FIGURE 8.1 Bird's-eye view of LEP storage ring shows how electron and positron bunches are accelerated by a series of smaller machines. Four bunches each of electrons and positrons circulate in opposite directions and collide inside the four gigantic detectors: Aleph, Delphi, L3, and Opal. The bunches also cross paths at four intermediate sites, where they are prevented from colliding by electrostatic separator plates. (From "The LEP Collider" by Stephen Myers and Emilio Picasso. Copyright © 1990 by Scientific American, Inc. All rights reserved.)

and independent body of physical law. Yet the Z particle, featured in electroweak theory, proved to offer the key to the problem of quark families.

The reason lay in the uncertainty principle of quantum mechanics. This principle states that when a particle exists for a vanishingly short time—for the Z, some 10^-25 seconds—there is a corresponding uncertainty in measuring its energy and therefore its mass. One might seek to determine the mass of a type of particle by making repeated measurements with great accuracy, but this principle would defeat the attempt. Any individual measurement might indeed show high precision, but no two measurements would agree. Different observations would show different values for the particle mass, with the ensemble of these values showing a bell-shaped curve, a distribution common in the field of statistics.