The Standard Model of particle physics is a theory that describes three ( electromagnetism, the weak interaction, and the strong interaction) of the four (gravitation) known fundamental interactions among the elementary particles that make up all matter.
The Standard Model falls short of being a complete theory of fundamental interactions, primarily because of its lack of inclusion of gravity, the fourth known fundamental interaction.
The particles of the standard model are organized into three classes according to their spin:
I dont understand what is meant by spin!
fermions (spin is ½ particles of matter), gauge bosons (spin is 1 force-mediating particles), and the (spin is 0) Higgs boson.
Particles of matter
Apart from their antiparticle partners, a total of twelve different fermions are known and accounted for. They are classified according to how they interact (or equivalently, what charges they carry): six of them are classified as quarks (up, down, charm, strange, top, bottom), and the other six as leptons (electron, muon, tau, and their corresponding neutrinos).The defining property of the quarks is that they carry color charge, and hence, interact via the strong force. The infrared confining behavior of the strong force results in the quarks being perpetually bound to one another forming color-neutral composite particles (hadrons) of either two quarks (mesons) or three quarks (baryons). The familiar proton and the neutron are examples of the two lightest baryons.
The remaining six fermions that do not carry color charge are defined to be the leptons. The three neutrinos do not carry electric charge either, so their motion is directly influenced only by means of the weak nuclear force. For this reason neutrinos are notoriously difficult to detect in laboratories. However, the electron, muon and the tau lepton carry an electric charge so they interact electromagnetically, too.
Force mediating particles
Forces in physics are the ways that particles interact and influence each other. At a macro level, the electromagnetic force allows particles to interact with one another via electric and magnetic fields, and the force of gravitation allows two particles with mass to attract one another in accordance with Newton's Law of Gravitation. The standard model explains such forces as resulting from matter particles exchanging other particles, known as force mediating particles. When a force mediating particle is exchanged, at a macro level the effect is equivalent to a force influencing both of them, and the particle is therefore said to have mediated (i.e., been the agent of) that force. Force mediating particles are believed to be the reason why the forces and interactions between particles observed in the laboratory and in the universe exist.
The known force mediating particles described by the Standard Model also all have spin (as do matter particles), but in their case, the value of the spin is 1, meaning that all force mediating particles are bosons. As a result, they do not follow the Pauli Exclusion Principle. The different types of force mediating particles are described below.
- Photons mediate the electromagnetic force between electrically charged particles. The photon is massless and is well-described by the theory of quantum electrodynamics.
- The W+, W−, and Z gauge bosons mediate the weak interactions between particles of different flavors (all quarks and leptons). They are massive, with the Z being more massive than the W±. The weak interactions involving the W± act on exclusively left-handed particles and right-handed antiparticles. Furthermore, the W± carry an electric charge of +1 and −1 and couple to the electromagnetic interactions. The electrically neutral Z boson interacts with both left-handed particles and antiparticles. These three gauge bosons along with the photons are grouped together which collectively mediate the electroweak interactions.
- The eight gluons mediate the strong interactions between color charged particles (the quarks). Gluons are massless. The eightfold multiplicity of gluons is labeled by a combination of color and an anticolor charge (e.g., red–antigreen).[9] Because the gluon has an effective color charge, they can interact among themselves. The gluons and their interactions are described by the theory of quantum chromodynamics.
The interactions between all the particles described by the Standard Model are summarized below.
The Higgs boson
The Higgs particle is a hypothetical massive scalar elementary particle predicted by the Standard Model,
and the only fundamental particle predicted by that model which has not been directly observed as yet.This is because it requires an exceptionally large amount of energy to create and observe at high energy colliders. It has no intrinsic spin, and thus, (like the force mediating particles, which also have integral spin) is also classified as a boson.
The Higgs boson plays a unique role in the Standard Model, and a key role in explaining the origins of the mass of other elementary particles, in particular the difference between the massless photon and the very heavy W and Z bosons. Elementary particle masses, and the differences between electromagnetism (caused by the photon) and the weak force (caused by the W and Z bosons), are critical to many aspects of the structure of microscopic (and hence macroscopic) matter. In electroweak theory it generates the masses of the massive leptons (electron, muon and tau); and also of the quarks.
As of September 2008, no experiment has directly detected the existence of the Higgs boson, but there is some indirect evidence for it. It is hoped that upon the completion of the Large Hadron Collider, experiments conducted at CERN would bring experimental evidence confirming the existence of the particle.
The Standard Model predicted the existence of W and Z bosons, the gluon, the top quark and the charm quark before these particles had been observed. Their predicted properties were experimentally confirmed with good precision.
The Large Electron-Positron Collider at CERN tested various predictions about the decay of Z bosons, and found them confirmed.
To get an idea of the success of the Standard Model a comparison between the measured and the predicted values of some quantities are shown in the following table:
| Quantity | Measured (GeV) | SM prediction (GeV) |
|---|---|---|
| Mass of W boson | 80.398±0.025 | 80.3900±0.0180 |
| Mass of Z boson | 91.1876±0.0021 | 91.1874±0.0021 |
Antiparticles
There are also 12 fundamental fermionic antiparticles which correspond to these 12 particles. The positron e+ corresponds to the electron and has an electric charge of +1 and so on:
First generation
| Second generation
| Third generation
|
It should be noted that the table below lists properties of a conceptual model that is still subject to research in modern physics.
| Interaction | Current Theory | Mediators | Relative Strength | Long-Distance Behavior | Range(m) |
|---|---|---|---|---|---|
| Strong | Quantum chromodynamics (QCD) | gluons | 1038 | 1 (see discussion below) | 10-15 |
| Electromagnetic | Quantum electrodynamics (QED) | photons | 1036 |  | infinite |
| Weak | Electroweak Theory | W and Z bosons | 1025 |  | 10-18 |
| Gravitation | General Relativity (GR) | gravitons (not yet discovered) | 1 | | infinite |
The modern quantum mechanical view of the three fundamental forces (all except gravity) is that particles of matter (fermions) do not directly interact with each other, but rather carry a charge, and exchange virtual particles (gauge bosons), which are the interaction carriers or force mediators. For example, photons are the mediators of the interaction of electric charges; and gluons are the mediators of the interaction of color charges.
The interactions
Gravitation
Gravitation is by far the weakest interaction, but at long distances gravity's strength relative to other forces becomes important. There are three reasons for this. First, gravity has an infinite range, like that of electromagnetism. Secondly, all masses are positive and therefore gravity's interaction cannot be screened like in electromagnetism. Finally, gravitational force cannot be absorbed or transformed, and so is permanent. Thus large celestial bodies such as planets, stars and galaxies dominantly feel gravitational forces. In comparison, the total electric charge of these bodies is zero because half of all charges are negative. In addition, unlike the other interactions, gravity acts universally on all matter. There are no objects that lack a gravitational "charge".
Because of its long range, gravity is responsible for such large-scale phenomena as the structure of galaxies, black holes and the expansion of the universe, as well as more elementary astronomical phenomena like the orbits of planets, and everyday experience: objects fall; heavy objects act as if they were glued to the ground; people are limited in how high they can jump.
Gravitation was the first kind of interaction which was described by a mathematical theory. In ancient times, Aristotle theorized that objects of different masses fall at different rates. During the Scientific Revolution, Galileo Galilei experimentally determined that this was not the case — if friction due to air resistance is neglected, all objects accelerate toward the ground at the same rate. Isaac Newton's law of Universal Gravitation (1687) was a good approximation of the general behaviour of gravity. In 1915, Albert Einstein completed the General Theory of Relativity, a more accurate description of gravity in terms of the geometry of space-time.
An area of active research today involves merging the theories of general relativity and quantum mechanics into a more general theory of quantum gravity. It is widely believed that in a theory of quantum gravity, gravity would be mediated by a massless spin 2 particle which is known as the graviton. Gravitons are hypothetical particles not yet observed.
Electromagnetism
Electromagnetism is the force that acts between electrically charged particles. This phenomenon includes the electrostatic force, acting between charges at rest, and the combined effect of electric and magnetic forces acting between charges moving relative to each other.
Electromagnetism is also an infinite-ranged force, but it is much stronger than gravity, and therefore describes almost all phenomena of our everyday experience, ranging from the impenetrability of macroscopic bodies, to lasers and radios, to the structure of atoms and metals, to phenomena such as friction and rainbows.
Electrical and magnetic phenomena have been observed since ancient times, but it was only in the 1800s that scientists discovered that electricity and magnetism are two aspects of the same fundamental interaction. By 1864, Maxwell's equations had rigorously quantified the unified phenomenon. In 1905, Einstein's theory of special relativity resolved the issue of the constancy of the speed of light, and Einstein also explained the photoelectric effect by theorizing that light was transmitted in quanta, which we now call photons. Starting around 1927, Paul Dirac unified quantum mechanics with the relativistic theory of electromagnetism; the theory of quantum electrodynamics was completed in the 1940s by Richard Feynman, Freeman Dyson, Julian Schwinger, and Sin-Itiro Tomonaga.
Weak interaction
The weak interaction or weak nuclear force is responsible for some phenomena at the scales of the atomic nucleus, such as beta decay. Electromagnetism and the weak force are theoretically understood to be two aspects of a unified electroweak interaction — this realization was the first step toward the unified theory known as the Standard Model. In electroweak theory, the carriers of the weak force are massive gauge bosons called the W and Z bosons.
Strong interaction
The strong interaction, or strong nuclear force, is the most complicated force because it behaves differently at different distances. At distances larger than 10 femtometers, the strong force is practically unobservable, which is why it wasn't noticed until the beginning of the 20th century.
After the nucleus was discovered, it was clear that a new force was needed to keep the positive protons in the nucleus from flying out. The force had to be much stronger than electromagnetism, so that the nucleus could be stable even though the protons were so close together, squeezed down to a volume which is 10-15 of the volume of an atom. From the short range of the force, Hideki Yukawa predicted that it was associated with a massive particle, whose mass is approximately 100 MeV. The pion was discovered in 1947 and this discovery marks the beginning of the modern era of particle physics.
QCD is a theory of fractionally charged quarks interacting with 8 photon-like particles called gluons. The gluons interact with each other, not just with the quarks, and at long distances the lines of force collimate into strings.
Challenges to the standard model
The Standard Model of particle physics has been empirically determined through experiments over the past fifty years. Currently the Standard Model predicts that there is one more particle to be discovered, the Higgs boson. One of the reasons for building the Large Hadron Collider is that the increase in energy is expected to make the Higgs observable. However, as of August 2008, there are only indirect experimental indications for the existence of the Higgs boson and it can not be claimed to be found.
There has been a great deal of both theoretical and experimental research exploring whether the Standard Model could be extended into a complete theory of everything. This area of research is often described by the term 'Beyond the Standard Model'. There are several motivations for this research. First, the Standard Model does not attempt to explain gravity, and it is unknown how to combine quantum field theory which is used for the Standard Model with general relativity which is the best physical model of gravity. This means that there is not a good theoretical model for phenomena such as the early universe.
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