  Electroweak theory joins electromagnetism with the weak force in a single
quantum field theory, ascribing the two fundamental interactions--so different
in their manifestations--to a common symmetry principle. How the electroweak
gauge symmetry is hidden is one of the most urgent and challenging questions
facing particle physics. The provisional answer incorporated in the "standard
model" of particle physics was formulated in the 1960s by Higgs, by Brout &
Englert, and by Guralnik, Hagen & Kibble: The agent of electroweak symmetry
breaking is an elementary scalar field whose self-interactions select a vacuum
state in which the full electroweak symmetry is hidden, leaving a residual
phase symmetry of electromagnetism. By analogy with the Meissner effect of the
superconducting phase transition, the Higgs mechanism, as it is commonly known,
confers masses on the weak force carriers W and Z. It also opens the door to
masses for the quarks and leptons, and shapes the world around us. It is a good
story--though an incomplete story--and we do not know how much of the story is
true. Experiments that explore the Fermi scale (the energy regime around 1 TeV)
during the next decade will put the electroweak theory to decisive test, and
may uncover new elements needed to construct a more satisfying completion of
the electroweak theory. The aim of this article is to set the stage by
reporting what we know and what we need to know, and to set some "Big
Questions" that will guide our explorations.
