| Themes > Science > Physics > Elementary particle physics > Elementary particle physics Today > Elementary particle physics Today > Introduction |
High-energy physics is the search for elementary particles and basic laws of nature. What are the smallest building blocks out of which protons, neutrons, atoms, and all matter are made? Do such elementary particles exist?; and if so, what are they? This search to unveil the elementary constituents of matter, along with the forces that link them, involves distances thousands of times smaller than nuclear sizes, about one ten trillionth of a centimeter, or 10-13cm. Accelerators must have very large energies to probe nature at such small distances. The ultimate goal of this quest is a view of the underlying first principles that govern our entire physical universe. In recent years, we have realized a strong and growing synergism between the physics of short distances and the properties and large- scale structure of the universe. This development reflects the unity of science as explored on both the high-energy and particle astrophysics frontiers. With this connection, we are now addressing some of the most basic questions one can ask: How did our physical universe begin? How did it evolve to its present state? What will be its final fate? Over the past several decades, experimental discoveries and theoretical insights have significantly advanced our understanding of the elementary particles and their forces. We now know that electrons, protons, and neutrons make up the visible matter all around us, but only the electron appears to be a point-like elementary particle. Protons and neutrons are bound states of more basic constituents, the up and down quarks. Those quarks are permanently bound or confined by what are called strong interactions or forces. The strong interactions are governed by a fundamental theory of quarks and gluons known as quantum chromodynamics (QCD). The gluons mediate the strong force that binds the quarks into protons and neutrons. QCD is an elegant theory that, in principle, is capable of explaining all observed strong interaction physics. On another front, two forces previously thought to be distinct, electromagnetism and the weak force that governs radioactive decay, are now properly described by a unified electroweak theory. This theory correctly predicted weak neutral currents as well as the observed properties of W and Z bosons, the carriers of the weak force and partners of the photon. The combination of QCD and the electroweak model provides a beautiful description of all known elementary particles down to distances of order 10-16 cm. The theory of strong and electroweak interactions can be unambiguously tested by comparing its predictions with precision measurements. Remarkably, a wealth of experimental data has been confronted at a high level of sensitivity, without any clear signal of disagreement or inconsistency. Those impressive successes have earned the theory its title as the "Standard Model," a label that describes its acceptance as a proven standard against which future experimental findings and alternative theories must be compared. Its discovery should be viewed as one of the great scientific triumphs of the twentieth century. Despite the successes of the Standard Model, it is believed not to be the final word. That conviction is based primarily on dissatisfaction with the electroweak sector which exhibits a number of shortcomings and leaves unanswered some basic questions: Why are there so many elementary particles and why do they have their observed pattern of masses? What is the origin of mass? Why and how is the symmetry between electromagnetism and weak interactions broken? Why is matter-antimatter symmetry broken and what does it have to do with the observed predominance of matter in our universe? Speculations abound, but physics is an experimental science, and only with new data will we be able to properly address these problems and uncover whatever new surprises lie ahead. |
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