Presentation Speech by Professor Carl Nordling of the Royal Swedish Academy of Sciences, December 10, 1995

Translation of the Swedish text

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

Physicists believe that all matter, for example the matter in our own bodies, consists of quarks and leptons. Quarks are heavy, and leptons are light. There are two types of quarks, which serve as the building blocks of atomic nuclei. Leptons, which occur outside of atomic nuclei, also come in two types: electrons, which have both an electrical charge and a measurable mass, and neutrinos, which lack both charge and mass. This one quark-lepton family, with its four members, is enough to account for all the matter in the universe today.

But the universe has a long history behind it. In its early stages, conditions were entirely different from today. There were very high temperatures and very high concentrations of energy. In such a climate, other quark-lepton families also thrived. With the aid of accelerators, physicists have re-created for brief moments the extreme conditions under which a second and even a third quark-lepton family can appear. But that was all. Physicists have shown that there is no fourth family of quarks and leptons within the existing paradigm of elementary-particle physics.

A long series of discoveries underpins the three-family model. Two of these discoveries are being honored with this year’s Nobel Prize in Physics. Both concern the discovery of leptons: one belonging to the first quark-lepton family, the other to the third such family. Both discoveries provide answers to deep, fundamental questions in physics.

Together with the late C.L. Cowan, Frederick Reines detected the neutrino, the sister lepton of the electron in the first quark-lepton family, even before the family concept had emerged. It was a long-awaited discovery. For nearly 25 years, physicists had been waiting for someone to accomplish this feat. Meanwhile the neutrino had been a mental construct that physicists had needed in order to “save” the law of conservation of energy in certain types of radioactive decay. But it seemed impossible to verify the neutrino’s actual existence. It flashed undetected past every observer, at the speed of light.

Reines realized that a nuclear reactor must emit copious quantities of neutrinos, although no one had noticed them before. During the 1950s he and Cowan developed a method for capturing at least a few of these elusive sub atomic particles.

After an unsuccessful trial, they devised a modified experiment that yielded favorable indications. A couple of years later, their findings were unequivocal. They had proved the existence of the neutrino. This discovery was a milestone of modern physics. It opened the way to a major new field of research, neutrino physics. Reines has also played a vital role in subsequent phases of this work.

There were now three known leptons. First was the electron, which had been discovered as early as 1897. Second was the newly discovered neutrino. Third was the muon, a heavier version of the electron and the odd man out among elementary particles. No quarks had entered the picture yet.

The idea that there might be a third, extremely heavy relative of the electron emerged during the 1960s, but such a particle fell outside the theoretical framework accepted at that time. The difficulties of experimentally proving the existence of such a heavy lepton also seemed almost insurmountable. But for many physicists, such almost insurmountable barriers exert an attraction all their own.

At Stanford, Martin Perl planned an experiment that might possibly resolve the matter. He needed a sufficiently strong energy source, and Stanford had the world’s most powerful accelerator for this purpose. He also needed to figure out in what way the new lepton might reveal its presence. There were few leads, however, and great inventiveness was required to design an experiment that would accomplish this.

When Perl and his coworkers wrote in 1975 that they had found a third lepton, nearly four thousand times heavier than the electron first identified in 1897, they were announcing a major discovery. They had identified the first member of the third and final quark-lepton family.

The tau, or tau lepton, as this super-heavy cousin of the electron was designated, provided the key to a definitive understanding of the family structure of elementary particles. Some day the tau neutrino, its sister lepton, may prove to account for a large portion of the missing mass in the universe and to play an important role in supernova explosions, and thus in cosmology. The tau lepton itself may be of crucial importance in testing future theories of how matter acquired the property that we call mass.

Professor Perl, Professor Reines,

You have been awarded the 1995 Nobel Prize in Physics for your outstanding contributions to lepton physics. It is a privilege and a great pleasure for me to congratulate you on behalf of the Royal Swedish Academy of Sciences, and I now ask you to receive your Nobel Prizes from the hands of His Majesty the King.