Presentation Speech by Professor Gösta Ekspong of the Royal Swedish Academy of Sciences

Translation from the Swedish text

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

The Royal Swedish Academy of Sciences has decided to award this year’s Nobel Prize in Physics jointly to Dr Leon Lederman, Dr Melvin Schwartz and Dr Jack Steinberger. The citation has the following wording, “for the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino”.

The neutrino figures in George Gamow’s entertaining book “Mr Tompkins Explores the Atom”, written in the 1940’s. Gamow describes how Mr Tompkins in a dream visits a woodcarvers shop, where the building blocks of the elements – protons, neutrons and electrons – are stored in separate caskets. Mr Tompkins sees many unusual things, but above all a carefully closed, but apparently empty casket labelled: “NEUTRINOS, Handle with care and don’t let out”. The woodcarver does not know whether there is anything inside. The friend, who had presented the casket to him, must have been Wolfgang Pauli, Nobel Laureate in Physics in 1945, who proposed the existence of the neutrino in the early 1930’s.

The neutrino is electrically neutral and almost or totally massless – hence the name. It cannot be seen and it interacts only weakly with atoms. It travels with the speed of light or nearly so. It is impossible to completely stop a beam of neutrinos. To do so would require a wall of several hundred thousands of steel blocks stacked in depth one after the other, each with a thickness corresponding to the distance from here to the sun.

Our sun is a source of neutrinos, which are copiously produced in its hot central region. They pass through the whole sun without much difficulty. Every square centimeter on Earth is bombarded by many billion solar neutrinos every second and they pass straight through the Earth without leaving a noticeable mark. The neutrinos are – if I may say so – “lazy”, they do almost nothing but steal energy, which they carry away.

The great achievement of the Nobel prize winners was to put the “lazy” neutrinos to work. Lederman, Schwartz and Steinberger are famous for several other important discoveries concerning elementary particles. At the time of the neutrino experiment they were associated with Columbia University in New York. They and their co-workers designed the world’s first beam of neutrinos at the Brookhaven National Laboratory, using its large proton accelerator as a source. Their neutrinos had considerably more energy than usual, because they were produced from the decay in flight of fast moving mesons. Such neutrinos are much more apt to interact with matter and the collisions with atomic particles become much more interesting. Although a neutrino collision is a rare event it can be spectacular at high energy – and very informative.

In their pioneering experiment the prizewinners dealt with a total of about 10¹⁴, i.e. a hundred thousand billion neutrinos. To catch just a few dozen collisions from all these, the research team invented and built a huge, sophisticated detector with the weight of 10 tons. Other unwanted particles in the beam had to be prevented from entering the detector. An enormous 13-meter thick steel wall served this purpose. To save time and money, the wall material was taken from scrapped battleships. Unwanted particles came also from the outside in the form of cosmic ray muons. Various tricks were used to prevent these muons from playing a role as false neutrinos. The first neutrino beam experiment was a bold endeavor, which proved successful. The method has since been much used as a tool for investigating the weak force and the quark structure of matter. It has also been used to investigate the neutrino itself.

At the time of the prizewinners’ experiment, physicists were puzzled by the fact that a possible, alternative decay of the muon particle did not happen. No known law forbade it, and there is a general principle which says that a process must occur unless it is explicitly forbidden by law. The mystery was solved when the prizewinners’ team discovered that Mother Nature provides two completely different species of neutrino, as had been suggested by a theoretical analysis. The old type of neutrino is paired with and may be transformed into an electron, the newly discovered type of neutrino is similarly paired with the muon. The two pairs constitute two separate lepton families, which never mix with each other. Thus, a new law of Nature had been discovered.

Cosmologists and physicists alike want to know how many different lepton families, i.e. how many neutrino species there are in Nature. Present ideas about the birth and early evolution of our universe cannot tolerate more than four. A third is already on the books. One of the goals of the experimental program at the large LEP accelerator ring at CERN, which will be ready to start operation next summer, is to give a precise answer as to the number of neutrino species and thus the exact number of lepton families in the universe.

Professors Dr Lederman, Dr Schwartz and Dr Steinberger,

You started a bold new line of research, which gave rich fruit from the beginning by establishing the existence of a second neutrino. Furthermore, problems which could not even be formulated at the time of your experiment, have been successfully elucidated in later experiments using your method. The pairing of the leptons, which you discovered, is also of much wider applicability than could be foreseen at the time and is now an indispensible ingredient in the standard model for quarks and leptons.

On behalf of the Royal Swedish Academy of Sciences, I have the privilege and the great honour to extend to you our warmest congratulations. May I now ask you to receive the 1988 Nobel Prize in physics from the hands of His Majesty the King.