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The Standard Model provides a description of how particles interact with each other, but John Womersley, of Fermilab, points out that this model doesn't really tell us why the particles actually exist.
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We have discovered three families of quarks and leptons,
families of fundamental particles that differ only in
their masses, which range from less than a millionth of
the mass of an electron to the mass of an atom of gold.
Just as quantum mechanics led to an understanding of the
organization of the periodic table, we look to new
theories to explain the patterns of elementary particles.
Why do three families of particles exist, and why do
their masses differ so dramatically?
Current investigations focus on developing a detailed
picture of the existing patterns in the particle world.
Remarkable progress has been made, especially in
characterizing the quarks. But why are the patterns for
leptons and quarks completely different? Detailed studies
of quarks and leptons at accelerator experiments will
provide the clearest insight into these issues.
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| | CLEO DETECTOR CLEO detects, measures and analyzes electron-positron collision events generated by CESR. Credit: Cornell–Wilson Laboratory
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Physicists have so far identified 57 species of elementary
particles. In particular, the Standard Model contains
quarks and leptons, grouped into three families that differ
only in their masses. Why the pattern of particles is
repeated three times with enormous variations in mass but
with other properties seemingly identical is an open
question. Quantum physics has shown that three families are
the minimum necessary to accommodate CP violation in the
Standard Model. Such CP violation is necessary for matter to
predominate over antimatter in the universe, but its effects
observed so far are insufficient to explain this predominance.
The current program of experiments focuses on developing a
detailed understanding of the existing patterns and searching
for signs that the patterns of the three families are not
identical.
The CDF and D0 experiments at the Tevatron are
measuring the properties of the top quark to see if its
enormous mass gives it a special role in the particle
world. The BaBar and Belle experiments at SLAC and
KEK are using their data samples, containing millions of
b and c quarks, as well as τ leptons, to make precision
measurements of the masses and decay modes of all
of these objects, in order to look for subtle deviations
from the predicted patterns of their decays. The third-
generation particles-top, bottom and tau-offer the best
hope for discovery, because their large masses allow them
to couple most effectively to undiscovered physics.
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| Something is missing in physicists’ understanding of how the universe evolved into its current state. At the big bang, equal quantities of matter and antimatter... read more |
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BaBar and Belle can study only two types of B mesons,
bound states of the bottom quark with up or down quarks.
However, many theories suggest signifi cant effects in
the bound state with the strange quark, Bs.
Physicists are currently studying the properties of the Bs meson
at the Tevatron. The future hadron B-factories, BTeV and LHC-b, will
explore the Bs meson with far greater precision.
Properties of individual quarks are experimentally difficult to study,
because they are always bound to other quarks. Lattice Computational
Facilities offer great promise for the calculation of the effects of
the strong interactions. As an example, lattice calculations will
provide sufficient precision to extract quark parameters, such as
those that describe flavor mixing, from the experimental data.
Experimental studies with CLEO-c will establish and validate the
precision of the lattice calculations for use in heavy quark systems.
Neutrinos have opened a surprising new window on the physics of lepton
generations, since neutrino masses are not necessary in the Standard Model. The presence of
neutrino masses may be telling us something about physics beyond
the Standard Model. The decay properties of the light leptons,
the electron and the muon, may also hold surprises. The MECO
experiment proposes to look for a conversion of a muon to an
electron and is sensitive to very-high-mass physics that might
affect that process.
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