ACCELERATORS, COLLIDING BEAMS: ELECTRON-PROTON

The electron and the proton, along with the neutron (a close relative of the proton), are the particles that make up all of the observed universe. The study of their structure and properties is therefore fundamental.

One of the consequences of relativity is that a truly elementary particle, that is, one with no internal structure, must be of zero size. The electron satisfies this criterion as far as we can tell. Measurements at the highest energies probe the smallest distances and show that the electron is certainly smaller than 10^-18 m across. The proton, by contrast, is 10^-15 m across and has internal structure. It consists of three quarks, which give it its properties, plus a fluctuating sea of quark-antiquark pairs, strongly bound together by gluons, which are the carriers of the strong force, which binds protons and neutrons together into the nuclei of atoms. The very strength of the strong force makes it hard to study: it is hard to isolate one feature of it at a time. Any disturbance one makes in a strongly interacting system has massive side effects from the collision that can confuse the features under investigation.

The Need for High Energies

One way to open up the strong force to study is to use the highest possible energies. This allows us to single out one interaction at a time while the effect of the underlying, less energetic interactions can be ignored. The key to high energies is to use colliding beams. Early experiments to create high-energy collisions in the laboratory used a single beam of high-energy particles hitting a stationary target. This has the disadvantage that a large fraction of the energy of the beam is wasted in uselessly imparting momentum to the target particle that is struck. The law of conservation of momentum makes this loss unavoidable and severe. The need to add momentum to the target soaks up precious beam energy.

When the energies concerned become large compared to the masses of the particles involved (using E = mc² as a comparator), the useful collision energy grows only very slowly as the beam energy is increased. Thus, if a proton beam is used on a fixed proton target, a beam kinetic energy six times the proton mass produces a useful collision energy twice the proton mass. Worse, an energy 800 times the proton mass produces a useful energy less than 40 times the proton mass. The process of putting energy into reactions therefore becomes very inefficient.

Benefits of Colliding Beams

Colliding beams are much more efficient. Here two beams of particles collide head on. In the case where the two beams consist of identical particles and have equal energies, they will have equal and opposite momenta. The two momenta then cancel out, no energy is needed to set anything in motion, and 100 percent of the energy is available for reaction. One must be careful when asserting this when one of the beams is made of composite particles, such as protons. The constituent quarks, antiquarks, and gluons each take only a modest fraction of the momentum of the moving proton. If one thinks of an electron-proton collider as an electron-quark collider (with the remainder of the proton constituents acting as spectators), then it is the balance between the electron and quark momenta that matters.

Colliding Beam Options

Practical colliding beams in use in 2002 use recirculating beams of electrically charged particles. One can compare electron-positron colliders (the positron is the antiparticle of the electron) with proton-antiproton and electron-proton (or positron-proton) colliders. Properties of selected colliders are shown in Table 1. The collision energy listed in the table is the total energy available for interactions after allowing for the effects of momentum conservation. The typical energy is the energy available in a quark collision. (Recall the typical quark carries only a small fraction of the proton energy). For some purposes, such as searching for new particles, the collision energy may be more important than the typical energy.

Electron-positron colliders create new matter out of the energy of the collisions they create and address many questions, but since there is no proton present, they do not directly touch on the question of proton structure. Proton-antiproton collisions (or proton-proton collisions planned for the European Laboratory for Particle Physics [CERN] Large Hadron Collider) reach the highest attainable energies but involve colliding two internally complex objects, and so as far as the structure of the proton is concerned, are harder to interpret. By contrast, electron-proton colliders use the pointlike electron as a scalpel to dissect the proton. Nature holds the quarks strongly inside the proton. One can never see a free quark but only a "jet" of pions and other particles produced from electron-proton collisions.

Figure 1 shows an example of an electron-proton interaction. The single electron track is turned almost around by the vigor of the collision, and a jet of particles is produced by the quark that is struck.

Features of a Colliding Beam Facility

A colliding beam accelerator operates in the following way. One starts with a source of electrons (a hot wire in a vacuum under high voltage, for example) and protons (simply the nuclei of the atoms in a cylinder of hydrogen gas). (If positrons are required, the procedure is more complex: a beam of electrons with an energy of several million volts hits a target, producing many matter-antimatter electron-positron pairs. Positrons are then accumulated in a small storage ring

TABLE 1

FIGURE 1

until enough have been made.) Then the particles are accelerated to high energy using a synchrotron.

The basis of acceleration is the addition of energy to an electrically charged particle by passing it through a voltage difference. The trick is that this voltage difference is oscillating inside a metal cavity at very high frequency, switching sign a billion times per second. If a localized bunch of particles arrives at just the right moment, their energies are increased. The particles are then recycled in a circular path within a narrow vacuum pipe (using a ring of magnets to steer and focus them) and passed repeatedly through the same cavities, gaining energy at each revolution. The magnetic field must be increased in step with the energy in order to keep the particles on the same circular path. (Synchronization of the magnetic field to the beam energy at each step in the acceleration process gives the synchrotron its name.) In practice, a set of accelerator rings is used, each ring stepping up the energy a certain amount until the large storage ring is filled with a train of bunches of electrons and protons (some 200 bunches in the case of HERA, the storage ring at the Deutsches Elektronen-Synchotron Laboratory [DESY] in Hamburg, Germany). The process of filling and accelerating the two separate electron and proton beams to the required energies normally takes up to an hour. The magnet rings are then kept filled at the maximum energy while the beams make repeated orbits, with the possibility of reactions occurring at each collision of an electron bunch with a proton bunch.

The need to recycle the particles during acceleration and storage governs the size of the machine. Using superconducting magnet technology, the HERA magnets (built in 1992) achieve a maximum magnetic field of over 5 Tesla around the circular arcs of the accelerator (more than twice that attainable using copper conductors). For a given proton beam energy, this dictates the radius of the arcs and hence the circumference of the machine. To achieve a smaller arc radius, a higher magnetic field would be required. The Large Hadron Collider magnets will run at 8.3 Tesla in 2007.

The HERA Collider

Figure 2 gives a schematic of the HERA facility showing the succession of booster rings. The electron (or positron) and proton beams circulate in opposite directions in separate magnet rings of 6.4 km

FIGURE 2

circumference. Each ring has circular arcs of magnets (for the acceleration and recycling of the beams) and straight sections containing the electrical cavities and focusing elements. The electron and proton beams are brought into collision at two points north and south of the ring where the collisions can be studied.

Colliding beams offer a big advantage over fixed targets in terms of energy but are much more problematic in terms of intensity. Protons and electrons are accumulated individually: the total mass of protons circulating at any time is only 10^-11 g, so precious few are available for the electrons to hit. The key lies in focusing, to increase the local density at the collision point, and recycling. At the interaction points the electron and proton beams are focused down to a spot only 0.05 mm by 0.2 mm across. Each individual particle, traveling close to the speed of light, makes some 47,000 circuits per second. In a typical 8 hour running period, each proton has 2,700,000,000 opportunities to interact collide and interact. Scientists select about five interactions per second where interesting processes have occurred for detailed study.

See also:Accelerator; Beam Transport; Cooling, Particle; Extraction Systems; Injector System

Bibliography

Fraser, G. The Particle Century (IoP Publishing, Bristol, UK, 1998).

Wille, K. The Physics of Particle Detectors, an Introduction (Oxford University Press, Oxford, UK, 2000).

Wolf, G. Proceedings International School of Subnuclear Physics, Erice, Vol. 37, edited by A. Zichichi (World Scientific, Singapore, 1999).

David H. Saxon