Modern Elementary Particle Physics Explaining and Extending the Standard Model

Before we consider further the tests and predictions of the Standard Model, it is useful to discuss the experimental facilities. Some understanding of the accelerators and detectors is essential to understand how the experiments are carried out, and why some experiments are done and not others. We do not discuss in detail how accelerators work, but it seems appropriate to have three goals: (1) to indicate how high energy beams of quarks and gluons and electrons are obtained, (2) to survey the available machines for doing particle physics, including those recently available but now closed, and (3) to describe the plans and hopes for accelerators over approximately the next decade or even longer. It does not require much of a crystal ball to do the latter, since the large construction times and large costs of new facilities imply that existing decisions almost completely determine what can happen for several years, and plans currently under discussion greatly restrict what can happen for even longer.

The accelerators provide the beams and make them collide. Then it is necessary to detect the collision products and interpret what happened. Detectors do that, and Chapter 13 will be devoted to a description of how they do it.

Parameters of Accelerators

From the point of view of the physics purpose of an accelerator, four of its properties are the main ones. The first is the type of particle being accelerated. Since the acceleration is achieved by electromagnetic fields, any long-lived, electrically charged particle can qualify. In practice, for particle physics the main possibilities are electrons (e−), positrons (e+), protons (p), and antiprotons. Beams of other particles can be obtained by hitting a target with a proton beam, which causes all varieties of particles to emerge, and carefully selecting particles of the desired type by bending or stopping the others. This has led to experiments with neutrinos, pions, kaons, photons, lambdas, muons, and others.

The second basic property is the final energy to which the particles are to be accelerated. The initial protons or electrons are obtained from ion generators and have energies in the eV to keV range. They are accelerated as they travel through regions with electric fields.