The standard model of particle physics is the quantum theory that describes all of the known particles and three of the four forces they feel. Although it is called a "model," it is a full-fledged scientific theory that has been extremely well-verified over the past thirty years. Currently, all laboratory and particle collider experiments have agreed with the predictions of the standard model, making it a great success. On the other hand, the only clues pointing to more fundamental physical theories come from disagreements with the standard model, which have been frustratingly hard to find.
The standard model is composed of three parts. The first part is the matter particles, which are called quarks and leptons. The second part is the forces, or interactions, which include the electromagnetic, weak, and strong forces. (Currently, there is no verified quantum theory of gravity.) The third part is the rules of quantum mechanics, which can be used to calculate observable numbers (such as decay rates and cross sections) using the properties of the particles and forces as input.
The matter particles make up the everyday matter we see around us. Protons and neutrons are made of quarks, and electrons combine with protons and neutrons to make atoms. The matter particles are further divided into quarks and leptons, which are spin-1/2 fermions. Quarks are particles with fractional electric charge and another type of charge referred to as color. There are two types of quarks in everyday matter, the up with electric charge +2/3 e and the down with electric charge -1/3 e. Just as negatively charged electrons combine with positively charge protons to make a charge-neutral atom, three quarks combine to form a color-neutral proton or neutron.
The other type of matter particles are called leptons. The electron is the most familiar lepton, with electric charge -e. The other common lepton is the electron neutrino, which is electrically neutral and is massless. (Whether or not neutrinos are actually massless is not known, but in the standard model they are treated as massless particles. In any case, the mass is extremely small.)
The forces, or interactions, in the standard model arise due to some symmetry principles. Under a set of global transformations, where the basic objects of the theory are redefined in the same way at each point in spacetime, the theory remains unchanged. However, to make the theory invariant under local transformations, where the basic objects are redefined differently at different points in space-time, an extra set of objects are added to the theory to offset the changes. These new objects are referred to as gauge bosons and they are said to mediate the interactions between particles.
In a quantum field theory, particles are considered to interact by exchanging gauge bosons. For example, two electrons scattering may occur the following way: the first electron emits a photon and recoils, and then the second electron absorbs the photon and changes its motion appropriately. The benefit of this viewpoint is that there cannot be any instantaneous transmission of force, which would violate the principles of relativity. The gauge bosons of the weak force are called the W-and Z-bosons, and the gauge bosons of the strong force are referred to as gluons.
The symmetry transformations that lead to the existence of a force are called gauge symmetries. Gauge symmetry is a very powerful idea, because it predicts that all particles interacting through a given force interact with the same strength. Without the gauge symmetry, it would be perfectly acceptable for the electromagnetic interaction between two protons to be stronger than between a proton and an electron. However, the gauge symmetry requires the same coupling strength to all particles with the same electric charges. This idea is even more useful in the theories of the weak and strong interactions, which we have less intuition about.
The standard model also incorporates two heavier copies of each matter particle already mentioned. It does not explain why they exist, but it does describe their interactions and how they can decay into the everyday matter particles. The heavier quarks similar to the down quark are the strange and bottom quarks, those similar to the up quark are called charm and top, and the heavier leptons similar to the electron are the muon and the tau. There are also two copies of the electron neutrino, but they also appear to be massless.
The standard model includes one more type of particle, known as the Higgs boson. The existence of the Higgs boson is necessary to explain why matter particles and certain gauge bosons have mass. The Higgs boson is the only missing piece of the puzzle, because it has not yet been discovered. However, particle physicists are sure of its existence and expect it to be discovered soon at either the newly refurbished Tevatron collider at Fermilab near Chicago, or at the new Large Hadron Collider at CERN in Geneva, Switzerland. In reality, there may be many Higgs bosons, because there are various models of Higgs physics that explain the existence of mass.
Aside from the problem of detecting the Higgs particle, all other known experimental results can be explained using the standard model. However, physicists desire to go further and explain some of the facts the standard model can only describe, such as why there are three families of particles. In addition, there are some theoretical loopholes in the renormalization theory that can be closed by introducing extra particles and symmetries. Because of these and other developments, theoretical physicists are convinced the standard model is not the whole story, but until they receive some signals that disagree with the standard model they will have to wait for verification of any new scenarios.
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