of
all matter. The elementary particle known for the longest time was the electron.

New particles were added when the atomic nuclei were explored. It was found that the
atomic nuclei consist of positively charged protons and electrically neutral neutrons.
These particles are held together in the atomic nuclei by enormously strong forces called
nuclear forces which do not distinguish between protons and neutrons. This symmetry of the
nuclear forces was expressed by saying that the nuclear forces are charge-independent. A
proton and a neutron have further very nearly the same mass. They form a doublet of
particles and have been given the common name of nucleons.

An increase already expected and desired occurred in the family of elementary particles at
the end of the 1940's, when new particles called pi-mesons were discovered. They were
named mesons because they have a mass between the electron and the nucleon masses. The pi
- mesons had been predicted by the Japanese physicist Yukawa. They form a triplet of
particules having nearly the same mass but different charges which are + 1, 0 and -1 in
units of the proton charge. Their interaction with the nucleons is strong and
charge-independent. Their most important task is to be an intermediary agent for the
strong interactions between the nucleons.

A very remarkable discovery which marked a new area in particle physics was made by the
British physicists Rochester and Butler about the same time. They found new unstable
particles which did not fit in with the theoretical ideas developed so far. Some ofthe new
particles are heavier than the nucleons and were grouped together with them under the
common name of baryons. The others were lighter than the nucleons but heavier than the
electrons and were called K-mesons. The new particles were copiously produced when
high-energy pi-mesons collide with nucleons and were therefore assumed to interact
strongly with other particles. But they had such a long lifetime that some law must exist
which prevent the strong forces to act when they disintegrate into other particles.
Gell-Mann discovered this law after some preliminary results had been found by Pais.

It had been assumed earlier that the new baryons from doublets like the nucleons and that
the K-mesons form triplets like the pi-mesons. Gell-Mann made the fundamental new
assumption that the new baryons instead form a singlet, a triplet and a doublet, the
latter being different from the nucleon doublet, and that the new mesons form two kinds of
doublets, one consisting of the antiparticles of the other. Gell - Mann assumed further
that the principle of charge-independence was generally valid for strong interactions. He
could thereby explain the mysterious properties of the new particles. He introduced a new
fundamental characteristic of a multiplet called its hypercharge. This is defined as twice
the mean value of the charges in the multiplet. Gell-Mann's proposed the new rule:
Elementary particles can be transformed in others by the strong and the electromagnetic
interactions only if the total hypercharge is conserved. This rule reminds of the law of
conservation of the electric charge. It should be remarked that Gell-Mann initially used
instead of the hypercharge a closely related number called the strangeness.

This discovery by Gell-Mann was admirable considering in particular the very meagre
experimental material available to him. In the predicted baryon multiplets there occurred
empty places. Gell- Mann could on this ground predict two new baryons. One of them was
soon discovered but the other not until six years later.

This classification of the elementary particles and their interaction discovered by
Gell-Mann has turned out to applicable to all strongly interacting particles found later
and these are practically all particles discovered after 1953. His discovery is therefore
fundamental in elementary particle physics.

It should be added that two Japanese physicists, Nakano and Nishijima, published a similar
classification some months later than Gell-Mann.

Many theoretical physicists tried during the following years to find new symmetries which
should give relations between the particle multiplets. Initiated by Sakata a series of
papers were published in particular by Japanese physicists. They indicated that a certain
kind of symmetry could be of interest. Gell - Mann showed in a new fundamentally important
paper of 1961 that this symmetry which had since long been studied in pure mathematics
could be used for the classification of all strongly interacting particles. Assuming the
validity of the new symmetry which includes the symmetry corresponding to
charge-independence, Gell-Mann found that his earlier multiplets could be brought together
into larger groups called supermultiplets each containing all baryons or all mesons which
have the same spin and the same parity, i. e. have the same measure for their rotation
around their axes and are transformed in the same way by reflections. Gell-Mann called
this classification "The Eightfold Way". The nucleons were found to belong to a
supermultiplet of eight particles *i.e.* an octet. For the mesons an octet was
proposed were the pi- and *K*-mesons filled seven places. Because one place was empty
a new meson was predicted. Its existence had been suspected already by some of the
Japanese physicists mentioned above. It was soon discovered which meant that Gell-Mann's
theory was strongly supported. Still more famous is Gell-Mann's prediction in 1962 of a
new baryon called omega minus.

A similar classification was proposed by Y. Néeman somewhat later than Gell-Mann.

Gell-Mann has also found that "The Eightfold Way" can be described very simply
by assuming that all particles which interact strongly with each other are composed of
only three kinds of particles which he called quarks and of the corresponding
antiparticles. The quarks are peculiar in particular because their charges are fractions
of the proton charge which according to all experience up to now is the indivisible
elementary charge. It has not yet been possible to find individual quarks although they
have been eagerly looked for. Gell-Mann's idea is none the less of great heuristic value.

And interesting application of "The Eightfold Way" is the so-called current
algebra which was founded by Gell-Mann. It has e.g. made evident that there are important
connections between the different kinds of elementary particle interactions.

Gell - Mann has given many fundamental contributions to the theory of ele mentary
particles besides those which have been mentioned here. He has during more than a decade
been considered as the leading scientist in this field.

Professor Gell-Mann. You have given fundamental contributions to our knowledge of mesons
and baryons and their interactions. You have developed new algebraic methods which have
led to a far-reaching classification of these particles according to their symmetry
properties. The methods introduced by you are among the most powerful tools for further
research in particle physics.

On behalf of the Royal Swedish Academy of Science, I congratulate you on your successful
work and ask you to receive your Nobel Prize from the hands of His Majesty the King.

From*Nobel Lectures*, Physics
1963-1970.

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Foundation

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