Proc. Nat. Acad. Sci. USA Vol. 70. No. 2, pp. 611-618, February 1973 Colliding Beams Versus Beams on Stationary Targets: Competing Tools for Elementary Particle Physics W. K. H. PANOFSKY Stanford University Linear Accelerator Center, Stanford, California 94305 High energy accelerators have been the primary tools in ad- Fig. 1 shows the state of the accelerators in the world, in an vancing elementary particle physics ever since J. J. Thompson admittedly oversimplified manner, by a plot of their energy discovered the electron, using what in modern terms might and intensity; the power of accelerators cannot be measured well be called the first accelerator. Although many of the by one, or even two, parameters. Particle energy remains, qualitative findings, including the discovery of new unstable of course, the foremost quantity of interest, but intensity, particles, originated from cosmic ray studies, quantitative beam quality and geometry, and many other factors determine measurements have required accelerators as particle sources. the usefulness of these instruments to experimental physicists. In fact, as the energies of accelerators have pushed to The National Accelerator Laboratory currently holds the higher and higher values, the relative importance of cosmic world's record in energy (400 GeV) and aspires to further rays as a tool for the study of any aspect of elementary particle increasing values; the collaborative international laboratory physics has sharply diminished, although study of cosmic at Geneva (CERN) has an accelerator under construction ray physics remains an important activity in its own right aiming at similar energies. These two machines are proton as a diagnostic tool of the cosmos. machines; the energy record for electrons is held by SLAC (22 TABLE 1 Energy Machine (GeV) Fig. 1 Name Institution Location Remarks 1 Frascati Laboratori Nazionali del CNEN Rome, Italy Shut down 1 Lund University of Lund Lund, Sweden 1 Tokyo Tokyo University Tokyo, Japan Shut down 1.5 CalTech Cal Tech Pasadena, Calif. Shut down 2 Cornell Cornell University Ithaca, N.Y. Shut down Electron 2.5 Bonn University of Bonn Bonn, W. Germ. synchrotron 4 NINA Daresbury Nuclear Physics Lab. Daresbury, U.K. 6 Yerevan Yerevan Physical Institute Yerevan, USSR 6 CEA Cambridge Electron Accelerator Boston, Mass. 7 DESY Deutsches Elektronen-Synchrotron Hamburg, W. Germ. 10 Cornell Cornell University Ithaca, N.Y. 1 Stanford High-Energy Physics Lab., Stanford Stanford, Calif. Shut down 2 Kharkov Physico-Technical Institute Kharkov, USSR Electron linac 2 Orsay University of Paris Orsay, France 22 SLAC SLAC, Stanford University Stanford, Calif. 50 SLAC-RLA SLAC, Stanford University Stanford, Calif. Under design 3 Cosmotron Brookhaven National Laboratory Upton, L.I., N.Y. Shut down 3 Saturne Centre D'Etudes Nuclieres Saclay, France 3 PPA Princeton-Pennsylvania Accel. Princeton, N.J. Shut down 6 Bevatron LBL, U. of Cal., Berkeley Berkeley, Calif. 7 Moscow Instit. of Exper. & Theor. Phys. Moscow, USSR 7 Nimrod Rutherford Lab. Harwell, U.K. Proton 8 Tsukuba National Lab for High-Energy Phys. Tsukuba, Japan Ready in 1974 synchrotron 10 Dubna Joint Instit. for Nuclear Research Dubna, USSR 12 ANL-ZGS Argonne National Lab. Chicago, Ill. 28 CERN-PS European Organ. for Nuclear Res. Geneva, Switz. 30 BNL-AGS Brookhaven National Laboratory Upton, L.I., N.Y. 76 Serpukhov Instit. for High Energy Phys. Serpukhov, USSR 200 NAL National Accelerator Lab. Batavia, Ill. Ready in 1972 300 CERN II European Organ. for Nuclear Res. Geneva, Switz. Ready in 1976 611 Downloaded by guest on September 27, 2021 612 Panofsky Proc. Nat. Acad. Sci. USA 70 (1973) HIGH ENERGY ACCE L ERATORS 1000 o USA o EUROPE OPEN ELECTRONS ' USSR BLACK PROTONS V JAPAN X SHUTDOWN (CERN 31) (NAL) ( ) UNDER CONSTRUCTION 1976 1972 (( )) PROPOSED , 100 S CD ft ((SLAC-RLA)) (.9 lANL-0.BNL-G 1975 w z w CERN-PSLA ANL-ZGS w DUBNA CD10 -0 CORNELL 0 (TSUKUBA) MoscowMOSCOW U NIMROD V 1974 0 DESY BEVATRON * CEA YEREVAN 0 NINA COSMOTRON PPA SATURNE 0 BONN CALTECH X CORNELL D0 ORSAY FRASCATI M KHARKOV LUND 'W TOKYO w X.I, , ,,1TOy, ,, ,,,,1 STANFORDg, , ,,,,,1 0.001I0I 0.01 0.1 10 100 ELECTRON OR PROTON INTENSITY (microamperes) FIG. 1. Energy versus intensity of high-energy accelerators. The facilities shown are described more fully in Table 1. GeV), which aspires to expand the energy to about 50 GeV by a recirculation scheme. Elementary particle physics has been exceedingly produc- tive throughout this century, and it is fair to say that the time interval between new discoveries in this field that have 100 GeV- changed man's basic view of nature has not given any indica- tion of stretching out. Yet, the magnitude of the tools and the concomitant cost of operating them have grown steadily, and the question is being raised with increasing frequency 10 GeV 4 as to when and how this evolution might stop. As shown in Fig. 2, the trend in the increase of accelerator energy has not stopped. New inventions have sustained an almost expo- nential increase in energy of one decade every 6 years, even 1 GeV__ though the scaling laws pertaining to each type of accelerator have in the past forced a leveling off of the energy attainable by any one method. This increase in energy has been bought at a serious social cost; because of the high price of each ac- celerator, the total number of installations worldwide that operate at the frontiers of the field has been steadily decreas- 100MeV PI ing. Therefore, elementary particle physicists at academic and other institutions have had to perform their experimental observations away from home. Yet, the consensus remains that involvement of academic physicists within the field should be maintained or even strengthened despite this diffi- culty: elementary particle physics, since it is among the most 1 MeV (5 basic of the sciences, remains an essential ingredient of the educational program in physics at major universities. A possible departure from the pattern outlined above has been introduced by the emergence of colliding-beam tech- 1930 1940 1950 1960 1970 1980 niques. It has been recognized for a very long time that the FIG. 2. The energy growth of accelerators from 1930 to the threshold for a reaction among particles to occur is set by present. the "center-of-mass energy," that is, by the energy measured Downloaded by guest on September 27, 2021 Proc. Nat. Acad. Sci. USA 70 (1978) Colliding Versus Single Beams 613 in that frame in which the center-of-mass of the colliding 10,000 system is at rest. At highly relativistic particle energies, the center-of-mass energy increases only with the square root of the energy (as measured in the laboratory) of the particle that bombards a stationary target; the rest of the energy 1000 is converted into the kinetic energy of motion of the center- of-mass of the combined system. The relationship between the center-of-mass energy and the laboratory energy of par- ticle beams striking stationary targets is shown in Fig. 3. z 100 Clearly this decreasing efficiency in terms of center-of-mass (I) C,) energy could be circumvented by two beams colliding with one another from opposite directions. This idea is an old one; it is, in fact, difficult to document how it originated. However, 0 the question here is not that of the idea but of its execution. 10 z The problem is that the density of particle beams is vastly C-) inferior to that or ordinary condensed matter and is, in fact, comparable to that of practically attainable vacua; thus, the reaction rates in colliding-beam experiments are apt to be very much lower than those encountered when particle beams strike stationary targets. Quantitatively, this factor is measured by a quantity that colliding-beam physicists call the "luminosity." This is the number by which one multi- 0.1 10 100 1000 10,000 plies the cross section of the reaction under investigation in ENERGY OF (EACH) BEAM-GeV order to obtain the reaction rate. Therefore luminosity is generally measured in units of cm-2 sec1. During the last decade there have been many develop- FIG. 3. Center-of-mass energy versus beam energy for accel- ments that have demonstrated that the luminosities of col- erators and storage rings. liding-beam devices can reach a range practical for important elementary particle physics experiments. The first such de- energies that were previously unattainable. The most im- monstration was made in the Stanford-Princeton collabora- portant and also most ambitious single step in colliding-beam tion, in which two electron beams, each with energy up to technology was taken in Europe at CERN in their ISR (In- 550 MeV, were made to collide in the common section be- tersecting Storage Ring) project, which became operational tween two magnetic storage rings arranged in a figure-eight in 1970. In this installation, particles are injected into two pattern. This installation resulted in a pioneering demon- rings at an energy of 26 GeV from CERN's proton synchro- stration that quantum electrodynamics remained valid up to tron; the energy in each ring can be slightly raised above that TABLE 2 Energy of Particles each beam stored (GeV) Fig. 4 Name Institution Location Remarks Electrons + 0.14 VEPP-1 Institute of Nuclear Physics Novosibirsk, USSR Shut down electrons 0.55 Stanford- High-Energy Phys. Lab., Stanford Stanford, Calif. Shut down Princeton 0.55 ACO-Orsay University of Paris Orsay, France 0.75 VEPP-2 Institute of Nuclear Physics Novosibirsk, USSR 0.75 VEPP-2' Institute of Nuclear Physics Novosibirsk, USSR Ready in 1973 1.5 Adone-Frascati Laboratori Nazionali del CNEN Rome, Italy 1.5 DCI-Orsay University of Paris Orsay, France Ready in 1975 Electrons + 2.5 SPEAR SLAC, Stanford Univ.