4.1: Unification Phenomena
Explore unification phenomena. Other Information:
Unification is simplicity at the heart of matter and energy. The complex patterns of particles and forces we see today emerged
from a much more symmetric universe at the extremely high energies of its first moments. Indications of this simpler world
must occur at energies just beyond the reach of current accelerators. A principal strategy is to find out how our complex
patterns merge into a unified picture at higher energies. The Standard Model of particles and forces asserts that all matter
is made of elementary particles called fermions. These are of two types: quarks and leptons, each of which comes in six “flavors.”
Four fundamental interactions are known: strong, weak, electromagnetic, and gravitational, which vary substantially in strength
and range. The first three interactions are carried by another class of particles called gauge bosons. No quantum theory of
gravity has been established and gravity is not included in the Standard Model. At energies above one trillion electron volts
(1 TeV), the electromagnetic and weak interactions are unified into the electroweak interaction, and two of its bosons are
massless. At about 1 TeV, this electroweak symmetry is broken and the bosons acquire mass. The Standard Model attributes this
to a new field called the Higgs, but the Higgs boson has not yet been observed. Three of the leptons are neutrinos, which
feel only the weak interaction, were thought to be massless, and barely interact with matter. Recent experiments have shown
that a neutrino produced in one flavor oscillates among all three flavors as it travels. This can only happen if neutrinos
do have mass, which has important consequences for the Standard Model and for the universe. The Standard Model explains many
observations at the energies our particle accelerators can reach today, but is known to have problems at higher energies.
The theory requires 18 arbitrary and independent parameters whose values are unexplained. It is clear that the Standard Model
must be substantially extended. Physicists are striving to develop a quantum field theory for gravity, using “string theories,”
which explain particles as vibration modes of a tiny string-like bit of energy. String theories involve supersymmetry, a deep
connection between fermions and bosons at high energies. Supersymmetry predicts that every known fermion has a boson partner
and vice versa. Some of these partners must have masses low enough to be created at the TeV energy scale. Thus, our highest
energy accelerators should be able to test supersymmetry by searching for the lightest supersymmetric particles. All string
theories require several extra spatial dimensions beyond the three we now observe. These may be detected at accelerators by
giving particles enough energy that they feel the effects of extra dimensions. A direct discovery of extra dimensions would
be an epochal event. Our strategy includes the following emphases: • Use the Tevatron protonantiproton collider at the Fermi
National Accelerator Laboratory to make detailed studies of the top quark discovered there in 1995. • Search for evidence
of unification at the Tevatron, such as the Higgs boson, supersymmetric particles, and extra dimensions. • Use the B-Factory
at the Stanford Linear Accelerator Center to improve our knowledge of the weak interactions of quarks. • Study neutrino oscillation
and double beta decay to learn more about lepton flavor mixing and neutrino masses. • Develop a string theory that explains
the observed particles and includes a quantum theory of gravity. • Continue our collaboration with the CERN laboratory in
Switzerland to complete construction of the Large Hadron Collider there and then use it to study unification. When it begins
operations in 2007, this proton-proton collider will extend the energy frontier well beyond the reach of the Tevatron. • Participate
in the development of an international linear electron-positron collider for research at the TeV energy scale. Such a facility
has been recommended by HEPAP and by expert panels in Asia and Europe as an essential tool for exploring unification. • Pursue
advanced accelerator development aimed at finding better ways to accelerate particles, with the promise of increasing their
energies beyond one TeV.
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