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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

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. From LEP to the LHC of the phenomenon could throw light for us on the underlying Our experience of the everyday world shows an enormous reasons bringing it about and thereby clarify one of the most complexity and variety of phenomena whose connections are controversial questions of present-day physics. hard to identify. The microcosm, on the other hand, is governed In interactions involving energies in the 1 TeV range quarks by simple laws describing the properties and interactions among radiate W and Z bosons which end up by colliding. Quantum the fundamental constituents of matter, which are displayed in fluctuations permit occasional collisions at high energies. Conse— modern particle accelerators with spatial resolutions of 10"8 m. quently, in a high-luminosity accelerator of some 10 TeV a These laws have been brought together in a unified theory suitable number of interactions should occur at an energy high known as the Standard Model (SM), describing the point-like enough to give rise to a new range of phenomena in which the constituents of matter, electrons and quarks, and the basic mechanisms governing the values of the basic constituents of interactions that give rise to more complex systems such as matter should be identifiable. These predictions go back to hadrons, nucleons and, on a larger scale, the atoms and concepts introduced by Peter Higgs t0 the effect that, as with 102 molecules. The SM, like any other physical law, is subjected to beliefs held about the ether in past centuries, in a vacuum- constant testing in order to elucidate its range of validity. pervaded universe the Higgs particles create quantum fields that 0 Br ianti and W Seandale

interact with matter and give every particle the valtre of its observed mass. Another reason driving the physicist to investigate the TeV energy range has to do with the constant effort to unify the laws of nature evermore closely. Following the extraordinary success achieved in unifying the electromagnetic and weak forces in the electroweak theory, it is natural to try and extend our unifying vision still further by finding out whether leptons and quarks have symmetrical behaviour due to manifestations of a single form of interaction. There are serious grounds for believing that this symmetry could effectively exist in phenomena involving energies > 10'2 TeV described by the Grand Unified Theory (GUT). Such situations existed in the primaeval universe at [0’43 s after the big bang but are not accessible in laboratory conditions. The vestiges of this powerful series of events which, together with the cooling of the universe, led to the break—up of primaeval unity, should still be present today and the effects of the unified interactions should be measurable in the energy range that can be attained by present—day accelerators. This has yet to be proven, however, and no residual perturbation of the powerful Pictorial view 01 the LHC in the LEP tunnel phenomena occurring at high energies has so far been observed. There may be compensation mechanisms in play that imply the existence of new particles and interactions; even if our way of interpreting these aspects of the natural world still remains several years CERN has pursued a research and development vague, there are indications that encourage one to think that this programme in close collaboration with industry, both for the new range of phenomena could become noticeable at 1 TeV development of NbTi wires with thinner superconductor fila— energies. ments than ever before (~ 5 pm), and the construction of models All these expectations make us want to explore the natural and prototypes. phenomena in interactions at energies in the TeV range, and they The motivation of the thin filaments is to reduce as much as fully justify the effort to build a hadron collider of the LHC type. possible the persistent currents which distort the field especially at the injection of the particles, when the field is ~ 0.6 T, namely 2.1 Brief history of the project sixteen times lower than the top field. Since the beginning of the LEP project, it was decided to The construction and testing of models and prototypes is construct a tunnel of the largest possible circumference, not only mandatory because the technology of LHC magnets, charac- in order to obtain the maximum collision energy for e+ and e‘, terized by the very high field of [O T and cooling with superfluid but also to install a proton—proton collider of very high energy in He at 1.9 K constitute a large extrapolation of the one used for the the same tunnel in the future. HERA magnets. Several models (full cross section ~ 1 m long) Such an energy was tentatively fixed at several TeV per beam and one full magnet (10 m long) have already been constructed in order to probe matter down to 10’") m and go back in time to and tested and the results are reported below. some l0“'2 s after the big bang. The LHC project, which is being The concept adopted for the overall collider design incor- developed at CERN fulfils this requirement. It is based on high— porates not only the highest possible beam energy for the given field superconducting magnets (~ l0 T) which will be installed circumference but also the largest possible luminosity. The moti— 1.2 m above LEP (fig. I) all along the 27 km tunnel. With an vation of the latter is that the proton beams being broad-band operational dipole field of ~ 9.5 T, the beam energy will be beams in terms of parton energy distribution, the high luminosity ~ 7.7 TeV and hence provide a centre—of-mass energy of enhances the probability of parton collisions with a large fraction 15.4 TeV. Such a high-magnetic field can only be obtained with of the proton energy. superconductors retaining a large current density in the high—field In this way, it is possible to increase the so—called “physics region and suitable for mass production both from the technical reach" of the collider, namely the mass of observable objects, for and economical point of view. the given proton energy. In general, one can indeed trade lu— Indeed NbTi conductors, which have been used both for the minosity for energy in the sense that a factor of ten increase in Tevatron and for HERA, are adequate for the purpose provided luminosity is approximately equivalent to an energy twice as 103 they are cooled down at the very low temperature of 1.9 K. For large. G. Brianti and W Scandale

The luminosity for bunched beams is given by Other limitations find their origins in the interaction of the _ k2 beam with the metallic vacuum chamber walls, which produces a destabilizing effect when the beam current exceeds a given limit. 41c ox 6y ’ They can be attenuated by a strong feed—back system. where f is the particle revolution frequency, k is the number of Technological limitations concern, for example, the cryogenic bunches per beam, N is the number of particles per bunch and GK, system, which must be capable of absorbing the synchrotron 0' are the beam radii at the collision point expressed in r.m.s. radiation power emitted by the beam following a circular trajec- values of the transverse spatial distributions, assumed to be tory, or the tremendous beam energy which must be safely ab— gaussian. sorbed in an external dump at the end of each experimental ses— It should be noted at this point that the cross section of a sion or in case of emergency. given type of event decreases with the square of the mass of the object produced, so that luminosity is at premium for very high 2.2 Magnets energy colliders. At variance with particle—antiparticle colliders (like LEP and But what limits the luminosity? SppS, the two LHC proton beams must be guided by two distinct The most important limit originates from the beam—beam magnet channels with vertically opposite fields. About 24 km out interaction, due to the action on the particles of one beam pro- of the 27 km of the circumference will be occupied by magnetic duced by the electromagnetic field of the other beam. This elements. interaction is a strongly non—linear function of the transverse The repetitive structure is composed of 192 modular cells, coordinates and the momentum of the particles, and leads to a each made up of 6 double dipoles, 2 double quadrupoles, several stochastic beam behaviour when the bunch particle density is single correctors and a beam position monitor element (fig. 2). higher than a limiting value determined by careful and extended The collision points are located at the centre of the 8 straight studies at the proton—antiproton colliders, both at CERN and sections, which are connected to the arcs by means of additional Fermi Laboratory. magnets, partly double (two apertures) and partly single (one

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Half-cell (with lattice): MB. main twin dipole with S (sextupole) and D (decapole) small correctors; MQF/D. main twin quadrupole (focusing/defocusing); BPH/V. beam position monitor (horizontal/vertical); QTDF/D. tuning quadrupole (focusing/defocusing) with octupole corrector and SBH/V. sextupole and dipole corrector (horizontal/vertical). aperture) elements. These magnets fulfill various functions such as: — suppression of the dispersion (momentum variation) at the collision points, — recombination of the two beams for collisions, * strong focusing of the beams at the collision points (“low— beta” section). The field distribution in superconducting magnets is essen— tially determined by the conductor position and hence is intrinsi— cally less precise than in the classical magnets, where it is governed by the iron pole boundaries. This means that one has to complement the main guiding dipoles and the main focusing quadrupoles with an important set of correcting magnets, namely small dipoles to control the beam orbits and multipoles (sextu— poles, octupoles and decapoles) to compensate the corresponding errors of the main magnets (fig. 2). In total there will be ~ 6000 magnetic elements for each of the beams. It should, however, be noted that the main dipoles and quadrupoles are double elements, in which the two distinct

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centred on the construction of several magnet models (full cross order to keep a sufficient margin with respect to the lambda line, section, ~ 1 m long) and often l0 m long prototypes was launch— namely the transition between He II and He 1. ed several years ago. The magnetic structure is immersed in a static bath of He II To date, seven models and one 10 m long prototype with pressurized slightly above 1 bar. which is kept at the temperature HERA coils have been built. The magnet performance is eval— of 1.9 K by a tubular heat exchanger passing through all magnets uated in terms of: of a half-cell and carrying boiling He II at a pressure of 16 mbar. (i) the maximum field with respect to the so—called “short— This boiling He, which stabilizes the temperature of the magnets, sample” field, namely the field corresponding to the maxi— is locally produced in each half—cell by expansion from a conduit mum current as measured in a short cable sample; carrying He at 2.2 K (fig. 4). (ii) the behaviour of “training”, which is characterized by the The scheme has the great advantage of stabilizing at a number of current cycles necessary to reach the maximum constant temperature all the magnets around the ring, indepen— field; dently of their distance from the 8 large refrigerators, located at (iii) the uniformity of the field distribution. the 8 access points. Four of these refrigerators are being installed The results so far can be summarized as follows: for LEP 200, namely the project aimed at increasing the LEP 0 all models and the prototype with the HERA coils have beam energy to more than 90 GeV per beam. It is interesting reached their respective short sample field at 4.2 K practically to note that the total mass to be cooled down to —271°C is without training; ~ 30 000 t and that the LHC will be the largest and most extended - at 1.8 K, one model. fully measured, has reached the short— cryogenic installation in Europe. sample field of 10 T after a long training, and the other models have attained 90—92% of their short-sample fields after some training, 70% of the quenches are in the coil ends, 60% of the 3. The LHC as a multiparticle collider quenches show voltage spikes indicating a displacement of the The LHC can provide three types of collisions: whole coil structure in the collars with vibrations at fixed (i) proton—proton collisions with a nominal energy of l5.4 TeV frequencies; and very high luminosities up to 4.5 X 1034 cm—Zs—l in a ' at 1.8 K the 10 m long prototype with HERA coils has reached single experiment or 1.7 X 1034 cm—zs—l in each of three its short—sample field of almost 8 T practically without simultaneous experiments; training; (ii) collisions between heavy ions, such as lead with an energy 0 the field distribution is satisfactory. of 6.3 TeV per nucleon pair and a luminosity of While the achievement of the unprecedented field of 10 T for 1027 cm—zs"; accelerator magnets must be emphasized, it has become clear that (iii) electron—proton collisions (one LHC proton beam against the more work has to be done on the detailed cable configuration and LEP electron beam) with a maximum energy of 1.7 GeV or on the collars in order to improve the training behaviour at the top 1.3 GeV at a maximum luminosity of some 1032 cm—Zs—l. field. This will be done by means of new models under con- Six out of the eight potential collision points could be used struction at CERN and by suitably modifying some of the already effectively for experiments, the last two being reserved for ma- ordered industrial prototypes. chine functions namely one for the double extraction system to The whole programme will culminate with the installation lead the beams towards external dumps at the end of each run or and testing in a CERN hall of a full cell, consisting of 8 dipoles in case of malfunctioning and one for the so-called cleaning and 2 quadrupoles, which corresponds to 100 m of the magnetic section which has the purpose of intercepting the beam halo structure as it will be installed in the machine. before it can spray the superconducting magnets. It goes without saying that the last important point to be At present, four of the six possible collision points are assessed is the field precision which can be obtained in the full occupied by the LEP detectors and two (point 1 and point 7) are magnets in the entire field range from injection (0.58 T) to full available for the construction of new experimental areas for field. The required accuracy is close to 1041 in the magnet field proton—proton experiments. Of course, any of the present LEP region to be occupied by particles. areas can be used for a LHC experiment either at the end of the useful scientific life of the corresponding LEP experiment or in 2.3 Cryogenic system alternation with it. In this latter case, the area must be modified The purpose of the cryogenic system is to keep the coils and essentially by adding a second detector garage. the whole magnetic structure at the temperature of the superfluid He, both to maximize the current carrying capacity of the superconducting cables and to profit from the exceptional physical 4. Conclusions 106 characteristics of He 11, such as its large thermal conductivity and In conclusion, it can be said that the installation in the LEP low viscosity. The working temperature is chosen to be 1.9 K in tunnel of the second magnetic structure in the form of very G Brianti and W Scandale

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advanced double superconducting magnets will provide the particle physicists not only with a superb tool to investigate an Addresses: energy domain of 1 TeV or more, but with important additional possibilities for ion physics and in the more distant future an G. Brianti extension by a factor of five of the energy of electron—proton Associate Director for Future Accelerators collisions which have just started at HERA. CERN All these features have given rise to enormous interest among CH—l21 1 Geneva (Switzerland) the physicists, who met from 5—8 March 1992 in Evian (France) under the auspices of ECFA and CERN to present and discuss the W. Scandale first experimental proposals. CERN On 20 December 1991 the CERN Council unanimously CH—1211 Geneva (Switzerland) 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. Received on March 1992. 107