Storage Rings And Colliders

Type of physical science: Elementary particle (high-energy) physics

Field of study: Techniques

Storage rings and colliders are similar devices that high-energy physicists use to increase the range of energies that can be studied. Because experiments are conducted in the center-of-mass frame of reference, all the energy is available to produce new phenomena.

Overview

High-energy physicists exploit Albert Einstein's relationship between matter and energy by using very energetic collisions to produce new forms of matter. The more energy available for a reaction, the more likely something new and unique can be produced. Since much of the matter studied by particle physicists is unstable and decays in a few billionths of a second or less, scientists are required to produce this matter in order to study its behavior. Physicists have employed particle accelerators, such as the synchrotron, to produce particle beams rivaling the most energetic particles found in nature. As scientists have learned more and more, there has been a push to higher energies. To achieve more energetic collisions, large accelerators have been built. Colliders provide a means to achieve very high-energy collisions at a modest cost.

The fundamental idea behind a collider is to perform the experiment in the center-of-mass frame of reference. The maximum amount of energy available for a collision is the energy in the center of mass. In any other frame of reference, some of the energy of the colliding particles is needed to keep the reaction products moving. This reserved energy is not available to the reaction.

This concept of the relativity of the amount of energy is familiar. For example, if one is standing on the side of a road, then the cars that move past have a considerable amount of energy. A person riding in a car, however, is not aware of this energy. Consider a collision between a very massive object, such as a bowling ball, and a light object, such as a tenpin. In the collision, the direction of the bowling ball is not deflected very much; it retains most of its kinetic energy, and momentum and very little of this energy is transferred to the tenpin. A collision between a light object and a very heavy one has similar results. For example, when one bounces a ball off of the ground (off of the earth), the ball reverses its direction and rises almost to its initial height. Again, very little of its energy is transferred to the earth (though twice its momentum is transferred). Collisions between equal-mass objects maximize the amount of energy transferred between them. This concept of equal-mass collisions must be generalized because, for relativistic reactions, the mass is no longer constant. The maximum energy is available for the reaction if the sum of the momenta of all the particles in the reaction is zero. In a reaction of two particles, this criterion is met when the collision is head-on and the particles have equal but opposite momenta. If the two particles have the same mass, then they will also have the same energy if their momenta are matched.

Colliders are instruments to produce head-on collisions between elementary particles.

If the momenta of the interacting particles are the same, then the maximum energy will be available to create new particles. The traditional way of carrying out high-energy physics experiments has been with "fixed targets." In fixed-target experiments, a very energetic beam of particles, from an accelerator, is directed at a target. The target is a piece of material at rest in the laboratory. Collisions then occur between the high-energy beam and the at-rest target particles.

Relativity dictates that, for very high energies, the center-of-mass energy available for the reaction only increases as the square root of the beam energy. Therefore, if the accelerator beam energy were doubled, then the center-of-mass energy would only increase by a factor of 1.41. In a collider, if the accelerator beam energy were doubled, then the center-of-mass energy would increase by a factor of four, that is, a factor of two for each beam.

Colliders provide a very high center-of-mass energy per collision, but for careful studies, physicists need many collisions. In order to achieve many collisions between the two energetic particle beams, the beams are stored. A storage ring is a device very much like a particle accelerator, except that the beams are allowed to circulate for many hours or days. The circulating beams are then periodically, perhaps several times per cycle, brought into collision with each other. A measure of the collision rate is a quantity called luminosity. A machine's luminosity indicates how many collisions can be recorded for a given process. The luminosity is proportional to the number of particles in either beam. Clearly, if the number of particles circulating in one beam is doubled, then the chance that the beam will interact is doubled as well.

The luminosity is also inversely proportional to the cross-sectional area of the beams.

If the particles can be squeezed down to a very small area, then there is a higher probability that they will interact with a particle in the other beam. Particles in storage rings are often stored in bunches. The luminosity also depends on the frequency with which the bunches cross. In a very large ring, the bunches may cross infrequently. Many bunches can also be stored, one behind the other in the ring, in order to increase the number of collisions.

In most colliders, two beams are brought to a head-on collision. The storage ring must be capable of storing beams of both particles for long periods of time, which means that the beams must be composed of such stable particles as protons and electrons, and their antiparticles (antiprotons and positrons). A substantial engineering benefit is realized if the collisions are between a particle and its antiparticle. The ring stores particles magnetically, so that if the momentum and charge of a stored particle are reversed, then the particle will follow the same path but in a reverse direction. If the two beams are particle and antiparticle, then both can be stored in the same ring. This is not always the optimal design choice, however, since one of the beams is composed of antimatter, which must be produced in high-energy collisions because it is not found in nature. This requirement limits the ultimate intensity of the antiparticle beam.

Applications

Storage rings and colliders have been successfully employed for the discovery and study of many new particles. Electron-positron colliders were the first to be widely exploited.

The positron, which is the antiparticle to the electron, is relatively easy to produce. Since it is positively charged, it will circulate opposite to the electron in a storage ring and only one ring is needed. Electron-positron colliders produce reactions through the electromagnetic interaction.

The fact that this interaction is well understood makes it much easier to design and carry out an experiment and to interpret the results.

Electron-positron colliders have been used to discover particles containing the quantum numbers of charm and beauty quarks. These are new unstable forms of matter that can be studied only as they are produced and allowed to decay in the laboratory. A very convenient feature of electron-positron colliders is that they are very effective at producing particles that have the same quantum numbers as the photon. Recall that the photon is produced in the collision of an electron and an antielectron (a positron), so that neutral quark-antiquark states should have comparable quantum numbers and be easily produced. In general, quark-antiquark bound states are meson resonances. Many new particles can be found in the decay of these easily produced resonances.

For example, the excited states of a charm-anticharm quark bound state (known as the ψ) decay copiously to charmed particles. Similarly, an excited bound state of beauty and antibeauty (known as the υ) is used as a "B factory," a convenient and copious source of beauty particles. Electron-positron storage rings have been most effective when the operating energy has been chosen to take advantage of these channels for the production of new particles.

Electron-positron colliders were also used in the discovery of the τ lepton. The tau lepton is a particle which is very similar to the muon and the electron. It differs from these particles only in that it has a much higher mass, about thirty-six hundred times that of the electron. It also has a new quantum number which restricts how it may interact or decay. It can be most easily produced in the reaction in which an electron and positron annihilate to make a τ and an antiτ lepton. In that way, no net change in the τ quantum number is produced because the τ and antiτ have opposite values.

Electron-positron storage rings have a number of limitations. As the energy needed to effect the collisions rises, the cost of building and running the collider rises very rapidly. Much of the added expense is the cost of storing such energetic beams. As the electrons and positrons circulate in the storage ring, they undergo acceleration to keep them moving in a circle.

Accelerated charged particles radiate energy. In a storage ring, this energy is known as "synchrotron radiation." The amount of energy radiated depends on the accelerated particle's mass and energy. This synchrotron radiation is a unique, intense source of ultraviolet light and X rays. A number of electron storage rings have been built to be used as a source of this radiation, which is a major tool in the study of materials. Synchrotron radiation is the major source of energy loss in electron-positron storage rings.

To avoid the limitations of synchrotron radiation, the concept of the linear collider has been proposed. A linear collider (also called a single pass collider) would have the center-of-mass energy advantage of a storage ring but would not store the beams. The beams are accelerated toward each other. They collide and then are discarded. In order for such a method to produce the high luminosity needed to obtain enough collisions, the beams themselves must be very intense because they will only cross once and are not circulated or stored for reuse.

Proton-antiproton colliders are used to achieve energies five to five hundred times those that are accessible to electron-positron colliders. The proton is eighteen hundred times heavier than the electron, so synchrotron radiation, which depends on the particle mass to the inverse fourth power, is no longer a serious problem. On the other hand, since they are so heavy, antiprotons are very hard to produce and store. Antiprotons are generated by nuclear collisions, but they make up only a small part of what is produced. Because they are produced in energetic collisions, they come out with a very wide range of momenta. In order to be stored, all antiprotons must have a very similar momentum. A technology called "cooling" has been developed to collect and store a suitable number of antiprotons to make collisions reasonably intense. Because of the difficulty of collecting and storing antiprotons, a number of colliders have been built with two separate storage rings, so that two beams of protons may be collided.

Since the two proton beams have the same charge but are moving in opposite directions, they cannot be contained in the same magnetic storage ring.

Proton-antiproton colliders have been used to discover the very massive particles responsible for the weak nuclear force. These particles weigh from eighty to ninety times the mass of the proton. Since the proton-antiproton interaction is dominantly the strong nuclear force, the collisions produce many particles, and sophisticated detectors are needed to sift through the products to find events of interest.

The weak nuclear force is the dominant interaction for electrons and positrons when they have a center-of-mass energy near the mass of the neutral weak boson, known simply as the Z. This feature has been employed to produce and study this neutral weak boson. In this case the benefits outweighed the problems associated with electron-positron colliders at high energy.

In spite of the gains in center-of-mass energy to be had from equal energy in each beam, it is sometimes useful to have beams of different energy. Such machines are called asymmetric colliders. Asymmetric colliders are used when it is important to push the particles resulting from the interaction in a specific direction in order to observe them better. For example, if one is studying the lifetime of unstable particles, then this push will time-dilate (slow) the lifetime so that it may be more easily observed. Colliders have also been built with dissimilar particles, such as an electron-proton collider. These provide the advantages of a collider but the ability to study the proton structure with an electromagnetic probe.

Context

The fundamental concepts involved in colliders are fairly basic in physics, but realizing the concept required much technology. The technology of particle accelerators themselves had to be developed to a high degree, as storage rings are basically a kind of accelerator. Any residual gas present in the ring would interact with the stored beam and eventually destroy it. Therefore, storage rings require a very high vacuum in order to operate reliably. The storage time, or beam lifetime, for conventional accelerators is on the order of seconds, while storage rings must maintain a stable beam for ten thousand times longer or more. The requirement of stability also makes severe demands on the focusing system of the accelerator. In order to keep a collection of particles together in a beam, all the particles must travel on nearly the same path for days. This places high stability requirements on the equipment and necessitates high sensitivity to engineering tolerances in the construction. The demand for high luminosity means that very high beam currents must be stored. Interactions between the colliding beams and within the bunches themselves lead to destabilizing effects for which the accelerator focusing system must compensate.

Storage rings became major tools in the 1970's and 1980's because physicists had a clear picture of the phenomena that they wanted to study and could identify the center-of-mass energy that would be most effective. These energies were beyond what could be conveniently reached with conventional, fixed-target devices, so there was a clear need for innovation.

Principal terms

ANTIMATTER: a form of matter which has the same mass as normal matter (electrons and protons) but reversed charges

CENTER-OF-MASS FRAME: the (usually moving) coordinate system in which the interaction is at rest

COLLIDER: a device which brings energetic particles into close vicinity so that they may interact and their interactions may be observed; generally refers to devices in which both particles are in motion

DETECTORS: devices for observing the elementary particles produced in reactions

LUMINOSITY: a measure of the intensity of beam-beam interaction in a colliding-beam machine

QUARKS: the fundamental components of strongly interacting particles; a baryon is made up of three quarks, and a meson is made up of a quark and an antiquark

STORAGE RING: a device which employs magnetic fields to trap charged particles for extended periods, the particles generally moving in a closed path or a ring

Bibliography

Carrigan, Richard A., Jr., and W. Peter Trower, eds. PARTICLES AND FORCES AT THE HEART OF MATTER. New York: W. H. Freeman, 1990. This is a collection of reprints from SCIENTIFIC AMERICAN on particle physics. Several articles extend the discussion of collider techniques and many present the motivation for and the results of collider experiments.

Cline, David B., Carlo Rubbia, and Simon van der Meer. "The Search for Intermediate Vector Bosons." SCIENTIFIC AMERICAN 246 (March, 1982): 48-59. This article tells the story of the first proton-antiproton collider and its use to discover the fundamental particles that mediate the weak interaction.

Jackson, J. David, Maury Tignier, and Stanley Wojcicki. "The Superconducting Supercollider." SCIENTIFIC AMERICAN 254 (March, 1986): 66-77. Describes the motivation and design of the Superconducting Supercollider, a very ambitious storage ring project. This device is designed to collide two proton beams at an energy of 20 trillion electronvolts each.

Mistry, Nariman B., Ronald A. Poling, and Edward H. Thorndike. "Particles with Naked Beauty." SCIENTIFIC AMERICAN 249 (July, 1983): 106-115. This article discusses the use of electron-positron collisions to produce particles with the beauty property. Since these particles are unstable, they can only be studied as they are produced. Electron-positron collision is the best method known for the production and study of beauty quarks.

Perl, Martin L., and William T. Kirk. "Heavy Leptons." SCIENTIFIC AMERICAN 238 (March, 1978): 50-57. Discusses the discovery of the τ lepton with electron-positron collisions. The τ lepton is a member of the third family of elementary particles.

Wilson, Robert R. "The Next Generation of Particle Accelerators." SCIENTIFIC AMERICAN 242 (January, 1980): 42-57. This article describes the evolution of the particle accelerators that are used for high-energy physics research. Explains the difference between fixed-target experiments and collider experiments.

Detectors on High-Energy Accelerators

Forces on Charges and Currents