Modern Elementary Particle Physics Explaining and Extending the Standard Model

This chapter provides a brief description of how information is obtained about the particles that emerge from a collision. In order to understand fully how the theory and its predictions are tested, it is necessary to have some understanding of how the experiments function.

A further goal is to identify the main detectors from which results could emerge during the next decade. At colliders the products of an interaction can emerge in any direction, so detectors must cover essentially all of the 4π solid angle – they are called “4π” detectors. The cost of a detector that can identify hadrons, photons, electrons, and muons, and also know when something has escaped, which is crucial to find (or not) new physics, has become so large that only a few detectors at each machine can be funded or find manpower for construction and operation and analysis, and they generally will have a rather long lifetime, being upgraded once or twice to do more physics as the machine improves or the physics goals change because of new knowledge. Consequently the detectors have taken on a life of their own, and results will often be quoted as “CMS (or ATLAS) reports …” or “MARK II discovered ….” We list the main detectors which have been important for the Standard Model, and those which will be the most common sources of results over the next decade. The size and complexity of detectors implies that they are very costly and take several years to build, so it is now possible to specify most or all of the detectors which will provide results over the next decade.

What Emerges from a Collider

The beams available to collide quarks, leptons, and gauge bosons are electrons and positrons (e±), quarks (q), and gluons (g), where the quarks and gluons are carried in hadrons. When the accelerated hadrons are p±, the quarks are mainly u and d quarks. In addition, quarks and electrons emit photons and W± and Z0 particles at sufficient rates effectively to make photon and W and Z beams under appropriate conditions. We do not distinguish quarks from antiquarks for our purposes. Various particles will emerge from the collisions. By detecting what emerges, with what momentum, at what angle, and how often, we hope to test the present understanding of particles and their interactions and to find any new physics effects.