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Particle Accelerators and Detectors


Van de Graaff Generator Particle accelerators were invented to investigate objects with size less then 10-12 cm. Accelerators are to particle physics what telescopes are to astronomy, or microscopes are to biology. These instruments all reveal and illuminate worlds that would otherwise remain hidden from our view. They are the indispensable tools of scientific progress. The earliest accelerators were simple vacuum tubes in which electrons were accelerated by the voltage difference between two oppositely charged electrodes. From these evolved the Cockcroft-Walton and van de Graaff machines (Figure 03), larger and more elaborate, but using the same principle. The modern example of this type of device is the linear accelerator, a sophisticated machine used in many scientific and medical applications. All such straight-line accelerators suffer from the disadvantage that the finite length of flight path limits the particle energies that can be achieved.

Figure 03 Van de Graaff Generator [view large image]

Cyclotron Synchrotron The great breakthrough in accelerator technology came in 1920 with Ernest O. Lawrence's invention of the cyclotron (Figure 04). In the cyclotron, magnets guide the particles along a spiral path, allowing a single electric field to apply many cycles of acceleration. Soon unprecedented energies were achieved, and the steady improvement of Lawrence's simple machine has led to today's proton synchrotrons (PS, Figure 05), whose endless circular flight paths allow protons to gain huge energies by passing millions of times through the electric fields that accelerate them.

Figure 04 Cyclotron
[view large image]

Figure 05 Synchrotron [view large image]

Collider Until twenty-five years ago, all accelerators were so-called fixed-target machines, in which the speeding particle beam was made to hit a stationary target of some chosen substance. But early in the 1960ís physicists had gained enough experience in accelerator technology to be able to build colliders, in which two carefully controlled beams are made to collide with each other at a chosen point (Figure 06). Several colliders exist around the world today, and the technology for them is by now well established. Colliders are more demanding to build, but the effort pays off handsomely. In a fixed-target machine, most of the projectile particles continue the forward motion with the debris after impact on the target. In a collider, on

Figure 06 Collider
[view large image]

the other hand, two particles of equal energy coming together have no net motion, and collision makes all their energy available for new reactions and the creation of new particles.

High Energy Collision It is realized that the mass-energy relation (E = mc2) provides a new way to get information about particles. If particles could be made very energetic and then used to collide with other particles, some of their energy could be converted into the creation of previously unknown particles. When particles are produced in a collision, they are not particles that were somehow inside the colliding ones. They are really produced by converting the collision energy into mass, the mass of other particles (Figure 07). Which particles will be produced is partly determined by their mass - the lighter they are, the easier it is to produced them, other things being equal

Figure 07 High Energy Collision [view large image]

- and also by the probabilities calculated from the Feynman diagrams

Top Quark Events Particle with energy about 1 Gev (109 ev) is required to probe the structure inside proton. Higher energy is required for smaller system - about 1000 Gev is needed to probe into the quarks. The same amount of energy is required to create many of the hypothetical particles. Currently, the Fermilab's Tevatron has enough energy to produce the top quark (~170 Gev). Figure 08 shows a schematic diagram of a top quark event and the actual observation. Since there is no free quark, the result is a jet of hadrons (particles affected by strong interaction) emerging in the direction of the original quarks. Up to 14 Tev (1012 ev) will be available by the

Figure 08 Top Quark Events [view large image]

LHC at CERN in 2008. Table 01 below summarizes some features of the major accelerators in the world (all of them are colliders):

Accelerator Colliding Particles Total Energy Major Accomplishment
Stanford Linear Collider (SLC) in Palo Alto electron, positron 100 Gev provided first look at Z0
Large Electron Positron Collider
(CERN-LEP) in Geneva
electron, positron 200 Gev discovered Z0, W in 1983;
shut down in 2002
Relativistic Heavy Ion Collider
(BNL-RHIC) in Brookhaven
heavy ions 200 Gev created quark-gluon plasma
(Mini Big Bang)
Tevatron (FNAL) in Chicago proton, antiproton 2 Tev confirmed Z0, W,
discovered top quark
Large Hadron Collider
(CERN-LHC) in Geneva
proton-proton; ion-ion 14 Tev planned for 2008 - LEP replacement

Table 01 High Energy Accelerators

Note: CERN = European Organization for Nuclear Research, BNL = Brookhaven National Laboratory,
             FNAL = Fermi National Accelerator Laboratory.

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