The Standard Model of particle physics is a collection of quantum theories that describes the charges, masses, and spins of the fundamental particles that make up matter, and how those particles interact. The Standard Model incorporates theories for three forces self-consistently: electromagnetism, which describes interactions between charged particles, light, and currents; electroweak theory, which in addition describes the weak force that governs radioactive decays involving neutrinos and positrons; and quantum chromodynamics, which describes interactions between quarks and the nuclear particles that make up matter.
In the Standard Model, there are three basic classes of particles: the leptons, the quarks that combine to make up hadrons, and the bosons that are responsible for communicating the fundamental forces. Most particles in the Standard Model have anti-matter particles. When particles and anti-particles combine, they annihilate and release energy.
Gravity has not been incorporated into the Standard Model. Gravity's effects are too small to influence the structure of matter at sub-atomic scales. However, mass in the Standard Model is produced by the Higgs mechanism, although the associated particle has not yet been detected.
The Standard Model was developed to explain the diversity of particles and their interactions. It emerged during the early to mid 20th century out of concurrent theoretical and experimental developments.
On the theoretical side, physicists developed a theoretical framework that unified electromagnetism, quantum mechanics, and special relativity. The result was referred to as a field theory, in which mathematically symmetries and conservation laws could be used derive how particles would interact.
On the experimental side, physicists began to systematically categorize particles that bombard the Earth's atmosphere from space (cosmic rays) and particles that were created when electrons and ions were accelerated in strong electric and magnetic fields (created in, for example, van der Graff generators and cyclotrons). Physicists found particles with a wide range of masses, most of which were unstable and decayed into lighter particles, releasing their excess energy.
The Standard Model is a set of field theories that describe the numbers of fundamental particles, their masses, and how they interact. The theory divides particles into three types:
In addition, each of the three fundamental forces covered by the Standard Model have an associated charge.
The initial strength of the theory is that it explained a wide range of interactions and particle decays using a relatively small set of parameters (values for particle masses and charges) and a handful of conservation laws (rules for combining particles with different charges).
The Standard Model of particle physics has developed through an iterative process, in which one of its component theories was developed and later confirmed by experiments. For instance, the success of the idea that hadrons were made of quarks was confirmed by the demonstration in particle collider experiments that protons had structure. Later experiments demonstrated that quarks and gluons can exist independently of the particles they compose, in the form of a quark-gluon plasma.
When the electroweak and quantum chromodynamic theories were developed, it was realized that explaining the diverse range of interactions between particles required invoking the existence of particles for which physicists at the time had no other evidence. For instance, under electroweak theory, it was predicted that a neutrino should accompany decays of muons; this was later found.
In some cases, it was possible to predict the masses of new particles. In the second half of the 20th century, particle accelerators allowed physicists to measure these masses, for the W and Z bosons, and the top and bottom quark.
The fundamental particles that are the subject of the Standard Model have not been incorporated into technology.
The Standard Model incorporates electromagnetism, special relativity, quantum mechanics (the three of which compose quantum electrodynamics); our understanding of the weak forces that regulate radioactive decays, and quantum chromodynamics that determine how quarks form hadrons, and how the particles within atomic nuclei interact. As such, the Standard Model represents our best current understanding of the sub-atomic world.
The Standard Model has found remarkable applications in astrophysics. The nuclear reactions within the Sun produce neutrinos, and studies of them are being used to test &mdash or more properly, challenge &mdash the Standard Model. The Standard Model is also a key component of the Big Bang theory. It is used to calculate the relative number of protons and neutrons in the early Universe, which in turns predicts the amount of hydrogen, helium, and lithium in the Universe.
There are several large open questions in the Standard Model. For instance, there is still uncertainty as to why the W and Z Bosons have masses. The Higgs mechanism invokes the presence of a new particle that interacts with them to give mass. The newest accelerator experiments are designed to identify this Higgs Boson.
Observations of neutrinos from the Sun suggest that neutrinos have mass, which allows them to change from one type to another. While this can be accommodated by general field theories, it is unexpected in the Standard Model.
The mathematical framework for the Standard Model contains many symmetries, in which things like a particle and an anti-particle are seen to be mathematically equivalent. Violations of these symmetry rules are being searched for, because they would imply that particles that are thought to be massless might in fact have mass, or that new particles might exist. These violations are suspected to be responsible for the fact that the universe contains much more matter than antimatter. Otherwise, this fact is not explained by the Standard Model.
Finally, the theories that make up the Standard Model have not yet been fully integrated &mdash the strong force and the electroweak forces have yet to be reconciled to form a grand unified theory.
Like science itself, these pages are under construction. OK, so they are in a lot worse shape than science. I welcome your comments.
