EvolutionThe of Particle Accelerators & Colliders by WOLFGANG K. H. PANOFSKY

HEN J. J. THOMSON discovered the electron, he did not call the instrument he was using an accelerator, but an accelerator it Wcertainly was. He accelerated particles between two electrodes to which he had applied a difference in electric potential. He manipulated the resulting beam with electric and magnetic fields to determine the charge-to- mass ratio of cathode rays. Thomson achieved his discovery by studying the properties of the beam itself—not its impact on a target or another beam, as we do today. Accelerators have since become indispensable in the quest to understand Nature at smaller and smaller scales. And although they are much bigger and far more complex, they still operate on much the same physical prin- ciples as Thomson’s device. It took another half century, however, before accelerators became entrenched as the key tools in the search for subatomic particles. Before that, experi- ments were largely based on natural radioactive sources and cosmic rays. Ernest Rutherford and his colleagues established the existence of the atomic nucleus— as well as of protons and neutrons—using radioactive sources. The positron, muon, charged pions and kaons were discovered in cosmic rays. One might argue that the second subatomic particle discovered at an accel- erator was the neutral pion, but even here the story is more complex. That it existed had already been surmised from the existence of charged pions, and the occurrence of gamma rays in cosmic rays gave preliminary evidence for such a particle. But it was an accelerator-based experiment that truly nailed down the existence of this elusive object.

36 SPRING 1997 There followed almost two decades of accelerator-based discoveries of other subatomic particles originally thought to be elementary, notably the antiproton and the vector mesons. Most of these particles have since turned out to be composites of quarks. After 1970 colliders—machines using two accelerator beams in collision—entered the picture. Since then most, but certainly not all, new revelations in particle physics have come from these colliders.

N CONSIDERING the evolution of accelerator and collider technology, we usually think first of the available energy such tools provide. Fundamen- Itally, this is the way it should be. When the study of the atomic nucleus stood at the forefront of “particle physics” research, sufficient energy was needed to allow two nuclei—which are positively charged and therefore repel one another—to be brought close enough to interact. Today, when the components of these nuclei are the main objects of study, the reasons for high energy are more subtle. Under the laws of quantum mechanics, particles can be described both by their physical trajectory as well as through an associated wave whose behavior gives the probability that a particle can be localized at a given point in space and time. If the wavelength of a probing particle is short, matter can be examined at extremely small distances; if long, then the scale of things that can be investigated will be coarser. Quantum mechanics relates this wavelength to the energy (or, more precisely, the momentum) of the colliding particles: the greater the energy, the shorter the wavelength.

BEAM LINE 37 This relationship can be expressed across a potential difference of one quantitatively. To examine matter at volt.) At the scale of the nucleus, en- − the scale of an atom (about 10 8 cen- ergies in the million electron volt— timeter), the energies required are in or MeV—range are needed. To ex- the range of a thousand electron amine the fine structure of the basic A “Livingston plot” showing accelerator energy versus time, updated to include volts. (An electron volt is the energy constituents of matter requires en- machines that came on line during the unit customarily used by particle ergies generally exceeding a billion 1980s. The filled circles indicate new or physicists; it is the energy a parti- electron volts, or 1 GeV. upgraded accelerators of each type. cle acquires when it is accelerated But there is another reason for us- ing high energy. Most of the objects of interest to the elementary parti- cle physicist today do not exist as free 1000 TeV particles in Nature; they have to be created artificially in the laboratory. The famous E = mc2 relationship gov- 100 TeV erns the collision energy E required to produce a particle of mass m. Proton Storage Rings Many of the most interesting parti- 10 TeV (equivalent energy) cles are so heavy that collision energies of many GeV are needed to create them. In fact, the key to under- 1 TeV standing the origins of many para- meters, including the masses of the Proton known particles, required to make Synchrotrons 100 GeV today’s theories consistent is believed to reside in the attainment of colli- sion energies in the trillion electron volt, or TeV, range. 10 GeV Electron Synchrotrons Our progress in attaining ever Particle Energy Electron Linacs Synchrocyclotrons higher collision energy has indeed Betatrons been impressive. The graph on the 1 GeV Proton Linacs left, originally produced by M. Stan- ley Livingston in 1954, shows how Sector-Focused the laboratory energy of the parti- Cyclotrons 100 MeV cle beams produced by accelerators Cyclotrons Electrostatic has increased. This plot has been up- Generators dated by adding modern develop- 10 MeV ments. One of the first things to no- tice is that the energy of man-made Rectifier accelerators has been growing ex- Generators 1 MeV ponentially in time. Starting from the 1930s, the energy has increased— roughly speaking—by about a fac- tor of 10 every six to eight years. A 1930 1950 1970 1990 second conclusion is that this spec- Year of Commissioning tacular achievement has resulted

38 SPRING 1997 M. Stanley Livingston and Ernest O. Lawrence, with their 27-inch cyclotron at Berkeley Radiation Laboratory. (Courtesy Lawrence Berkeley National Laboratory)

from a succession of technologies rather than from construction of big- ger and better machines of a given type. When any one technology ran out of steam, a successor technology usually took over. In another respect, however, the Livingston plot is misleading. It suggests that energy is the primary, if not the only, parameter that defines the discovery potential of an accel- erator or collider. Energy is indeed provide a steady flow of particles, already well understood and contain required if physicists wish to cross a generally because that would require no new information. Accelerators new threshold of discovery, provid- too much electric power; instead, the differ in terms of the presence or ab- ed that this threshold is defined by beam is pulsed on and off. When sence of both kinds of backgrounds; the energy needed to induce a new physicists try to identify what reac- their discovery potential differs ac- phenomenon. But there are several tion has taken place, one piece of ev- cordingly. The amount and kinds of other parameters that are important idence is whether the different par- background are directly related to the for an accelerator to achieve—for ex- ticles emerge from a collision at the ease of data analysis, the type of ample, the intensity of the beam, or same time. Thus electronic circuits detector to be built, or whether the the number of particles accelerated register the instant when a particle desired results can be extracted at all. per second. traverses a detector. But if the ac- In general, as the energy increases, When the beam strikes a target, celerator’s duty cycle is small, then the number of possible reactions also its particles collide with those in the all the particles will burst forth dur- increases. So does the burden on the target. The likelihood of producing a ing a short time interval. Therefore discriminating power of detectors reaction is described by a number a relatively large number of acci- and on the data-analysis potential of called the cross section, which is the dental coincidences in time will oc- computers that can isolate the effective area a target particle pre- cur, caused by particles emerging “wheat” from the “chaff.” With the sents to an incident particle for that from different individual reactions, growth in energy indicated by the reaction to occur. The overall inter- instead of from real coincidences due Livingston plot, there had to be a par- action rate is then the product of the to particles emerging from a single allel growth in the analyzing poten- beam intensity, the density of target event. If time coincidence is an im- tial of the equipment required to particles, the cross section of the re- portant signature, a short duty cycle identify events of interest—as well action under investigation, and the is a disadvantage. as a growth in the number of peo- length of target material the incident Then there is the problem of back- ple involved in its construction and particle penetrates. This rate, and grounds. In addition to the reaction operation. therefore the beam intensity, is ex- under study, detectors will register And finally there is the matter tremely important if physicists are two kinds of undesirable events. of economy. Even if a planned ac- to collect data that have sufficient Some backgrounds arise from parti- celerator is technically capable of statistical accuracy to draw mean- cles generated by processes other providing the needed energy, ingful conclusions. than the beam’s interaction with the intensity, duty cycle, and low back- Another important parameter is target or another beam—such as with ground, it still must be affordable what we call the duty cycle—the per- residual gas, from “halo” particles and operable. The resources re- centage of time the beam is actual- traveling along the main beam, or quired—money, land, electric pow- ly on. Unlike Thomson’s device, even from cosmic rays. Other back- er—must be sufficiently moderate most modern accelerators do not grounds stem from reactions that are that the expected results will have

BEAM LINE 39 The first colliding-beam machine, a double-ring electron-electron collider, built by a small group of Princeton and Stanford physicists. (Courtesy Stanford University)

reference to their possibility stems from a Russian publication of the 1920s; it would not be surprising if the same idea occurred indepen- dently to many people. The first col- lider actually used for particle- physics experiments, built at Stanford in the late 1950s, produced electron-electron collisions (see pho- tograph on the left). Other early ma- chines, generating electron-positron collisions, were built in Italy, Siberia and France. Since then there has been commensurate value. Of course “val- the creation of new particles. This a plethora of electron-positron, ue” has to be broadly interpreted in collision energy is less than the lab- proton-proton and proton-antiproton terms not only of foreseeable or con- oratory energy of the particles in a colliders. jectured economic benefits but also beam if that beam strikes a station- There is another problem, how- of cultural values related to the in- ary target. When one particle hits an- ever. If the particles participating crease in basic understanding. In other at rest, part of the available en- in a collision are themselves view of all these considerations, the ergy must go toward the kinetic composite—that is, composed of choice of the next logical step in ac- energy of the system remaining after constituents—then the available celerator construction is always a the collision. If a proton of low en- energy must be shared among these complex and frequently a contro- ergy E strikes another proton at rest, constituents. The threshold for new versial issue. Energy is but one of for example, the collision energy is phenomena is generally defined by many parameters to be considered, E/2 and the remaining E/2 is the ki- the collision energy in the con- and the value of the project has to be netic energy with which the protons stituent frame: the energy that be- sufficiently great before a decision to move ahead. At very high energies comes available in the interaction go ahead can be acceptable to the the situation is complicated by rel- between two individual con- community at large. ativity. If a particle of total energy stituents. Here there are major dif- All these comments may appear E hits another particle of mass M, ferences that depend on whether the fairly obvious, but they are frequently then the collision energy is given accelerated particles are protons, 2 11/2 forgotten. Inventions that advance by Ecoll ~ (2Mc E) , which is much deuterons, electrons or something just one of the parameters—in par- less than E/2 for E much larger than else. Protons are composed of three ticular, energy—are often proposed Mc2. quarks and surrounded by various sincerely. But unless the other pa- If two particles of equal mass trav- gluons. Electrons and muons, as well rameters can be improved at the eling in opposite directions collide as quarks and gluons, are considered same time, to generate an overall ef- head on, however, the total kinetic pointlike, at least down to distances − ficient complex, increasing the energy of the combined system after of 10 16 centimeter. Recognizing energy alone usually cannot lead to collision is zero, and therefore the these differences, we can translate fundamentally new insights. entire energy of the two particles be- the Livingston plot into another comes available as collision energy. chart (top right, next page) showing HE ENERGY that really This is the basic energy advantage of- energy in the constituent frame ver- matters in doing elementary fered by colliding-beam machines, or sus year of operation for colliding- Tparticle physics is the colli- colliders. beam machines. sion energy—that is, the energy avail- The idea of colliding-beam ma- But the idea of generating higher able to induce a reaction, including chines is very old. The earliest collision energy via colliding beams

40 SPRING 1997 Right: The energy in the constituent 10 TeV frame of electron-positron and hadron colliders constructed (filled circles and squares) or planned. The energy of Hadron Colliders hadron colliders has here been derated e+e– Colliders by factors of 6–10 in accordance with LHC (CERN) the fact that the incident proton energy 1 TeV is shared among its quark and gluon constituents. TEVATRON NLC (Fermilab) – LEP II SPPS is worthless unless (as discussed (CERN) SLC LEP above) higher interaction rates can 100 GeV (SLAC) (CERN) be generated, too. To succeed, the TRISTAN density of the two beams must be (KEK) PETRA PEP high enough—approaching that of (DESY) (SLAC) atoms in ordinary matter—and their ISR CESR (Cornell) (CERN) interaction cross sections must be 10 GeV VEPP IV (Novosibirsk) sufficient to generate an adequate SPEAR II Constituent Center-of-Mass Energy data rate. In colliding-beam machines SPEAR DORIS VEPP III (SLAC) (DESY) (Novosibirsk) the critical figure is the luminosity ADONE L, which is the interaction rate per (Italy) second per unit cross section. The 1 GeV bottom graph on this page illustrates PRIN-STAN VEPP II ACO (Stanford) (Novosibirsk) (France) the luminosity of some of these ma- chines. In contrast to the constituent collision energy, which has contin- 1960 1970 1980 1990 2000 2010 ued the tradition of exponential Year of First Physics growth begun in the Livingston plot, the luminosity has grown much more slowly. There are good reasons for this trend that I will discuss shortly. 1038 e+e– Collider (operational) Hadron Collider (operational) Naturally there are differences e+e– Collider (planned) Hadron Collider (planned) that must be evaluated when choos- – ing which particles to use in accel- e p Collider 1036 erators and colliders. In addition to ) –1 the energy advantage mentioned for s –2 electrons, there are other factors. As 34

10 NLC

protons experience the strong inter- PEP-II/KEKB action, their use is desirable, at least LHC in respect to hadron-hadron inter- CESR 32 ISR BEPC actions. Moreover, the cross sections 10 PEP involved in hadron interactions are Luminosity (cm LEP LEP-II generally much larger than those en- PETRA DORIS ADONE PS

30 SPEAR HERA 10 – TRISTAN countered in electron machines, SP TEVATRON which therefore require higher lu- SLC minosity to be equally productive. 1028 Proton accelerators are generally 1 10 100 1000 10000 much more efficient than electron Center-of-Mass Energy (GeV) machines when used to produce sec- ondary beams of neutrons, pions, kaons, muons, and neutrinos. But Peak luminosities achieved at existing colliders and values projected for planned or upgraded machines. The dashed line electrons produce secondary beams indicates luminosity increasing as the square of the center-of- that are sharply concentrated in the mass energy Note that the rated machine energy has been forward direction, and these beams used in calculating the abscissa. (Data updated courtesy are less contaminated by neutrons. Greg Loew, SLAC)

BEAM LINE 41 Far left: William Hansen (right) and colleagues with a section of his first linear electron accelerator, which operated at Stanford University in 1947. Eventually 3.6 m long, it could accelerate electrons to an energy of 6 MeV. (Courtesy Stanford University)

Left: Ernest Lawrence’s first successful cyclotron, built in 1930. It was 13 cm in diameter and accelerated protons to 80 keV. (Courtesy Lawrence Berkeley National Laboratory)

compatible with this fact. In addi- tion, the remnants of the muons that When discussing the relative merits Beyond this problem is the matter decay during acceleration and stor- of electron and proton colliders, the of interpretability. When we use age constitute a severe background. background situation is complex be- electrons to bombard hadron targets, Thus, while the idea of muon col- cause the factors that cause them are be they stationary or contained in an liders as tools for particle physics has quite different. When accelerated, opposing beam, we are exploring a recently looked promising, there is and especially when their path is complex structure with an (as-yet) no example as yet of a successful bent by magnets, electrons radiate elementary object whose behavior is muon collider. X rays in the form of synchrotron well understood. Thus the informa- radiation. Protons usually have more tion about the structure of the proton UT THERE is an overarching serious interactions with residual gas resulting from electron-proton colli- issue of costs that dominates atoms, and those that deviate from sions, for example, tends to be easi- Bthe answer to the question, the nominal collider orbit are more er to interpret than the results from “How large can accelerators and col- apt to produce unwanted backgrounds proton-proton collisions. All the liders become, and what energy can from such causes. above observations are generalities, they attain?” The relationship of size A much more difficult—and to of course, and there are numerous and cost to energy is determined by some extent controversial—subject and important exceptions. For in- a set of relations known as scaling is the comparison of the complexi- stance, if neutrinos or muons— laws. Accelerators and colliders can ties of events initiated by electrons copiously produced as secondary be broadly classified into linear and with those induced by hadrons in beams from proton machines—are circular (or nearly circular) machines. general, and protons in particular. used to explore the structure of With classical electrostatic acceler- Today particle physicists are usually, hadrons, the results are comple- ators and proton or electron radio- but not always, interested in the re- mentary to those produced by elec- frequency linear accelerators, the sults of “hard” collisions between tron beams. scaling laws imply that the costs and the elementary constituents (by Everything I have said about elec- other resources required should grow which I mean entities considered to trons is also true of muons. The use about linearly with energy. Although be pointlike at the smallest observ- of muon beams offers significant ad- roughly true, linear scaling laws tend able distances). Because protons are vantages and disadvantages rela- to become invalid as the machines composite objects, a single hard col- tive to electrons. The two lightest approach various physical limits. The lision between their basic con- charged leptons, the electron and old electrostatic machines became stituents will be accompanied by a muon, experience essentially the too difficult and expensive to con- much larger number of extraneous same interactions. But muons, being struct when electrical breakdown “soft” collisions than is the case for heavier, radiate far less electromag- made it hard to devise accelerating electrons. Thus the fraction of in- netic energy than do electrons of columns able to withstand the nec- teresting events produced in an elec- equal energy; therefore backgrounds essary high voltages. And radio- tron machine is generally much larg- from radiative effects are much frequency linear accelerators indeed er than it is for proton machines. So lower. On the other hand, muons obey linear scaling laws as long as the analysis load in isolating the have a short lifetime (about 2 there are no limits associated with “needle” from the “haystack” tends microseconds), whereas electrons are their required luminosity. to be considerably more severe at stable. Colliding-beam devices using The scaling laws for circular ma- hadron machines. muons must be designed to be chines are more complex. Ernest

42 SPRING 1997 Ernest Courant, M. Stanley Livingston, and Hartland Snyder (left to right), who conceived the idea of strong focussing. (Courtesy Brookhaven National Laboratory)

Lawrence’s cyclotrons obeyed an future accelerators approximately cubic relationship be- would eventually be tween size or cost and the momen- linear. But the question tum of the accelerated particle. The remained, “Where is magnet’s radius grew linearly with the crossover in costs the momentum, and the magnet gap between circular and also had to increase accordingly to linear machines?” provide enough clearance for the New inventions, par- higher radio-frequency voltages re- ticularly strong focus- quired to keep particles and the volt- ing, raised the predict- age crest synchronized. All this ed crossover to much changed in 1945 with the invention higher energy. More- of phase stability by Edwin McMillan over, strong focusing also made the radius increases as the square of the and Vladimir Vexler. Their indepen- scaling law for high energy proton energy. dent work showed that only mod- synchrotrons almost linear. The Such a consideration therefore in- erate radio-frequency voltages are re- transverse dimensions of the beam dicates that linear electron machines quired in circular machines because aperture do not need to grow very should eventually become less ex- all the particles can be “locked” in much with energy; thus the cost of pensive than circular ones. But what synchronism with the accelerating large circular proton colliders grows is meant by the word “eventually?” fields. roughly linearly with energy. The answer depends on the details. Then came the 1952 invention of While the scaling laws for proton As designers of circular electron ma- strong focusing, again independently machines are not affected signifi- chines have been highly resource- by Nicholas Christophilos and by cantly by radiation losses (although ful in reducing the costs of compo- Ernest Courant, Livingston, and such losses are by no means negli- nents, the crossover energy between Hartland Snyder (see photograph on gible for the largest proton colliders), circular and linear colliders has been the right). Conventional wisdom says they become the dominant factor for increasing with time. But it appears that a magnetic lens to focus parti- circular electron machines. The likely that CERN’s Large Electron- cles both horizontally and vertically radiation energy loss per turn of a cir- Positron collider (LEP), with its 28 cannot be constructed— in contrast culating electron varies as the fourth kilometer circumference, will be the to optical lenses, which can. But the power of the energy divided by the largest circular electron-positron col- principle of strong focusing showed machine radius. It is also inversely lider ever built. that, while a magnetic lens indeed proportional to the mass of the cir- The only reasonable alternative is focuses in one plane and defocuses culating particle, which tells you to have two linear accelerators, one in the orthogonal plane, if two such why electrons radiate much more with an electron beam and the oth- lenses are separated along the beam profusely than protons. In an elec- er with a positron beam, aimed at one path, then their net effect is to focus tron storage ring, certain costs are another—thereby bringing these in both planes simultaneously. This roughly proportional to its radius beams into collision. This is the es- breakthrough made it possible to while others are proportional to the sential principle of a linear collider; squeeze beams in circular (and also radiation loss, which must be com- much research and development has linear!) accelerators to much tighter pensated by building large and ex- been dedicated to making such a ma- dimensions, thus reducing magnet- pensive radio-frequency amplifiers. chine a reality. SLAC pioneered this ic field volumes and therefore costs. As the energy grows, it therefore be- technology by cheating somewhat on Because the basic linear scaling comes necessary to increase the the linear collider principle. Its lin- laws apply to linear machines for radius. The total cost of the radio- ear collider SLC accelerates both elec- both electrons and protons, promi- frequency systems and the ring itself tron and positron beams in the same nent physicists predicted that all will be roughly minimized if the two-mile accelerator; it brings these

BEAM LINE 43 beams into collision by swinging one pinches the other, usually in- positrons or for protons, become?” them through two arcs of magnets creasing its density; but if that pinch- As indicated, the costs of electron- and then using other magnets to ing action becomes too severe, the positron linear colliders may be lin- focus the beams just before collision. beam blows up! In addition, the ear for awhile, but then costs increase In the SLC (and any future linear col- extremely high electric and magnetic more sharply because of new physi- lider), there is a continuing struggle fields that arise in the process cause cal phenomena. The situation is sim- to attain sufficient luminosity. This the particles to radiate; the energy ilar for proton colliders. The cost problem is more severe for a linear thereby lost diversifies the energy of estimates for the largest proton col- collider than a circular storage ring, the different particles in the bunch, lider now under construction— in which a single bunch of particles which makes it less suitable for CERN’s Large Hadron Collider—and is reused over and over again thou- experiments. for the late lamented SSC are rough- sands of times per second. In a linear And there is an additional feature ly proportional to energy. But this collider the particles are thrown that aggravates the problem. As the will not remain so if one tries to build away in a suitable beam dump after energy of colliders increases, the machines much larger than the SSC, each encounter. Thus it is necessary cross sections of the interesting re- such as the speculative Eloisatron, to generate and focus bunches of ex- actions decrease as the square of the which has been discussed by certain ceedingly high density. energy. Therefore the luminosity— European visionaries. At the energy An extremely tight focus of the and therefore the density of the in- under consideration there, 100 TeV beam is required at the point of col- teracting bunches—must increase per beam, synchrotron radiation be- lision. There are two fundamental sharply with energy. Thus all the comes important even for protons limits to the feasible tightness. The problems cited above will become and looms as an important cost com- first has to do with the brightness even more severe. ponent. Indeed, physical limits will of the sources that generate electrons As a result of all these factors, a cause the costs eventually to rise and positrons, and the second is linear collider is not really linear in more steeply with energy than lin- related to the disruption caused by all respects; in particular, the bright- early for all kinds of machines now one bunch on the other as they pass ness of the beam must increase as a under study. through each other. According to a high power of its energy. This fact But before that happens the ques- fundamental physics principle that is difficult to express as a simple cost- tion arises: “To what extent is soci- is of great importance for the design scaling law. It suffices to say that ety willing to support tools for par- of optical systems, the brightness (by all these effects eventually lead to ticle physics even if the growth of which I mean the intensity that il- a very serious limit on electron- costs with energy is ‘only’ linear?” luminates a given area and is prop- positron linear colliders. Where this The demise of the SSC has not been agated into a given angular aperture) limit actually lies remains in dispute. a good omen in this regard. Hopefully cannot be increased whatever you do At this time an upper bound of sev- we can do better in the future. with a light beam—or, for that mat- eral TeV per beam is a reasonable ter, a particle beam. Thus even the estimate. We can hope that human fanciest optical or magnetic system ingenuity will come to the rescue cannot concentrate the final beam again—as it has many times before spot beyond certain fundamental when older technologies appeared to limits set by the brightness of the approach their limits. original source and the ability of the accelerating system to maintain it. HIS DISCUSSION of linear The second limit is more complex. electron-positron colliders is The interaction between one beam Ta part of a larger question: and another produces several effects. “How big can accelerators and col- If beams of opposite charge collide, liders, be they for electrons and

44 SPRING 1997