The science behind the 300 GeV project

A report on the "status of the project for a European 300 GeV proton synchrotron" was presented to the CERN Council at its December meeting. The following extracts are from the sections covering the scientific background to the proposal.

Up to now, the most significant discovery in high-energy physics is that the proton, neutron and electron are not the only basic particles of nature. There are many more – difficult to observe because, when produced, they disintegrate almost immediately. But they are as important as the proton, neutron or electron when one tries to understand the nature of the strong and weak forces.

The first of these new, highly unstable particles were discovered in cosmic rays. The high-energy accelerators constructed in the last 15 years have revealed the existence of close to a hundred more. These have often been called elementary particles – a name that is getting more and more questionable as their properties are better known.

As the list of the known particles grew, their astounding variety at first created bewilderment and discouraged systematic interpretations. Quite remarkably, however, the last five years have brought us to the stage where the very multiplicity of particles has revealed a novel order, characterized by well defined mathematical principles of symmetry (usually denoted by the symbols SU3 and SU6). Particles that, at first sight, are completely different from each other have now been recognized as belonging to the same family and as having deep-lying similarities. The proton and neutron cannot be understood separately, they are only two members of a larger family containing perhaps 18 particles. Also, the interpretation of the strong and weak forces is profoundly affected by these new principles of symmetry, which allow single interpretations of experimental facts that would have been wholly unrelated a few years ago. Finally, most physicists now tend to believe that the new symmetries may be the manifestation of a remarkable internal structure of the proton, neutron and many other particles, which were earlier regarded as elementary. If this is true, the proton may contain even more fundamental objects (for which the name of "quark" has been proposed) – a fact that would open up once more completely new viewpoints in physics.

It is in the light of this general development that the significance of the 300 GeV project can best be evaluated. Large-scale improvement and extension programmes have been undertaken to increase the potentialities of the CERN and Brookhaven proton synchrotrons. These will ensure that the advanced positions reached by Europe and the USA can be maintained for some 10 years. But the next step must be prepared now, because projects on this scale take a decade to construct and bring into use. The community of particle physicists agrees that this step consists of building much larger proton synchrotrons – such machines have the advantage of offering simultaneously higher energies and higher beam intensities. Thus, the USSR is approaching completion of a 70 GeV proton synchrotron and the USA is preparing the final decision to build a 200 GeV proton synchrotron. In Europe, both ECFA (the European Committee for Future Accelerators) and the Scientific Policy Committee of CERN have agreed that the 300 GeV proton synchrotron, with its higher energy compensating its longer construction time, would provide our continent with a suitable instrument to take over in the second half of the next decade.

• Compiled from the article on pp231–233.

Compiler’s Note

The article quoted a statement by Nobel Laureate Edwin McMillan to the US Joint Committee on Atomic Energy, 1965: "In no case in the past have scientists been disappointed in the results following an increase in available energy."

The 300 GeV Super Proton Synchrotron (SPS), finally built at CERN, made available not only an increase in energy but also an impressive variety of physics. Planned as a 300 GeV proton accelerator, a beam energy of 400 GeV was announced within minutes of turn-on in 1976. In 1981 it was transformed into a proton–antiproton collider, where the W and Z were discovered. In the 1990s it alternated between providing 20 GeV electrons and positrons for the Large Electron–Positron Collider (LEP), and accelerating heavy ions to energies well beyond 100 GeV per nucleon. While the Standard Model was being tested with outstanding precision in LEP collisions, Big Bang matter – the quark-gluon plasma – was being created in fixed-target experiments.

Today the SPS is accelerating protons to 450 GeV on their way to the LHC, designed to reach 7 TeV per beam – another energy increase that will surely not disappoint, whatever the results.