Concerning the Discovery of the Quantum Number «Colour»
Recently, the outstanding American physicist Murray Gell-Mann visited Moscow and Dubna. In his interview to The Moscow News weekly (2007, No. 40) he ranked the discovery of"colour" as one of the greatest achievements of the last decades. As co-author of this discovery together with Academician N.N. Bogoliubov and Professor B.V. Struminsky, I would like to make some comments of historic nature with the aim of emphasizing the priority of the scientists of the Joint Institute for Nuclear Research (Dubna) in this field. However, it is worth elucidating first the importance of this discovery and its relevance to the modern scientific picture of the world.
At present, the dominant point of view is that physical phenomena, both terrestrial and cosmological, are governed by three fundamental forces - gravitational, electroweak, and chromodynamic. Matter itself is the source of gravitational forces, and their action provides stability of the Solar System and evolution processes in the Cosmos. The electromagnetic forces are produced by the electric charge that particles of matter possess. The results of the action of the electromagnetic forces are widely used in our everyday life and together with the nuclear forces they ensure stability of atoms, i.e., the existence of the world we live in. Owing to the weak forces peculiar to elementary particles of matter, for instance neutrinos are produced bearing, together with electromagnetic radiation, unique information on the evolution of the Universe The electromagnetic and weak forces are manifestations of the unified electroweak interaction.
The chromodynamic ("colour dynamic") forces are due to quarks and gluons that compose protons and neutrons and possess a specific fundamental characteristic - the quantum number "colour". Noteworthy that protons and neutrons themselves do not have this quantum number, i.e., they are "colourless" or "white". The notion "colour" gives a convenient physical picture for understanding how "colourless" objects result from "coloured" ones by combining three different main colours, e.g., red, blue, and green. The chromodynamic forces are responsible for the interaction between quarks and gluons in protons and neutrons as well as for the nuclear interaction between them in the nucleus. The strong (nuclear) interaction of protons and neutrons in the nucleus is secondary much as the intermolecular interaction is not fundamental but is due to the electromagnetic forces.
The quantum number "colour" was first discovered by N.N. Bogolubov, B.V. Struminsky and me, and independently by American scientists Y. Nambu and M. Han at the beginning of 1965.
The fact that the atomic nucleus consists of protons and neutrons was established in the 1930s. The atomic nucleus was thought to be stable due to the existence of strong interaction between protons and neutrons. A detailed study and understanding of this interaction, apart from being important for fundamental physics, acquired also practical significance dictated by the necessity of developing nuclear power engineering for both peaceful and military purposes.
By the beginning of the 1960s it had been established that hundreds of different particles, regarded as elementary at that time, take part in nuclear interactions. This number was steadily increasing with improvement of experimental possibilities. There arose a problem of defining more accurately the very concept of a particle's elementarity.
A turning point was the hypothesis put forward in 1964 by Gell-Mann and Zweig that protons, neutrons, and all so-called hadrons (particles involved in nuclear interactions) are not elementary but are composed of more fundamental constituents called quarks. The quark model of hadrons allowed one to reduce their study to the consideration of properties of only three types of quarks and on this basis to classify all hadrons.
It should be noted that in the Gell-Mann and Zweig model quarks were treated as purely mathematical objects. Besides, being fermions by their nature, they did not obey the Pauli principle, i.e., they could be in one and the same state simultaneously. Moreover, the model did not consider the problem of forces confining quarks in hadron and preventing them from escaping from it.
In the autumn of 1964, Professor A. Salam, in his review report at the Rochester Conference in Dubna, in particular, dwelled upon the Gell-Mann and Zweig quark model. While discussing that presentation, during a walk, N.N. Bogoliubov said thoughtfully: "You know, Albert Nikiforovich, after all, quarks are not mathematical but real physical objects".
That was precisely the starting idea of our paper to explore quarks which ultimately let to the introduction of quantum numbers subsequently termed "colour". We assumed that quarks making up colourless hadrons may exist in three possible colour states. This made it possible to satisfy the Pauli principle, and, what is no less important, the introduction of colour did not change the dynamic electromagnetic characteristics of hadrons calculated within the framework of SU(6) symmetry.
In our above-mentioned paper with N.N. Bogoliubov and B.V. Struminsky, the dynamic model was proposed that predicted a quite unusual, paradoxical property of interaction between quarks: in hadrons they are in a quasi-free state; however, it is impossible to "pull" them out from there, i.e., as it is said at present, quarks are in "eternal confinement" in hadrons.
In May 1965 I reported about these investigations, carried out at the Laboratory of Theoretical Physics of the Joint Institute for Nuclear Research (Dubna), at a representative international conference in Trieste, one of the largest centres for theoretical physics. It was widely appreciated at that time that an additional quantum number ("colour") saves the situation, and quarks may manifest themselves as real physical objects in experiments. In the autumn of the same year, American physicist Y. Nambu, in his presentation at a conference in the USA, pointed out that "colour" may play the role of a charge creating a surrounding chromodynamic force field that may establish interaction between quarks and is a prototype of modern gluons confining quarks in hadrons.
An important step in justifying the dynamic model of quasi-independent quarks was the derivation by V.A. Matveev, R.M. Muradyan and me of the "quark counting" formula brilliantly confirmed in experiments. In our paper with N.N. Bogoliubov and V.S. Vladimirov, a local nature of interaction between quarks was proved which may in fact ensure a quasi-free behaviour of quarks in hadrons.
In early 1970s, owing to the outstanding experimental and theoretical investigations carried out in nuclear physics centres of the world, the model of coloured quarks coupled by chromodynamic forces providing a quasi-free state of quarks in hadrons was generally recognized. Based on the results obtained by that time, M. Gell-Mann, H. Fritzsch and independently M. Leutwyller systematized the basic principles of quantum chromodynamics - the modern theory of nuclear forces.
For the series of publications "New quantum number - colour and establishment of dynamic regularities in the quark structure of elementary particles and atomic nuclei" a group of scientists from the Joint Institute for Nuclear Research (Dubna) and the Institute for Nuclear Research of the Russian Academy of Sciences (Moscow) were awarded the Lenin Prize in 1988.