2014 marks not just the centenary of the start of World War I, and the 75th anniversary of World War II, but on 29 September it is 60 years since the establishment of CERN, the European Centre for Nuclear Research or, in its modern form, Particle Physics. Less than a decade after European nations had been fighting one another in a terrible war, 12 of those nations had united in science. Today, CERN is a world laboratory, famed for having been the home of the world wide web, brainchild of then CERN scientist Tim Berners-Lee; of several Nobel Prizes for physics, although not (yet) for Peace; and most recently, for the discovery of the Higgs Boson. The origin of CERN, and its political significance, are perhaps no less remarkable than its justly celebrated status as the greatest laboratory of scientific endeavour in history.
Its life has spanned a remarkable period in scientific culture. The paradigm shifts in our understanding of the fundamental particles and the forces that control the cosmos, which have occurred since 1950, are in no small measure thanks to CERN.
In 1954, the hoped for simplicity in matter, where the electron and neutrino partner a neutron and proton, had been lost. Novel relatives of the proton were proliferating. Then, exactly 50 years ago, the theoretical concept of the quark was born, which explains the multitude as bound states of groups of quarks. By 1970 the existence of this new layer of reality had been confirmed, by experiments at Stanford, California, and at CERN.
During the 1970s our understanding of quarks and the strong force developed. On the one hand this was thanks to theory, but also due to experiments at CERN’s Intersecting Storage Rings: the ISR. Head on collisions between counter-rotating beams of protons produced sprays of particles, which instead of flying in all directions, tended to emerge in sharp jets. The properties of these jets confirmed the predictions of quantum chromodynamics – QCD – the theory that the strong force arises from the interactions among the fundamental quarks and gluons.
CERN had begun in 1954 with a proton synchrotron, a circular accelerator with a circumference of about 600 metres, which was vast at the time, although trifling by modern standards. This was superseded by a super-proton synchrotron, or SPS, some 7 kilometres in circumference. This fired beams of protons and other particles at static targets, its precision measurements building confidence in the QCD theory and also in the theory of the weak force – QFD, quantum flavourdynamics.
QFD brought the electromagnetic and weak forces into a single framework. This first step towards a possible unification of all forces implied the existence of W and Z bosons, analogues of the photon. Unlike the massless photon, however, the W and Z were predicted to be very massive, some 80 to 90 times more than a proton or neutron, and hence beyond reach of experiments at that time. This changed when the SPS was converted into a collider of protons and anti-protons. By 1984 experiments at the novel accelerator had discovered the W and Z bosons, in line with what QFD predicted. This led to Nobel Prizes for Carlo Rubbia and Simon van der Meer, in 1984.
The confirmation of QCD and QFD led to a marked change in particle physics. Where hitherto it had sought the basic templates of matter, from the 1980s it turned increasingly to understanding how matter emerged from the Big Bang. For CERN’s very high-energy experiments replicate conditions that were prevalent in the hot early universe, and theory implies that the behaviour of the forces and particles in such circumstances is less complex than at the relatively cool conditions of daily experience. Thus began a period of high-energy particle physics as experimental cosmology.
This raced ahead during the 1990s with LEP – the Large Electron Positron collider, a 27 kilometre ring of magnets underground, which looped from CERN towards Lake Geneva, beneath the airport and back to CERN, via the foothills of the Jura Mountains. Initially designed to produce tens of millions of Z bosons, in order to test QFD and QCD to high precision, by 2000 its performance was able to produce pairs of W bosons. The precision was such that small deviations were found between these measurements and what theory implied for the properties of these particles.
The explanation involved two particles, whose subsequent discoveries have closed a chapter in physics. These are the top quark, and the Higgs Boson.
As gaps in Mendeleev’s periodic table of the elements in the 19th century had identified new elements, so at the end of the 20th century a gap in the emerging pattern of particles was discerned. To complete the menu required a top quark.
The precision measurements at LEP could be explained if the top quark exists, too massive for LEP to produce directly, but nonetheless able to disturb the measurements of other quantities at LEP courtesy of quantum theory. Theory and data would agree if the top quark mass were nearly two hundred times that of a proton. The top quark was discovered at Fermilab in the USA in 1995, its mass as required by the LEP data from CERN.
As the 21st century dawned, all the pieces of the “Standard Model” of particles and forces were in place, but one. The theories worked well, but we had no explanation of why the various particles have their menu of masses, or even why they have mass at all. Adding mass into the equations by hand is like a band-aid, capable of allowing computations that agree with data to remarkable precision. However, we can imagine circumstances, where particles collide at energies far beyond those accessible today, where the theories would predict nonsense — infinity as the answer for quantities that are finite, for example. A mathematical solution to this impasse had been discovered fifty years ago, and implied that there is a further massive particle, known as the Higgs Boson, after Peter Higgs who, alone of the independent discoveries of the concept, drew attention to some crucial experimental implications of the boson.
Discovery of the Higgs Boson at CERN in 2012 following the conversion of LEP into the LHC – Large Hadron Collider – is the climax of CERN’s first 60 years. It led to the Nobel Prize for Higgs and Francois Englert, theorists whose ideas initiated the quest. Many wondered whether the Nobel Foundation would break new ground and award the physics prize to a laboratory, CERN, for enabling the experimental discovery, but this did not happen.
CERN has been associated with other Nobel Prizes in Physics, such as to Georges Charpak, for his innovative work developing methods of detecting radiation and particles, which are used not just at CERN but in industry and hospitals. CERN’s reach has been remarkable. From a vision that helped unite Europe, through science, we have seen it breach the Cold War, with collaborations in the 1960s onwards with JINR, the Warsaw Pact’s scientific analogue, and today CERN has become truly a physics laboratory for the world.