HOME

TheInfoList



OR:

Yang–Mills theory is a quantum field theory for nuclear binding devised by Chen Ning Yang and Robert Mills in 1953, as well as a generic term for the class of similar theories. The Yang–Mills theory is a
gauge theory In physics, a gauge theory is a type of field theory in which the Lagrangian, and hence the dynamics of the system itself, does not change under local transformations according to certain smooth families of operations (Lie groups). Formally, t ...
based on a special unitary group , or more generally any compact Lie group. A Yang–Mills theory seeks to describe the behavior of elementary particles using these non-abelian
Lie group In mathematics, a Lie group (pronounced ) is a group (mathematics), group that is also a differentiable manifold, such that group multiplication and taking inverses are both differentiable. A manifold is a space that locally resembles Eucli ...
s and is at the core of the unification of the electromagnetic force and weak forces (i.e. ) as well as
quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the study of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of ...
, the theory of the strong force (based on ). Thus it forms the basis of the understanding of the
Standard Model The Standard Model of particle physics is the Scientific theory, theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions – excluding gravity) in the unive ...
of particle physics.


History and qualitative description


Gauge theory in electrodynamics

All known fundamental interactions can be described in terms of gauge theories, but working this out took decades.
Hermann Weyl Hermann Klaus Hugo Weyl (; ; 9 November 1885 – 8 December 1955) was a German mathematician, theoretical physicist, logician and philosopher. Although much of his working life was spent in Zürich, Switzerland, and then Princeton, New Jersey, ...
's pioneering work on this project started in 1915 when his colleague
Emmy Noether Amalie Emmy Noether (23 March 1882 – 14 April 1935) was a German mathematician who made many important contributions to abstract algebra. She also proved Noether's theorem, Noether's first and Noether's second theorem, second theorems, which ...
proved that every conserved physical quantity has a matching symmetry, and culminated in 1928 when he published his book applying the geometrical theory of symmetry (
group theory In abstract algebra, group theory studies the algebraic structures known as group (mathematics), groups. The concept of a group is central to abstract algebra: other well-known algebraic structures, such as ring (mathematics), rings, field ( ...
) to quantum mechanics. Weyl named the relevant symmetry in Noether's theorem the "gauge symmetry", by analogy to distance standardization in railroad gauges.
Erwin Schrödinger Erwin Rudolf Josef Alexander Schrödinger ( ; ; 12 August 1887 – 4 January 1961), sometimes written as or , was an Austrian-Irish theoretical physicist who developed fundamental results in quantum field theory, quantum theory. In particul ...
in 1922, three years before working on his equation, connected Weyl's group concept to electron charge. Schrödinger showed that the group U(1) produced a phase shift e^ in electromagnetic fields that matched the conservation of electric charge. As the theory of
quantum electrodynamics In particle physics, quantum electrodynamics (QED) is the Theory of relativity, relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quant ...
developed in the 1930's and 1940's the U(1) group transformations played a central role. Many physicists thought there must be an analog for the dynamics of nucleons. Chen Ning Yang in particular was obsessed with this possibility.


Yang and Mills find the nuclear force gauge theory

Yang's core idea was to look for a conserved quantity in nuclear physics comparable to electric charge and use it to develop a corresponding gauge theory comparable to electrodynamics. He settled on conservation of isospin, a quantum number that distinguishes a neutron from a proton, but he made no progress on a theory. Taking a break from Princeton in the summer of 1953, Yang met a collaborator who could help: Robert Mills. As Mills himself describes:
"During the academic year 1953–1954, Yang was a visitor to Brookhaven National Laboratory ... I was at Brookhaven also ... and was assigned to the same office as Yang. Yang, who has demonstrated on a number of occasions his generosity to physicists beginning their careers, told me about his idea of generalizing gauge invariance and we discussed it at some length ... I was able to contribute something to the discussions, especially with regard to the quantization procedures, and to a small degree in working out the formalism; however, the key ideas were Yang's."
In the summer 1953, Yang and Mills extended the concept of gauge theory for abelian groups, e.g.
quantum electrodynamics In particle physics, quantum electrodynamics (QED) is the Theory of relativity, relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quant ...
, to non-abelian groups, selecting the group to provide an explanation for isospin conservation in collisions involving the strong interactions. Yang's presentation of the work at Princeton in February 1954 was challenged by Pauli, asking about the mass in the field developed with the gauge invariance idea. Pauli knew that this might be an issue as he had worked on applying gauge invariance but chose not to publish it, viewing the massless excitations of the theory to be "unphysical 'shadow particles'". Yang and Mills published in October 1954; near the end of the paper, they admit: This problem of unphysical massless excitation blocked further progress. The idea was set aside until 1960, when the concept of particles acquiring mass through symmetry breaking in massless theories was put forward, initially by Jeffrey Goldstone, Yoichiro Nambu, and Giovanni Jona-Lasinio. This prompted a significant restart of Yang–Mills theory studies that proved successful in the formulation of both electroweak unification and
quantum chromodynamics In theoretical physics, quantum chromodynamics (QCD) is the study of the strong interaction between quarks mediated by gluons. Quarks are fundamental particles that make up composite hadrons such as the proton, neutron and pion. QCD is a type of ...
(QCD). The electroweak interaction is described by the gauge group , while QCD is an Yang–Mills theory. The massless gauge bosons of the electroweak mix after spontaneous symmetry breaking to produce the three massive bosons of the weak interaction (, , and ) as well as the still-massless
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...
field. The dynamics of the photon field and its interactions with matter are, in turn, governed by the gauge theory of quantum electrodynamics. The
Standard Model The Standard Model of particle physics is the Scientific theory, theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions – excluding gravity) in the unive ...
combines the
strong interaction In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interaction, fundamental interactions. It confines Quark, quarks into proton, protons, n ...
with the unified electroweak interaction (unifying the weak and electromagnetic interaction) through the symmetry group . In the current epoch the strong interaction is not unified with the electroweak interaction, but from the observed running of the coupling constants it is believed they all converge to a single value at very high energies.
Phenomenology Phenomenology may refer to: Art * Phenomenology (architecture), based on the experience of building materials and their sensory properties Philosophy * Phenomenology (Peirce), a branch of philosophy according to Charles Sanders Peirce (1839� ...
at lower energies in quantum chromodynamics is not completely understood due to the difficulties of managing such a theory with a strong coupling. This may be the reason why confinement has not been theoretically proven, though it is a consistent experimental observation. This shows why QCD confinement at low energy is a mathematical problem of great relevance, and why the Yang–Mills existence and mass gap problem is a Millennium Prize Problem.


Parallel work on non-Abelian gauge theories

In 1953, in a private correspondence, Wolfgang Pauli formulated a six-dimensional theory of Einstein's field equations of
general relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...
, extending the five-dimensional theory of Kaluza, Klein, Fock, and others to a higher-dimensional internal space. However, there is no evidence that Pauli developed the Lagrangian of a gauge field or the quantization of it. Because Pauli found that his theory "leads to some rather unphysical shadow particles", he refrained from publishing his results formally. Although Pauli did not publish his six-dimensional theory, he gave two seminar lectures about it in Zürich in November 1953. In January 1954 Ronald Shaw, a graduate student at the
University of Cambridge The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209, the University of Cambridge is the List of oldest universities in continuous operation, wo ...
also developed a non-Abelian gauge theory for nuclear forces. However, the theory needed massless particles in order to maintain gauge invariance. Since no such massless particles were known at the time, Shaw and his supervisor Abdus Salam chose not to publish their work. Shortly after Yang and Mills published their paper in October 1954, Salam encouraged Shaw to publish his work to mark his contribution. Shaw declined, and instead it only forms a chapter of his PhD thesis published in 1956.


Mathematical overview

Yang–Mills theories are special examples of gauge theories with a non-abelian symmetry group given by the Lagrangian :\ \mathcal_\mathrm = -\tfrac\operatorname(F^2) = - \tfrac F^ F_^a\ with the generators \ T^a\ of the
Lie algebra In mathematics, a Lie algebra (pronounced ) is a vector space \mathfrak g together with an operation called the Lie bracket, an alternating bilinear map \mathfrak g \times \mathfrak g \rightarrow \mathfrak g, that satisfies the Jacobi ident ...
, indexed by , corresponding to the -quantities (the
curvature In mathematics, curvature is any of several strongly related concepts in geometry that intuitively measure the amount by which a curve deviates from being a straight line or by which a surface deviates from being a plane. If a curve or su ...
or field-strength form) satisfying :\ \operatorname\left( T^a\ T^b \right) = \tfrac \delta^\ , \qquad \left T^a,\ T^b \right= i\ f^\ T^c ~. Here, the are structure constants of the Lie algebra (totally antisymmetric if the generators of the Lie algebra are normalised such that \ \operatorname(T^a\ T^b)\ is proportional to \ \delta^\ ), the
covariant derivative In mathematics and physics, covariance is a measure of how much two variables change together, and may refer to: Statistics * Covariance matrix, a matrix of covariances between a number of variables * Covariance or cross-covariance between ...
is defined as :\ D_\mu = I\ \partial_\mu - i\ g\ T^a\ A^a_\mu\ , is the
identity matrix In linear algebra, the identity matrix of size n is the n\times n square matrix with ones on the main diagonal and zeros elsewhere. It has unique properties, for example when the identity matrix represents a geometric transformation, the obje ...
(matching the size of the generators), \ A^a_\mu\ is the vector potential, and is the coupling constant. In four dimensions, the coupling constant is a pure number and for a group one has \ a, b, c = 1 \ldots n^2-1 ~. The relation :\ F_^a = \partial_\mu A_\nu^a - \partial_\nu A_\mu^a + g\ f^\ A_\mu^b\ A_\nu^c\ can be derived by the
commutator In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory. Group theory The commutator of two elements, ...
:\ \left D_\mu, D_\nu \right= -i\ g\ T^a\ F_^a ~. The field has the property of being self-interacting and the equations of motion that one obtains are said to be semilinear, as nonlinearities are both with and without derivatives. This means that one can manage this theory only by perturbation theory with small nonlinearities. Note that the transition between "upper" ("contravariant") and "lower" ("covariant") vector or tensor components is trivial for ''a'' indices (e.g. \ f^ = f_\ ), whereas for μ and ν it is nontrivial, corresponding e.g. to the usual Lorentz signature, \ \eta_ = (+---) ~. From the given Lagrangian one can derive the equations of motion given by :\ \partial^\mu F_^a + g\ f^\ A^\ F_^c = 0 ~. Putting \ F_=T^aF^a_\ , these can be rewritten as :\ \left( D^\mu F_ \right)^a = 0 ~. A Bianchi identity holds :\ \left( D_\mu\ F_ \right)^a + \left( D_\kappa\ F_ \right)^a + \left( D_\nu\ F_ \right)^a = 0\ which is equivalent to the Jacobi identity :\ \left D_, \left[ D_,D_ \right\right">D_,D__\right.html" ;"title="D_, \left[ D_,D_ \right">D_, \left[ D_,D_ \right\right+ \left D_, \left[ D_,D_ \right\right">D_,D__\right.html" ;"title="D_, \left[ D_,D_ \right">D_, \left[ D_,D_ \right\right+ \left D_, \left[ D_,D_ \right\right">D_,D__\right.html" ;"title="D_, \left[ D_,D_ \right">D_, \left[ D_,D_ \right\right= 0\ since \ \left[ D_,F^a_ \right] = D_\ F^a_ ~. Define the Hodge star operator, dual strength tensor \ \tilde^ = \tfrac\varepsilon^F_\ , then the Bianchi identity can be rewritten as :\ D_\tilde^ = 0 ~. A source \ J_\mu^a\ enters into the equations of motion as :\ \partial^\mu F_^a + g\ f^\ A^\ F_^c = -J_\nu^a ~. Note that the currents must properly change under gauge group transformations. We give here some comments about the physical dimensions of the coupling. In dimensions, the field scales as \ \left \right\left L^ \right and so the coupling must scale as \ \left ^2\right= \left ^\right~. This implies that Yang–Mills theory is not renormalizable for dimensions greater than four. Furthermore, for the coupling is dimensionless and both the field and the square of the coupling have the same dimensions of the field and the coupling of a massless quartic scalar field theory. So, these theories share the scale invariance at the classical level.


Quantization

A method of quantizing the Yang–Mills theory is by functional methods, i.e. path integrals. One introduces a generating functional for -point functions as :\ Z = \int mathrmA \exp\left \tfrac \int \mathrm^4x\ \operatorname\left( F^\ F_\right) + i\ \int \mathrm^4x\ j^a_\mu(x)\ A^(x) \right , but this integral has no meaning as it is because the potential vector can be arbitrarily chosen due to the gauge freedom. This problem was already known for quantum electrodynamics but here becomes more severe due to non-abelian properties of the gauge group. A way out has been given by Ludvig Faddeev and Victor Popov with the introduction of a ''ghost field'' (see Faddeev–Popov ghost) that has the property of being unphysical since, although it agrees with Fermi–Dirac statistics, it is a complex scalar field, which violates the spin–statistics theorem. So, we can write the generating functional as :\begin Z ,\bar\varepsilon,\varepsilon& = \int mathrm\ A mathrm\ \bar c mathrm\ c \exp\Bigl\ \\ &\exp\left\ \end being :S_F=- \tfrac \int \mathrm^4 x\ \operatorname\left( F^\ F_ \right)\ for the field, :S_ = -\frac \int \mathrm^4 x\ (\partial\cdot A)^2\ for the gauge fixing and :\ S_g = -\int \mathrm^4 x\ \left(\bar c^a\ \partial_\mu\partial^\mu c^a + g\ \bar c^a\ f^\ \partial_\mu\ A^\ c^c \right)\ for the ghost. This is the expression commonly used to derive Feynman's rules (see Feynman diagram). Here we have for the ghost field while fixes the gauge's choice for the quantization. Feynman's rules obtained from this functional are the following These rules for Feynman's diagrams can be obtained when the generating functional given above is rewritten as :\begin Z ,\bar\varepsilon,\varepsilon&= \exp\left(-i\ g\int \mathrm^4x\ \frac\ f^\partial_\mu\ \frac\ \frac \right)\\ & \qquad \times \exp\left(-i\ g\int \mathrm^4x\ f^\partial_\mu\frac\frac\ \frac\right)\\ & \qquad \qquad \times \exp\left(-i\ \frac\int \mathrm^4x\ f^\ f^\frac\ \frac\ \frac \frac \right) \\ & \qquad \qquad \qquad \times Z_0 ,\bar\varepsilon,\varepsilon\end with : Z_0 ,\bar\varepsilon,\varepsilon= \exp \left( -\int \mathrm^4x\ \mathrm^4y\ \bar\varepsilon^a(x)\ C^(x-y)\ \varepsilon^b(y) \right)\exp \left( \tfrac \int \mathrm^4x\ \mathrm^4y\ j^a_\mu(x)\ D^(x-y)\ j^b_\nu(y) \right)\ being the generating functional of the free theory. Expanding in and computing the functional derivatives, we are able to obtain all the -point functions with perturbation theory. Using LSZ reduction formula we get from the -point functions the corresponding process amplitudes, cross sections and decay rates. The theory is renormalizable and corrections are finite at any order of perturbation theory. For quantum electrodynamics the ghost field decouples because the gauge group is abelian. This can be seen from the coupling between the gauge field and the ghost field that is \ \bar c^a\ f^\ \partial_\mu A^\ c^c ~. For the abelian case, all the structure constants \ f^\ are zero and so there is no coupling. In the non-abelian case, the ghost field appears as a useful way to rewrite the quantum field theory without physical consequences on the observables of the theory such as cross sections or decay rates. One of the most important results obtained for Yang–Mills theory is asymptotic freedom. This result can be obtained by assuming that the coupling constant is small (so small nonlinearities), as for high energies, and applying perturbation theory. The relevance of this result is due to the fact that a Yang–Mills theory that describes strong interaction and asymptotic freedom permits proper treatment of experimental results coming from deep inelastic scattering. To obtain the behavior of the Yang–Mills theory at high energies, and so to prove asymptotic freedom, one applies perturbation theory assuming a small coupling. This is verified a posteriori in the ultraviolet limit. In the opposite limit, the infrared limit, the situation is the opposite, as the coupling is too large for perturbation theory to be reliable. Most of the difficulties that research meets is just managing the theory at low energies. That is the interesting case, being inherent to the description of hadronic matter and, more generally, to all the observed bound states of gluons and quarks and their confinement (see hadrons). The most used method to study the theory in this limit is to try to solve it on computers (see lattice gauge theory). In this case, large computational resources are needed to be sure the correct limit of infinite volume (smaller lattice spacing) is obtained. This is the limit the results must be compared with. Smaller spacing and larger coupling are not independent of each other, and larger computational resources are needed for each. As of today, the situation appears somewhat satisfactory for the hadronic spectrum and the computation of the gluon and ghost propagators, but the glueball and hybrids spectra are yet a questioned matter in view of the experimental observation of such exotic states. Indeed, the resonance is not seen in any of such lattice computations and contrasting interpretations have been put forward. This is a hotly debated issue.


Open problems

Yang–Mills theories met with general acceptance in the physics community after Gerard 't Hooft, in 1972, worked out their renormalization, relying on a formulation of the problem worked out by his advisor Martinus Veltman. Renormalizability is obtained even if the gauge bosons described by this theory are massive, as in the electroweak theory, provided the mass is only an "acquired" one, generated by the
Higgs mechanism In the Standard Model of particle physics, the Higgs mechanism is essential to explain the Mass generation, generation mechanism of the property "mass" for gauge bosons. Without the Higgs mechanism, all bosons (one of the two classes of particles ...
. The mathematics of the Yang–Mills theory is a very active field of research, yielding e.g. invariants of differentiable structures on four-dimensional manifolds via work of
Simon Donaldson Sir Simon Kirwan Donaldson (born 20 August 1957) is an English mathematician known for his work on the topology of smooth function, smooth (differentiable) four-dimensional manifolds, Donaldson–Thomas theory, and his contributions to Kähl ...
. Furthermore, the field of Yang–Mills theories was included in the Clay Mathematics Institute's list of "
Millennium Prize Problems The Millennium Prize Problems are seven well-known complex mathematics, mathematical problems selected by the Clay Mathematics Institute in 2000. The Clay Institute has pledged a US $1 million prize for the first correct solution to each problem ...
". Here the prize-problem consists, especially, in a proof of the conjecture that the lowest excitations of a pure Yang–Mills theory (i.e. without matter fields) have a finite mass-gap with regard to the vacuum state. Another open problem, connected with this conjecture, is a proof of the confinement property in the presence of additional fermions. In physics the survey of Yang–Mills theories does not usually start from perturbation analysis or analytical methods, but more recently from systematic application of numerical methods to lattice gauge theories.


See also

* Aharonov–Bohm effect * Coulomb gauge * Deformed Hermitian Yang–Mills equations * Gauge covariant derivative *
Gauge theory (mathematics) In mathematics, and especially differential geometry and mathematical physics, gauge theory is the general study of Connection (mathematics), connections on vector bundles, principal bundles, and fibre bundles. Gauge theory in mathematics should ...
* Hermitian Yang–Mills equations * Kaluza–Klein theory * Lattice gauge theory * Lorenz gauge * = 4 supersymmetric Yang–Mills theory * Propagator * Quantum gauge theory * Field theoretical formulation of the standard model * Symmetry in physics * Two-dimensional Yang–Mills theory * Weyl gauge * Yang–Mills equations * F-Yang–Mills equations * Bi-Yang–Mills equations * Yang–Mills existence and mass gap * Yang–Mills–Higgs equations


References


Further reading

; Books * * * ; Articles * *


External links

* * * {{DEFAULTSORT:Yang-Mills Theory Gauge theories Symmetry