need has driven the construction of ever larger accelerator facilities and increasingly large and complex particle detectors.

The two most important properties characterizing the utility of a facility for EPP research are energy and luminosity. Today, physicists are able to produce collisions between elementary particles with energies close to 1 TeV (1012 electron volts)—the energy equivalent of 1012 V energy source. This represents a millionfold increase since the invention of the cyclotron 65 years ago (the difference in scale is shown in Figure 6.1). Until the 1960s, accelerator-based elementary-particle experiments relied exclusively on directing particle beams onto bulk matter—the so-called stationary target configuration. However, over the past 30 years, energy performance has been greatly enhanced by the development of the "particle collider"—an accelerator configuration in which particles and/or antiparticles collide head-on. As described in Chapter 2, the collider configuration provides the most efficient mechanism for translating beam energy into collision energy and thus provides the most direct access to the energy frontier.

The luminosity of a facility is a measure of the rate at which particles collide. Luminosity is directly related to the intensity of the particle beam (or beams) employed (and, in a collider, to the size of a spot onto which the beams are focused). Elementary-particle physicists measure luminosity in units of inverse square centimeters times inverse seconds (cm−2 s−1. This allows one to calculate an event rate by multiplying luminosity by the effective cross-sectional area of the particles that are colliding. Typical luminosities are in the range 1030 to 1035 cm−2 s−1. Such large luminosities are required because the effective areas of the colliding particles are so small—for example, high-energy electrons have an effective area for producing Z0 particles of only about 10−32 cm2, leading to an interaction rate of one every 10 seconds in a facility operating with a luminosity of 1031 cm−2 s−1.

The highest luminosities are attained at stationary target facilities where one can use a dense solid as a target. At the Brookhaven Alternating Gradient Synchrotron (AGS), for example, several trillion protons can be made to interact with stationary targets every second. Stationary target facilities are often used to produce intense beams of particles that can not be accelerated or stored in collider facilities because of their short lifetimes (muons, charged K and p mesons) or their lack of electric charge (neutrons and neutrinos).

Luminosity at collider facilities is much lower due to the low density of the beams relative to ordinary matter. However, for observations undertaken at these machines, the increased operating energy more than compensates for the lower luminosity. At the Fermilab Tevatron, for example, current operations produce about 500,000 annihilations every second between protons and antiprotons in the two countercirculating beams. Experimenters were recently able to identify approximately 100 examples of production of the top quark based on a year of operations at this facility. The observation and measurement of such



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