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Physicists describe the universe with a set of rules known as the Standard Model
(SM). According to these rules, force is transmitted between objects by the
exchange of particles (or "quanta") of the force. There are four known types of
forces - strong, electromagnetic, weak and gravitational.
(All other forces are variants of these basic four.)
Each force corresponds to the exchange of a different type of particle. The gluon generates the strong force, the photon (light) generates the electromagnetic force, the "vector bosons" (W and Z) generate the weak force and the graviton generates the gravitational force. (You may note already the physicist's predilection for peculiar names!) These forces act on a variety of particles making up matter. The SM, then, consists of the list of exchanged quanta, a description of how they attach themselves to the matter particles (fermions), a list of the fermions, and their properties. "Field theory" takes us from the particles and exchanged quanta to phenomena seen in the laboratory.
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Two problems arise: first, do we have the correct set of rules? - and second,
how do we apply the rules to actually calculate a real physical process as seen
in an experiment? Although the SM has been amazing in its ability to describe
observed phenomena and predict new phenomena, it is certainly not the whole
story. Gravitation is not described in a manner consistent with the other
forces, and there are aspects of the SM that theorists consider unpalatable. It
has too many free parameters that need arbitrary adjustment, and too many doors
that appear to lead nowhere. Thus, there is a continuing push to refine the SM.
However, this model is exceptionally successful in describing experimental results. Because of this, theorists have little guidance as to where problems might lie, and have been free to exercise their imagination in proposing extensions to the SM. Eventually, experiments - many of them here at TRIUMF - will have to winnow out the chaff.
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One of the few phenomena that suggest a problem in the SM, and hence that will help in the winnowing, is the deficit of neutrinos from the sun. The new Solar Neutrino Observatory (SNO) being built at Sudbury, Ontario, will help determine if this deficit is due to a problem with our model of the sun or to an inadequacy of the SM in describing neutrinos. An experiment on a nuclear reaction rate, planned for TRIUMF's future ISAC facility, will play an important role in interpreting the results of the solar neutrino experiment.
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As noted above, we call the carriers of the weak force "vector bosons". In the arcane language of particle physics, this is because they have one unit of what is known as angular momentum. A logical possibility exists that some particles may have no angular momentum. These hypothetical particles are called "scalars" (regardless of their aptitude for scaling mountains). The existence of scalar bosons would require major modifications to the SM. This would delight the particle physics community which, contrary to the usual stereotype, loves to prove old models wrong or incomplete! Some of the first experiments to be carried out using "TRINAT" - TRIUMF's Zeeman optical trap - will test for the existence of scalar bosons.
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The carriers of the weak force not only carry angular momentum themselves, but they interact in a peculiar way with the angular momentum of the particles they meet. The angular momentum of a particle must be pointed in the opposite direction to a particle's motion. Proposed experiments with the Zeeman optical trap may be able to detect interactions with particles whose angular momentum is in the direction of their motion. This would also result in a major change of the SM. In many cases it is far from trivial to see what the SM will predict in a given situation. For example the protons and neutrons which make up the nuclei of atoms consist of quarks held together by the strong force. Unfortunately the strong force is so strong that it makes calculations difficult. One approach is to use brute force and large amounts of computational power to solve the problem numerically, using a technique known as "Lattice Quantum Chromodynamics" (QCD). We have made considerable progress in calculating the mass of particles like the proton and pion. |
At higher energies, the strong force weakens, and hence is more amenable to
calculation. "QCD sum rules" allow us to relate low-energy properties to
high-energy properties, and using these may let us extrapolate correctly from
one energy region to the other.
An alternative approach to the region of strong forces is to use "effective theories" having properties (symmetries) that we know the real theory must have. We might then see what relationships can be derived between physical observables. "Chiral Perturbation Theory" uses this approach to study interactions between the strongly interacting particles at relatively low energies. It has been particularly successful in describing the interaction of pions with nucleons. The realistic description of nuclei usually requires models even farther removed from the fundamental rules or SM. One approach is to deal entirely with "effective" forces which are not related directly to the fundamental ones. We are, however, working toward applying our knowledge of the fundamental forces to reveal nuclear structure. |
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