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

Translation from the Swedish text

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

This year’s Nobel Prize for Physics has been awarded to Professor Carlo Rubbia and Dr. Simon van der Meer. According to the decision of the Royal Swedish Academy of Sciences the prize is given “for their decisive contributions to the large project, which led to the discovery of the field particles W and Z, communicators of the weak interaction”.

The large project mentioned in the citation is the antiproton project at CERN, the international centre for research devoted to the study of elementary particles, which has 13 European states as members. CERN straddles, in a unique way, the border between two countries, Switzerland and France, and has grown progressively in importance over the 30 years of its life. The international character is underlined by the fact that Carlo Rubbia is Italian, Simon van der Meer is Dutch and the collaborators in the various phases of the project are scientists, engineers, and technicians of many nationalities, either employed by CERN or in one of the many universities or research institutes involved in the experiments. The project has been made possible by collaboration, by the pooling of financial resources and of scientific and technical skill. When the antiproton project was proposed eight years ago the CERN ship had two captains – two Directors General, Professor Leon van Hove from Belgium and Sir John Adams, from the United Kingdom. Navigating through the high waves generated by the convincing enthusiasm of Rubbia but having van der Meer on board as pilot to steer through the more difficult waters, they directed their ship towards new challenging frontiers. The late Sir John Adams had been responsible for the construction of the two outstanding proton accelerators, which were called into action in new roles for the new project.

A former Nobel Laureate expressed his opinion about the CERN project with the following words: van der Meer made it possible, Rubbia made it happen. Looking closer one finds that two conditions had to be fulfilled in order to produce the W and Z in particle collisions: The first is that the particles must collide at sufficiently high energy so that the conversion of energy into mass could create the heavy W and Z particles. The second is that the number of collisions must be large enough to give a chance of seeing the rare creation process taking place. The name of Rubbia is connected with the first condition, that of van der Meer with the second. Rubbia’s proposal was to use the largest accelerator at CERN, the SPS, as a storage ring for circulating antiprotons as well as for protons, circulating in the opposite direction. The particles in the two beams would cross the French-Swiss border about 100,000 times every second for a whole day or more, to be repeated with new beams during months of operation. Antiprotons cannot be found in nature, in any case not on Earth. But they can be created at CERN where sufficient energy is available at the other accelerator, the PS. The antiprotons are accumulated in a special storage ring, built by a team led by van der Meer.

It is here that his ingenious method, called stochastic cooling, enables an intense antiproton beam to be built up. The signals from produced particles are recorded in huge detector systems set up around two collision points along the periphery of the SPS storage ring. The largest of these detectors was designed, built and operated by a team led by Rubbia. A second large detector was built by another team, operating it in parallel with the first one, nicely confirming the extremely important results.

An old dream was fulfilled last year when the discoveries of the W and Z were made at CERN – the dream of better understanding the weak interaction, which turns out to be weak just because the W and Z are so very heavy. The weak interaction is unique in that it can change the nature of a particle, for example transforming a neutron into a proton or vice versa. Such transformations are crucial for the sun and it is the weakness of the interaction which leads to the very slow burning of the nuclear fuel of the sun and thus creates the conditions on earth which can support life.

At first radioactive decays were the only weak interaction phenomenon available for study. Nowadays thanks to accelerators and storage rings this field of research is quite large. The theory which synthesizes a vast amount of knowledge and combines our understanding of the weak and electromagnetic interactions was honoured by the award of the Nobel Prize for physics in 1979 to Sheldon Glashow, Abdus Salam and Steven Weinberg. It also predicted new phenomena caused by the invented particle Z, introduced to make the theory consistent. Such phenomena were first observed in a CERN experiment about ten years ago. The only historical parallel goes back 120 years to Maxwells theory for electric and magnetic phenomena. In that case the theory was made consistent by a new ingredient, which contained the seed for the prediction of radio waves, discovered by Heinrich Herz almost 100 years ago. The modern electroweak theory contains not only the electromagnetic photons as communicators of force but also the communicators W and Z which act as a kind of shock-absorber, especially noticeable in hard collisions – such as those which must have occurred frequently during the Big Bang era at the early stage of the evolution of our universe. The collisions in the CERN collider may be hard enough to break loose the communicators, the shock-absorbers, for a short moment. The resulting fireworks of newly produced particles have been observed in the detectors, and the signs showing the presence of the W and Z have been seen and a start has been made on measuring their properties.

Professor Rubbia and Dr. van der Meer,

Your achievements in recent years, leading to the successful operation of the CERN proton-antiproton collider, have been widely admired in the whole world. The discovery of the W and Z particles will go down in the history of physics like the discovery of radio waves and the photons of light, the communicators of electromagnetism.

I know that you share your joy with many collaborators at CERN and in the participating universities. I also know that they congratulate you in many ways, also by setting new records for energy and for the rate of collisions, and by finding new interesting phenomena produced in the collisions. The discovery of the W and Z is not the end – it is the beginning.

On behalf of the Royal Swedish Academy of Sciences, I have the pleasure and the honour of extending to you our warmest congratulations. I now invite you to receive your prizes from the hands of His Majesty the King.