Standard Model
It was developed in stages throughout the latter half of the 20th century, through the work of many scientists worldwide,[1] 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 (1995), the tau neutrino (2000), and the Higgs boson (2012) 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[note 1] 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.
The model does not contain any viable dark matter particle that possesses all of the required properties deduced from observational cosmology.
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.
[8] In 1967 Steven Weinberg[9] and Abdus Salam[10] incorporated the Higgs mechanism[11][12][13] 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.
[17] The Higgs mechanism is believed to give rise to the masses of all the elementary particles in the Standard Model.
[22] The theory of the strong interaction (i.e. quantum chromodynamics, QCD), to which many contributed, acquired its modern form in 1973–74 when asymptotic freedom was proposed[23][24] (a development that made QCD the main focus of theoretical research)[25] and experiments confirmed that the hadrons were composed of fractionally charged quarks.
[26][27] The term "Standard Model" was introduced by Abraham Pais and Sam Treiman in 1975,[28] with reference to the electroweak theory with four quarks.
[29] Steven Weinberg has since claimed priority, explaining that he chose the term Standard Model out of a sense of modesty[30][31][32][better source needed] and used it in 1973 during a talk in Aix-en-Provence in France.
[33] The Standard Model includes members of several classes of elementary particles, which in turn can be distinguished by other characteristics, such as color charge.
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.
[40] 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 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.
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.
[34] 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 electroweak theory, the Higgs boson generates the masses of the leptons (electron, muon, and tau) and quarks.
Experiments to confirm and determine the nature of the Higgs boson using the Large Hadron Collider (LHC) 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[clarification needed] at energies above 1.4 TeV;[45] therefore, the LHC (designed to collide two 7 TeV proton beams) was built to answer the question of whether the Higgs boson actually exists.
Upon writing the most general Lagrangian, one finds that the dynamics depends on 19 parameters, whose numerical values are established by experiment.
The quantum chromodynamics (QCD) sector defines the interactions between quarks and gluons, which is a Yang–Mills gauge theory with SU(3) symmetry, generated by
is a three component column vector of Dirac spinors, each element of which refers to a quark field with a specific color charge (i.e. red, blue, and green) and summation over flavor (i.e. up, down, strange, etc.)
acquires a non-zero Vacuum expectation value, which generates masses for the Electroweak gauge fields (the Higgs mechanism), and
[59] Self-consistency of the Standard Model (currently formulated as a non-abelian gauge theory quantized through path-integrals) has not been mathematically proved.
While regularized versions useful for approximate computations (for example lattice gauge theory) exist, it is not known whether they converge (in the sense of S-matrix elements) in the limit that the regulator is removed.
If one insists on using only Standard Model particles, this can be achieved by adding a non-renormalizable interaction of leptons with the Higgs boson.
[61] On a fundamental level, such an interaction emerges in the seesaw mechanism where heavy right-handed neutrinos are added to the theory.
Inadequacies of the Standard Model that motivate such research include: Currently, no proposed theory of everything has been widely accepted or verified.
