Standard Model
The Standard Model of particle physics is the theory describing three of the four known fundamental forces in the universe and classifying all known elementary particles. It was developed in stages throughout the latter half of the 20th century, through the work of many scientists worldwide, with the current formulation being finalized in the mid-1970s upon experimental confirmation of the existence of quarks. Since then, proof of the top quark, the tau neutrino, and the Higgs boson have added further credence to the Standard Model. In addition, the Standard Model has predicted various properties of weak neutral currents and the W and Z bosons with great accuracy.
Although the Standard Model is believed to be theoretically self-consistent and has demonstrated some success in providing experimental predictions, it leaves some physical phenomena unexplained and so falls short of being a complete theory of fundamental interactions. For example, it does not fully explain why there is more matter than anti-matter, incorporate the full theory of gravitation as described by general relativity, or account for the universe's accelerating expansion as possibly described by dark energy. The model does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology. It also does not incorporate neutrino oscillations and their non-zero masses.
The development of the Standard Model was driven by theoretical and experimental particle physicists alike. The Standard Model is a paradigm of a quantum field theory for theorists, exhibiting a wide range of phenomena, including spontaneous symmetry breaking, anomalies, and non-perturbative behavior. It is used as a basis for building more exotic models that incorporate hypothetical particles, extra dimensions, and elaborate symmetries to explain experimental results at variance with the Standard Model, such as the existence of dark matter and neutrino oscillations.
Historical background
In 1928, Paul Dirac introduced the Dirac equation, which implied the existence of antimatter. In 1954, Yang Chen-Ning and Robert Mills extended the concept of gauge theory for abelian groups, e.g. quantum electrodynamics, to nonabelian groups to provide an explanation for strong interactions. In 1957, Chien-Shiung Wu demonstrated parity was not conserved in the weak interaction. In 1961, Sheldon Glashow combined the electromagnetic and weak interactions. In 1964, Murray Gell-Mann and George Zweig introduced quarks and that same year Oscar W. Greenberg implicitly introduced color charge of quarks. In 1967 Steven Weinberg and Abdus Salam incorporated the Higgs mechanism into Glashow's electroweak interaction, giving it its modern form.In 1970, Sheldon Glashow, John Iliopoulos, and Luciano Maiani introduced the GIM mechanism, predicting the charm quark. In 1973 Gross and Wilczek and Politzer independently discovered that non-Abelian gauge theories, like the color theory of the strong force, have asymptotic freedom. In 1976, Martin Perl discovered the tau lepton at the SLAC. In 1977, a team led by Leon Lederman at Fermilab discovered the bottom quark.
The Higgs mechanism is believed to give rise to the masses of all the elementary particles in the Standard Model. This includes the masses of the W and Z bosons, and the masses of the fermions, i.e. the quarks and leptons.
After the neutral weak currents caused by Z boson exchange were discovered at CERN in 1973, the electroweak theory became widely accepted and Glashow, Salam, and Weinberg shared the 1979 Nobel Prize in Physics for discovering it. The W± and Z0 bosons were discovered experimentally in 1983; and the ratio of their masses was found to be as the Standard Model predicted.
The theory of the strong interaction, to which many contributed, acquired its modern form in 1973–74 when asymptotic freedom was proposed and experiments confirmed that the hadrons were composed of fractionally charged quarks.
The term "Standard Model" was introduced by Abraham Pais and Sam Treiman in 1975, with reference to the electroweak theory with four quarks. Steven Weinberg has since claimed priority, explaining that he chose the term Standard Model out of a sense of modesty and used it in 1973 during a talk in Aix-en-Provence in France.
Particle content
The Standard Model includes members of several classes of elementary particles, which in turn can be distinguished by other characteristics, such as color charge.All particles can be summarized as follows:
Fermions
The Standard Model includes 12 elementary particles of spin, known as fermions. Fermions respect the Pauli exclusion principle, meaning that two identical fermions cannot simultaneously occupy the same quantum state in the same atom. Each fermion has a corresponding antiparticle, which are particles that have corresponding properties with the exception of opposite charges. Fermions are classified based on how they interact, which is determined by the charges they carry, into two groups: quarks and leptons. Within each group, pairs of particles that exhibit similar physical behaviors are then grouped into generations. Each member of a generation has a greater mass than the corresponding particle of generations prior. Thus, there are three generations of quarks and leptons. As first-generation particles do not decay, they comprise all of ordinary matter. Specifically, all atoms consist of electrons orbiting around the atomic nucleus, ultimately constituted of up and down quarks. On the other hand, second- and third-generation charged particles decay with very short half-lives and can only be observed in high-energy environments. Neutrinos of all generations also do not decay, and pervade the universe, but rarely interact with baryonic matter.There are six quarks: up, down, charm, strange, top, and bottom. Quarks carry color charge, and hence interact via the strong interaction. The color confinement phenomenon results in quarks being strongly bound together such that they form color-neutral composite particles called hadrons; quarks cannot individually exist and must always bind with other quarks. Hadrons can contain either a quark-antiquark pair or three quarks. The lightest baryons are the nucleons: the proton and neutron. Quarks also carry electric charge and weak isospin, and thus interact with other fermions through electromagnetism and weak interaction. The six leptons consist of the electron, electron neutrino, muon, muon neutrino, tau, and tau neutrino. The leptons do not carry color charge, and do not respond to strong interaction. The charged leptons carry an electric charge of −1 e, while the three neutrinos carry zero electric charge. Thus, the neutrinos' motions are influenced by only the weak interaction and gravity, making them difficult to observe.
Gauge bosons
The Standard Model includes 4 kinds of gauge bosons of spin 1, with bosons being quantum particles containing an integer spin. The gauge bosons are defined as force carriers, as they are responsible for mediating the fundamental interactions. The Standard Model explains the four fundamental forces as arising from the interactions, with fermions exchanging virtual force carrier particles, thus mediating the forces. At a macroscopic scale, this manifests as a force. As a result, they do not follow the Pauli exclusion principle that constrains fermions; bosons do not have a theoretical limit on their spatial density. The types of gauge bosons are described below.; Electromagnetism: Photons mediate the electromagnetic force, responsible for interactions between electrically charged particles. The photon is massless and is described by the theory of quantum electrodynamics.
; Strong interaction: Gluons mediate the strong interactions, which binds quarks to each other by influencing the color charge, with the interactions being described in the theory of quantum chromodynamics. They have no mass, and there are eight distinct gluons, with each being denoted through a color-anticolor charge combination. As gluons have an effective color charge, they can also interact amongst themselves.
; Weak interaction: The,, and gauge bosons mediate the weak interactions between all fermions, being responsible for radioactivity. They contain mass, with the having more mass than the. The weak interactions involving the act only on left-handed particles and right-handed antiparticles respectively. The carries an electric charge of +1 and −1 and couples to the electromagnetic interaction. The electrically neutral boson interacts with both left-handed particles and right-handed antiparticles. These three gauge bosons along with the photons are grouped together, as collectively mediating the electroweak interaction.
; Gravitation: It is currently unexplained in the Standard Model, as the hypothetical mediating particle graviton has been proposed, but not observed. This is due to the incompatibility of quantum mechanics and Einstein's theory of general relativity, regarded as being the best explanation for gravity. In general relativity, gravity is explained as being the geometric curving of spacetime.
The Feynman diagram calculations, which are a graphical representation of the perturbation theory approximation, invoke "force mediating particles", and when applied to analyze high-energy scattering experiments are in reasonable agreement with the data. However, perturbation theory fails in other situations. These include low-energy quantum chromodynamics, bound states, and solitons. The interactions between all the particles described by the Standard Model are summarized by the diagrams on the right of this section.
Higgs boson
The Higgs particle is a massive scalar elementary particle theorized by Peter Higgs in 1964, when he showed that Goldstone's 1962 theorem provides a third polarization of a massive vector field. Hence, Goldstone's original scalar doublet, the massive spin-zero particle, was proposed as the Higgs boson, and is a key building block in the Standard Model. It has no intrinsic spin, and for that reason is classified as a boson with spin-0.The Higgs boson plays a unique role in the Standard Model, by explaining why the other elementary particles, except the photon and gluon, are massive. In particular, the Higgs boson explains why the photon has no mass, while the W and Z bosons are very heavy. Elementary-particle masses and the differences between electromagnetism and the weak force are critical to many aspects of the structure of microscopic matter. In electroweak theory, the Higgs boson generates the masses of the leptons and quarks. As the Higgs boson is massive, it must interact with itself.
Because the Higgs boson is a very massive particle and also decays almost immediately when created, only a very high-energy particle accelerator can observe and record it. Experiments to confirm and determine the nature of the Higgs boson using the Large Hadron Collider at CERN began in early 2010 and were performed at Fermilab's Tevatron until its closure in late 2011. Mathematical consistency of the Standard Model requires that any mechanism capable of generating the masses of elementary particles must become visible at energies above ; therefore, the LHC was built to answer the question of whether the Higgs boson actually exists.
On 4 July 2012, two of the experiments at the LHC both reported independently that they had found a new particle with a mass of about , which is "consistent with the Higgs boson". On 13 March 2013, it was confirmed to be the searched-for Higgs boson.