DRbJECTS fogTHE FUTURE OF EUROPEAN nNce_ CERN’S LARGE HADRON COLLIDER: A NEW TOOL FOR INVESTIGATING THE MlCROCOSM G. Brianti and W. Scandale Abstract Following the success of its Large Electron Positron (LEP) collider, CERN is studying ways of building a Large Hadron Collider (LHC) using the underground tunnel housing LEP to keep costs down. Thus, by the year 2000 experimentalists may be able to shed light on some of the most fascinating physics problems of the microcosm. On 20 December 1991 the CERN Council unanimously adopted a resolution which recognized the LHC as the right machine for the advance of the field and the future of CERN, and has asked the Director-General to present a final plan and cost estimate within 1993 in View of final approval for construction. 1. Introduction nominal momentum of 30 GeV/c. In every interaction the energy Until the beginning of the nineteen—sixties accelerators were involved is equivalent to that of a proton of 2000 GeV/c striking a used purely to produce bunches of electrons or protons at higher fixed target, something still impossible to achieve with a single— and higher energies and intensities that could be fired at fixed beam accelerator. targets in the solid, liquid or gaseous state and the results studied At the end of the nineteen—seventies, CERN’s proton accel- with suitable experimental detectors. In such a configuration only erator Super Proton Synchrotron (SPS), originally designed to a small part of the kinetic energy released by the incident parti— accelerate individual bunches of protons up to 450 GeV, was cles colliding with nuclei in such targets is transformed into new turned into a proton—antiproton collider capable of developing matter. the rest being wasted by putting in motion the debris of energies of 630 GeV per interaction. the target. To get around the problem, colliders were devised This undertaking, based on a proposal by Carlo Rubbia going towards the end of the nineteen-fifties. These are accelerators in back to 1975, was completed in 1981 and led to the 1983 discov- which two beams of particles circulating in opposite directions ery of the intermediate vector bosons. are made to collide with one another. The centre-of—mass of two Some years later, the first proton accelerator, which was fitted colliding particles is stationary in the reference system of the with superconducting magnets, came into service at Fermilab accelerator so that all the initial kinetic energy can go towards the outside Chicago. Called the Tevatron, this is a revolutionary creation of new particles. machine with magnets cooled to a temperature of 4.5 K creating The idea of using electron colliders began making headway fields of 4.5 tesla, with the result that in a ring of practically the and was studied in greatest depth in the USA. However, it was same size as the SPS individual bunches can be accelerated up to contributions to thinking by Bruno Tuschek, an Austrian physi- energies of some 900 GeV and, when suitably modified, can cist working at the Italian National Physics Laboratory of Frascati produce proton—antiproton collisions with kinetic energies of that provided the greatest spur to the concept. 1.8 TeV in the centre—of—mass. By showing that intense beams at In 1961 Tuschek suggested that bunches of electrons and high—kinetic energies can be confined in a cryogenic environ- positrons should be accelerated in opposite directions around a ment, the Tevatron has paved the way for a new generation of single magnetic ring, and he demonstrated the various advantages hadron accelerators exploiting state-of—the-art technologies to of this solution, which in the next thirty years gave rise to succes- achieve high—field magnets while holding down costs. sive generations of lepton accelerators, starting from the small One major difference between lepton and hadron colliders device 2 m across called the Anello di Accumulazione (AdA), i.e. stems from the fact that leptons are particles with no structure accumulation ring built in 1962 and designed merely to check the while hadrons have a composite make-up, consisting of quarks concept’s feasibility, right up to the gigantic LEP collider ring, and gluons sharing the available energy. In interactions between some 9 km in diameter and completed at CERN in 1989. leptons, the energy involved is exactly the same as that of the Hadron colliders are of more recent origin. The first, called incident particles, whereas in collisions between hadrons only a the Intersecting Storage Ring (ISR), was born at CERN in the fraction of the initial energy is available for an interaction occur- second half of the nineteen—sixties, completed in 1971 and ring between a single quark or gluon in one hadron and a quark or 101 remained in operation until 1983. The two beams each had a gluon in the other. At first sight, this fact would appear to give an © 1992 Gordon and Breach Science Publishers S A. Photocopying permitted by license only Particle World, Vol. 3, N0. 2. p. lOl—l07, I992. G Biianti and W. Scandale unchallengeable edge to the lepton machines. But in practice, in CERN’s LEP accelerator is the ideal tool for bringing to fruition the hadron machines energies can be reached that are at least one a detailed research programme of this type. The energy range order of magnitude greater than those of the lepton machines, so accessible in the LEP experiments, on |00 GeV to 200 GeV, is that effective collision energies end up by being far greater with what is needed to study objects at sizes of It)‘8 In, and situations hadron collisions. This depends on the fact that a charged particle like those reigning in an expanding universe such as the one that travelling along a curved orbit emits electromagnetic energy in a existed at 10"0 s after the big bang. By investigating that far—off quantity that is inversely proportional to the fourth power of its moment in the past, results should emerge that would throw light own mass and to the radius of curvature, a phenomenon that in on our understanding of recent and astrophysical observations in the case of circular lepton accelerators determines the ultimate the contemporary macrocosm. limit on the available energy. Nevertheless, the success and predictive capacity of the SM A collider’s efficiency is measured by a parameter termed should not make us oblivious to the many unresolved problems luminosity, which is proportional to the number of particle pairs that still exist, the investigation of which requires energies and likely to collide in a given unit of time. More specifically, spatial resolutions inaccessible to present—day accelerators. It is luminosity is the interaction frequency per useful collision area, considered, for example, that in the timespan between [0’12 and also known as the cross section, and it is normally expressed in IO’l0 s after the big bang, the W and Z bosons, carriers of the the units cm’2s4. In processes in which a mass M is created, the weak forces, assumed masses of considerable size and were lost production cross section is inversely proportional to M2 Con— to the cosmic stage because of the steady cooling down of the sequently, if M is very large the cross section is very small and a universe. To verify this hypothesis, the energy and spatial resolu— high luminosity is therefore needed to obtain observable inter— tion available for the exploration of the phenomena involved action frequencies. would have to be increased by an order of magnitude above those From the foregoing observations it will be realized that high available today. energy and high luminosity are basic properties of a collider and The SM is based on the concept of symmetry as applied to the that they complement one another. High energy makes it possible quantum properties of particles. However, in phenomena where to produce “exotic” particles with ever greater masses while high the energy involved is modest, the laws of symmetry can be luminosity enables the experimentalist to increase production violated, and the hypothesis has been put forward that this kind of rates. violation might be closely connected to the properties of empty In the past few years CERN has been studying the prospects space. In describing the electroweak theory, which brings of providing researchers with a new and more powerful tool for together the properties of electromagnetism with those of radio- examining the microcosm: a hadron collider producing proton activity, or the theory of strong interactions, nowadays known as interactions up to a possible total energy of 15 TeV, at a lumi— quantum chromodynamics (QCD), account should be taken of the nosity of 1.6 X l034 cm‘zsd. A device of this kind would have to fact that empty space is not a state in which “there is an absence be fitted with high—field magnets produced by state-of—the-art of everything”. Rather, it behaves like a supporting medium, with technology. It could be built rather rapidly and costs could be properties that are very like those of a superconducting material held down, and it would, by the year 2000, be capable of shed- and, in the SM, cause radical changes in the characteristics of ding light on some of the most fascinating problems of the interactions upon temperature variations and introduce the physics of the microcosm. asymmetry of the expanding universe after a brief period of initial cooling. At temperatures higher than an energy of 200 GeV, the mass of W and Z bosons ought to disappear. Direct observation 2.