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The Science of Particle Physics

Elementary particle physics attempts to identify the fundamental constituents of nature and the fundamental interactions of these basic building blocks. This line of research is the frontier of the reductionist side of physics.

The Symmetry Paradigm

Over the last century, physicists have learned to replace basic assumptions about the dynamics of nature with assumptions about the symmetries of nature. The most familiar examples of this include conservation laws that can be recast as symmetries:

Dynamical statement Symmetry Principle
conservation of momentum spatial translation invariance
conservation of energy temporal translation invariance
conservation of angular momentum rotational translation invariance

Symmetry is a powerful and predictive organizing principle. It explains and predicts the basic forces (interactions) of nature very economically. Once physicists learned to take symmetry seriously as an organizing principle the fundamental forces of nature were understood as the consequences of symmetries.

DynamicsSymmetry Principle
Electricity & Magnetism
(Maxwell's Equations)
U(1) Gauge Invariance
Weak Interactions SU(2) Gauge Invariance
Strong Interactions SU(3) Gauge Invariance
Gravity General Covariance

The Standard Model

The figure below lists the know fundamental particles (which are the basic building blocks of everything we know) as we currently understand them.

particle chart

The theory which describes these particles and how they interact with each other is called the Standard Model of Particle Physics. As of this writing, all experimental tests of the fundamental forces (interactions) between the fundamental particles of nature are in excellent agreement with the Standard Model of particle physics. The Standard Model, is an example of a relativistic quantum field theory. Relativistic quantum file theories contain both the principles of special relativity and the principles of quantum mechanics. Einstein's special relativity, and Newtonian mechanics are both special limiting cases of this more encompassing theory.

In addition to the ingredients of special relativity, and quantization, the standard model is specified by a set of symmetries. These additional symmetries explain: electricity and magnetism, the weak force (responsible for nuclear beta decay), and the strong force (responsible for holding nucleons together to form a nucleus, and responsible for binding quarks together to form protons and neutrons). So far, it works very, very well ... the only missing ingredient is the elusive Higgs Boson.

Beyond the Standard Model

Despite the spectacular success of the Standard Model, it is generally accepted that the Standard Model can not be the ultimate, fundamental theory of nature. As the theory stands, it accommodates but does not explain the masses of fundamental particles, and while it explains the strengths of many of their interactions, the strengths of other interactions are unexplained. Moreover, the standard model of particle physics is currently incomplete because it does not contain a quantum theory of gravity. In our attempt to develop a more complete theory of nature, particle theorists attempted to embed the Standard Model in grander theories, and then study the testable predictions of these models. The predictions of the Standard Model, and theories which attempt to go beyond the Standard Model are tested at particle accelerators.

Our best window towards identifying the more fundamental theory of nature comes to us from particle accelerators. Theories which attempt to go beyond the standard model predict new particles which are not part of the standard model. Searching for these hypothetical new particles is one of the central missions of these accelerators. High energy physicists attempt to produce new particles through the high-energy collisions of other particles. Today's high-energy particle accelerators collide various particles: Collisions of protons with anti-protons (the anti-matter version of the proton) are made nearby at Fermilab's Tevatron. Future upgrades of this collider will continue to collide protons and anti-protons into the next century. In the next century, protons will be collided with themselves at tremendously high energies at CERN's LHC, in Europe. Collisions of electrons with positrons (the anti-particle of the electron) are currently being studied at CERN and have been studied at the SLC, a linear accelerator in Stanford, California, and in Europe at CERN's LEP collider. In the future a `next generation' electron-positron collider may be constructed in the USA, Japan, or Europe.

Roughly speaking, the heavier a particle is, the harder (with more energy) we need to smash lighter particles together to create it. Almost all theories which attempt to embed the Standard Model in a more complete theory predict new, heavy particles with masses several hundred times heavier than the proton. These heavy particles require so much energy to create that they could not have not been produced by previous collider experiments. The new experiments upcoming and underway discussed above will change this situation in the next decade.

The most popular and, many believe, the most promising extension of the Standard Model is supersymmetry. Supersymmetry predicts that every fundamental particle observed so far has a companion called its superpartner. The properties of these new superpartners are identical to the standard particles, except in two respects. These new superpartners are heavier than their companions and they differ in a quantum number called spin. Because they are heavy, superpartners have not yet been produced in high-energy collisions. We don't know exactly how heavy these particle are, however, the masses of these superpartner can't be heavier than the mass of a few W or Z particles (particles we have already seen) If supersymmetry is the next step towards the fundamental theory of nature, these new particles will be discovered at Fermilab's Tevatron collider, or at CERN's LEP II collider , or at CERN's large, hadron collider, the LHC.

One very interesting and far reaching idea in theoretical physics is grand unification. We know of four fundamental forces in nature; (1) electricity and magnetism, (2) the weak nuclear force, (3) the strong nuclear force, and (4) gravity. As mentioned above, the Standard Model is a quantum theory of the first three of these forces. Each of these forces, more appropriately referred to as interactions, can be characterized by its strength. When measured at low energies, the strengths of these interactions are very different. These strengths, however, depend on the energy at which they are measured. In some theories, if we extrapolate the measured strengths of these three interactions from low energies to high energies, all three forces appear to unify into a single force at very high energies. This unification is predicted by grand unified theories. If grand unification proves to be correct, all three of these forces can be understood as different manifestations of a single force. More ambitious still are string theories, which attempt to unify these forces with gravitation at and even higher scale.

For more information on particle physics try "The Particle Adventure" from the Particle Data Group and "The Science of Particle Physics" from Fermilab.

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