Joshua Eli Gates

Michael Quinn Ventura

The rise of modern particle accelerators has provided us with knowledge of the elementary particles and their interactions. These accelerators, linear and circular, move particles to tremendous speeds (with energies approaching 10 TeV at the largest accelerators) where they collide with similar particles moving in the opposite direction. Many experiments can be performed based on the outcome of these collisions. Detectors must be built to analyze the collisions. Detectors come in many forms. They fall into two main groups: counters and track recording devices.

There are two main types of counters, scintillation and Cherenkov. Scintillation counters are made of plastics or liquids that give off small flashes of light when impinged upon by a charged particle. These counters have an array of photomultiplier tubes that are sensitive to the light flashes. The counter functions by converting these flashes into an electric current that is analyzed by a computer. The main component of Cherenkov counters is a tank filled with liquid or gas at high pressure where the speed of light is significantly less than that in a vacuum. Particles leave the accelerator at speeds very near near the speed of light in a vacuum and actually greater than the speed of light in the counter's dielectric tank. Since the particles are moving faster than the speed of light in that medium a "light boom" is emitted (similar to when a plane breaks the sonic barrier). Another kind of array of photomultiplier tubes detects the pulses and a computer analyzes the collision. The current detector at the Stanford Linear Accelerator (SLAC), the Stanford Linear Accelerator Large Detector (SLD) is based on the Cherenkov effect. The goal of the SAC Large Detector is to find the mass of the Zo particle, the mediating massive gauge vector boson of the weak nuclear force. During 1992, eleven-thousand Zo particles were detected, at 22% polarity. During 1993, fifty-thousand of these particles were detected at 61% polarity. Electron polarization allows the tagging of bottom (beauty) quarks so that the collisions may be analyzed in greater depths. According to Stanford, asymmetry of the "Z-peak resonance" in hadrons is dependent upon the top (truth) quark mass and the mass of the Higgs boson, quantities that science is actively researching. The other large category of detectors consists of track recording detectors. These detectors give us actual pictures of the paths of the charged particles with them. Scientists use these pictures of high-energy interactions between incident streams of particles to decipher the identities and properties of the resultant particles and to discover the intricacies of the forces that control the interactions. One form of the detectors is the bubble chamber. There is a large tank filled with a liquid slightly lower in temperature than its boiling point. High-energy charged particles that move through this liquid cause the liquid to boil close to the particles, and the bubbles are photographed as they are created by the moving particles. Using conservation laws, scientists are able to deduce the existence of non-charged particles, as they do not show up in the photographs. The largest of these bubble chambers are being used at the Centre Européen de la Recherche Nucléaire, the European Centre for Nuclear Research (CERN). These are the most accurate of the tracking detectors. Spark chambers are much simpler detectors, but they make a less definite image than bubble chambers. They are a long series of thin, alternately charged metallic plates, separated by a gas. As the particles pass through the gas, they ionize the gas particles, just as in the bubble chambers, but the high electric field of the gas molecules cause the electrons released from the ionization to accelerate, and a chain of electrons are emitted, eventually causing a visible spark trail, retracing the path of the particles. Streamer chambers are similar to spark chambers, but they only have two charged plates, and when charged particles pass between the plates, the beginnings of sparks are formed between the plates and the particles, leaving faint trails. Proportional chambers are also cousins of the spark chamber; they have two parallel, negatively potentialled plates with a series of thin, parallel, positively potentialled wires between them. When a charged particle passes close to a wire, an electric discharge is produced near the wire. The current that the detector measures is proportional to the ionization of the gas, hence the name of the detector. A combination of scintillation counters with a proportional chamber is used at CERN's Super Proton Synchrotron (SPS). CERN's detector also contains a magnet that allows the momentum of the particles passing through the field to be measured, by measuring their deflection. In this way, immediate identification of particles can be made.

CERN is currently at work building the Large Hadron Collider (LHC). Three different detectors have been proposed: A Large Ion Collider (ALICE) experiment detector, A Toroidal Large Hadron Collider Apparatus (ATLAS), and theCompact Muon Solenoid (CMS), with a four Tesla magnetic field. For further detector information, browse http://www.cern.ch/cern/lhc/pgs/general/detectors.html.

The era of particle physics has brought about a need to experiment with high-energy particles. To bring particles to such energies as 10 TeV and higher, at which the similarities of the forces that control our world become apparent and the underlying structure of the matter of which it is composed is discerned, huge particle accelerators are employed. The high-energy streams of particles that are collided produce new particles and reveal details about the four forces, but we can only take advantage of these experiments using data obtained from detectors, often-large devices that give us pictures or other accounts of these collisions.

Sources

Beiser, Arthur. Concepts of Modern Physics, McGraw-Hill Book Company, NY. 1987.

Ohanion, Hans C. Modern Physics, Prentice-Hall, Inc. EngleWood Cliffs, NJ. 1987.