It consists of particles that carry the four forces among the basic units of matter. Like waiters flitting from kitchen to table in an excellent restaurant, the force-carrying particles do their jobs without our really noticing them. We know about photons , which ferry electromagnetic forces from place to place. The other force-carrying particles stay mostly hidden from view.

The various building blocks of matter combine to create hundreds of composite particles. You've heard of two of these--protons and neutrons . It's less likely that you're aware of pions, kaons , and omegas . Not to worry; the standard model has everything under control. It tells us how all particles arise and how they interact, just as surely as we can glean the properties of a chemical element by glancing at the periodic table. Physicists are as comfortable with the standard model as chemists are with their table of neat rows and columns.

However, there's a major difference between the periodic table and the standard model . The quantum mechanical behavior of electrons within atoms tells us why the periodic table looks the way it does. Its patterns are the direct results of how electrons act when atoms approach one another. the standard model , on the other hand, lacks such a tangible foundation. Instead, it prompts questions that physicists cannot yet answer. For example, why are there three families of matter ? Why aren't there two or four? What dictates the mass of each particle? A proton's mass is (continued)

1.67 x 10^­27 kilograms--that's 270 trillion trillion protons per pound. An electron is about 1,835 times less massive. The standard model is full of numbers like these, but no one knows exactly why the values are what they are.

One major answer may come in the form of a hypothetical particle with a slightly comical name: the Higgs boson . This force-carrying particle would create a field that permeates the universe, much like magnetic or gravitational fields. Flecks of matter would acquire their masses by experiencing this field to different degrees. Imagine wading through three pools filled respectively with air, water, and molasses. You'd "feel" light, then heavier, then heavier still as the substances dragged on you. The Higgs field works in a similar way. Fleeting neutrinos would interact with the field not at all or just barely, electrons a bit more, and quarks considerably more strongly.

Physicists are racing to find the Higgs boson within the blasts of particles created in their colliders. If they detect it, the standard model will become an ever more powerful tool for understanding how matter assumes its many forms in the universe.

Despite the successes of the standard model , nearly all physicists agree that it will be subsumed by a more fundamental theory of how the cosmos ticks. Rules of physics beyond the model's tight confines should illuminate other enduring mysteries. One of these is why matter exists at all. The Big Bang theory predicts that the fury of the (continued)