Detectors and Subsystems

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DETECTORS AND SUBSYSTEMS

The study of elementary particles and the forces between them is made possible by modern high-energy accelerators that bring beams of electrons or protons—or their antiparticles, positrons, and anti-protons—to nearly the speed of light and energies of hundreds or thousands of (GeV). (A GeV is the energy that an electron or proton would reach if accelerated across the terminals of a billion-volt battery).

Colliding beam accelerators focus two beams traveling in opposite directions onto each other; compared with the older technique of directing a single beam on a target of stationary nuclei, the energy available for creating new particles is much enhanced. The energy of modern colliders is larger than the energy associated with a particle's significant mass, E = mc², where c is the speed of light. The proton and antiproton have a mass energy of about 1 GeV; the electron and positron have approximately ½ MeV, where 1 GeV = 1,000 MeV. Thus, when a collision takes place between two particles of the opposite beams, sufficient energy is brought to the system that hundreds of particles may be created in the event. The particles created fly outward from the collision point, with the imprint of what occurred in the original collision encoded in their energies and momenta. The momentum of a particle is its mass times its velocity; it carries directional information. The magnitude of the momentum for the highly relativistic particles typical of accelerator collisions is related to the energy by E = cp .

In contrast to colliding beam accelerators, collisions of particles in a single beam with nuclei in a fixed target occur at lower effective energy. However, added flexibility exists in that a wider range of beam particles can be used—not only the stable electron or proton but also unstable particles such as pions, kaons, or hyperons and even the very weakly interacting neutrinos. Thus for some specific studies, fixed-target experiments are preferred.

It is the job of the particle detector to record the momenta and energies of particles produced in a collision and, to the extent possible, determine the particle's identity. For example, when high-energy electrons or muons emerge from collisions of protons and antiprotons, it typically signals that something rare and interesting has occurred, and it is important to flag their presence to help distinguish the interesting events from the large background of less interesting collisions. A collider detector surrounds the accelerator pipes, centered on the collision point. A fixed-target detector is arranged downstream of the interaction target, as the produced particles are almost all thrown forward. Either type of detector consists of a set of subdetectors, each providing specific information; these sub-detector types are the same for both collider and fixed-target use.

Tracking Detectors

The tracking subdetector system located closest to the collision point determines the momentum and direction of all the particles carrying electric charge. A strong magnetic field is imposed on the tracking region, usually by a solenoid magnet surrounding this detector in a collider experiment or by a dipole magnet in a fixed-target experiment. The charged particles bend in this field as a result of the Lorentz magnetic force. The bending radius is inversely proportional to the momentum. With care in design, a resolution of better than 0.1 percent can be achieved at low momentum, but for high-momentum particles the bending decreases and ultimately the resolution becomes very poor.

The tracking detectors sense the ionization trails left by the particle's disruption of atoms in their material. Several types of tracking detector may be used, often in concert. In the widely used drift chamber, wires with positive high voltage are arranged at regular intervals within a low-density gas environment. Electrons drift with constant velocity to these wire electrodes where they create detectable signals through an avalanche in the large electric field close to the wires. The time taken for the electrons to reach the wire measures the coordinate along the drift direction with precision, typically a few hundred microns. Alternate wires may be stretched at small angles to each other, allowing the measurement of both coordinates perpendicular to the particle's direction. An alternate detector type employs thin crystalline silicon layers with conducting narrow (typically 50 μm wide) strips or pixels prepared on the surfaces. A voltage between the two surfaces causes ionization electrons or holes to drift to the strip electrodes, allowing spatial measurements with precision better than strip widths. Still other choices for track-sensitive detectors exist, and for special purposes these may be considered. Organic plastics called scintillators are available that deliver visible light from the deexcitation of the molecules of the plastic after disruption by ionizing particles. This light is delivered to photo-optic devices such as photomultipliers or avalanche photodiodes through optical waveguides. The fast time response of scintillators makes them attractive possibilities. Scintillating fibers with diameters less than a millimeter have recently been employed, giving good spatial resolution for tracking detectors.

Typically, the silicon strip and outer tracking detectors are used together, with the higher-resolution but more expensive silicon detectors arrayed nearest to the collision point, and the drift chamber or scintillating fibers starting approximately 20 cm from the beams. In this way, the presence of particles that live for a short time and travel only several hundred microns before their decay can be sensed in the high-resolution silicon detector, whereas the larger depth of the drift chamber can be used for a more accurate measurement of the bending radius and hence, particle momentum. The knowledge that there are short-lived particles present—such as those containing bottom or charm quarks, or the tau lepton—is often crucial for an experiment, as these particles signal the presence of interesting events that have produced new heavy states of matter. A general requirement for tracking detectors is that they do not significantly degrade the energy of the particles traversing them, and do not create new particles through interactions with the material. It is important that the directions of the charged particles not be seriously affected by the material, so the measurement of momentum from the bending in the magnetic field is not compromised.

Particle Identification

In some cases, the ability to identify specific particle types is of enough importance to the experiment that some of the tracking detector volume may be devoted to special particle identification detectors. These employ methods to differentiate particles of different mass but the same momentum. Cerenkov detectors record the light emitted when a particle exceeds the speed of light within a material medium and have a response that depends on particle velocity. Together with the momentum measurement in the standard tracking detectors, they allow determination of the particle mass. A transition radiation detector, containing many closely spaced layers of a dielectric material, detects the X rays emitted when a highly relativistic particle passes through it. Since the lightest of all particles, the electron, is the most relativistic for a given energy, the transition radiation detector can make a useful electron identifier. A final particle identification choice is a time of flight detector made from thick scintillation counters at the outside of the tracking region; with their exceptional timing response, such counters are able to distinguish the velocity of particles of differing mass at a given momentum. It is especially important to distinguish electrons and muons from other charged particles; this is primarily achieved using the calorimeter and muon detectors discussed below. The special particle identifying detectors typically require the use of valuable space, and unless there are strong reasons to include them, cost considerations tend to disfavor their inclusion in collider detectors. The special-purpose fixed-target experiments often place a higher premium on particle identification and more often incorporate such subdetectors.

In tracking detectors, charged particle energies are poorly measured and neutral particles (photons, neutrons, long-lived neutral K mesons) are not seen at all. Thus, it is necessary to have detectors outside the tracking region to measure particle energies. Such detectors are called calorimeters, based on the analogy with the energy-measuring calorimeters of chemistry experiments at a much-lower-energy scale. A calorimeter is based on the showering of particles in a dense medium. A high-energy particle incident upon a material travels, on average, some characteristic interaction distance λ before suffering a collision where several new particles are produced. These daughter particles jointly carry the energy of the incident particle, so they are at lower energies. After a further distance of about λ, these too suffer collisions, resulting in the further multiplication of particles in the shower. This process continues until the particles in the shower are so low in energy that further multiplication is not possible, and the particles stop. The shower process is statistical; the locations of the collisions and the number of particles produced vary from shower to shower, but for high-energy incident particles the total number of particles (or their total travel distance in the medium) is, on average, proportional to the incident energy. Although particles that interact primarily through the electromagnetic interaction (electrons and photons) have a rather different scale λ from the hadrons that interact primarily by the strong nuclear interaction (protons, pions, K mesons, etc.), the principles are the same. Typically, calorimeters are subdivided into an initial first thinner section for electromagnetic particles and a thicker backing section for the hadrons.

In a calorimeter, the showering process is generated through collisions in a medium with large atomic number; lead, iron, or uranium are often used. Generally, the absorber material is made into sheets with interleaved gaps in which particle detectors are placed to record the signals left by the particles traversing it. The total signal from these 50 to 100 interleaved detectors samples the total number of particles in the shower; hence, it is proportional to the energy of the incident particle. Owing to the statistical nature of the showering process itself and the statistical sampling of its content, the relative energy resolution improves with energy like the square root of the energy. The active detectors are segmented in the directions perpendicular to the shower direction to differentiate the energy deposits from different particles in the event. Several choices for the active detector are possible; they differ in their stability, ease of calibration, ability to withstand radiation, ease of segmentation, and time response. A typical choice is scintillation counters, arranged in pads of a few centimeters across, with the light piped out to external photon detectors. Alternate choices are liquid argon or silicon wafers, in which the signal deposits are collected directly as electronic charges on transversely segmented electrodes at the surface of the detector. The energy resolutions achievable are in the range σ/E = 10 to 20 percent/√E for electromagnetic particles and σ/E = 40 to 70 percent/√E for hadrons, where σ is the standard deviation energy error, and E is measured in GeV. If better electromagnetic energy resolution is required, it is possible to make calorimeters with transparent heavy crystals such as cesium iodide or bismuth germanate. For these calorimeters, the absorber and active elements are combined, and the sampling fluctuations can be avoided. These more expensive options may be chosen for fixed-target experiments where the specific goals might require exceptional energy resolution. If improved spatial resolution is required, a separate, more finely segmented, section of the calorimeter called a preshower detector may be added before the main calorimeter.

Muon Detector

The final detector system of a modern collider or fixed-target detector measures the muon, which is capable of penetrating the calorimeter without causing showers and thus does not have its energy well measured. The muon is the only observable particle that can penetrate the material of the calorimeter without substantial degradation of its energy, so seeing a track after the calorimeter gives a clear signal for a muon. Its ionization trail is typically measured in detectors such as drift chambers or scintillation counters interleaved with magnetized iron plates. The magnetic field in the muon detector iron permits a second measurement of the muon's momentum, independent of that performed in the inner tracking. In a collider detector, the outermost muon system is very large, so there is a premium on cheap detectors.

Neutrino Detection

Neutrinos can be produced in the decays of several particles of particular interest, so it is important to know if they are present in a collider experiment event. Neutrinos have such small interaction probabilities that they pass through the detector without showing a detectable trace. However, their presence can be inferred from the balance of momentum in the event. Before a collision, the beam particles have no momentum components in the two directions perpendicular to the beams; momentum conservation ensures that after the collision, the sum of all momenta in these directions should also be zero. If a neutrino is present, it will result in an apparent imbalance in this transverse momentum and thus be revealed in the analyses of the event.

Event Selection

The rate at which collision events occur is very high—in excess of 1 million collisions per second at colliding beam accelerators using protons or antiprotons, and even higher in some fixed-target experiments. This rate is much too large to allow the recording of every event, so selection of only the most interesting must be made. This is the job of the trigger system. Information from each of the detectors is made available within a few microseconds after the collision to special electronics processors. These enable a quick, but somewhat crude, look at the pattern of activity in the detector. By finding evidence for particles such as electrons, muons, or short-lived particles, or by sensing the particularly interesting topologies of many particles, the first-level trigger can flag an event for more detailed scrutiny. On receipt of such a flag, more complete digitized information from subdetectors is collected, and a refined examination is made in dedicated micro-processors, leading to a decision on whether to transfer the full set of subdetector information to the online computer for logging the data to permanent storage. Typically, the rate of events saved for permanent archiving is about 100 per second in a collider experiment, where the total amount of information to be stored for each event is hundreds of thousands of bytes. In a fixed-target experiment where smaller event data sizes are typical, the rate of logged events may be even higher. The recorded events can then be analyzed off-line in great detail and studied in a multitude of specific physics analyses.

A full collider detector is extremely large and complex. Existing detectors at the 2,000 GeV Fermilab antiproton-proton collider in Batavia, Illinois, are

FIGURE 1

about 15m high and wide, and 20m long. Future detectors for the 14,000-GeV large hadron collider (LHC) (see Figure 1) at the European Laboratory for Particle Physics (CERN) in Geneva, Switzerland, will be twice this size. There are well over a million channels of electronic readout for the various subdetectors. The collaborations of physicists and engineers who design, build, and operate these detectors number in the hundreds; these individuals come together from universities and laboratories across the world.

Further information on particle detectors can be found for experiments currently under way at major research laboratories around the world (D0 and CDF at Fermilab, BaBar at Stanford in Palo Alto, California, CLEO at Cornell in Ithaca, New York, and Hl and ZEUS at the Deutsches Elcktroncn-Synchrotron Laboratory [DESY] in Hamburg, Germany). A description of the detectors being planned for the LHC (ATLAS and CMS) include very accessible explanations of the language of particle physics experiments and Web-based tours of the operational principles of subdetectors.