Yang–Mills theory

Unsolved problem in physics:

Yang-Mills theory in the non-perturbative regime: confinement. The equations of Yang-Mills remain unsolved at energy scales relevant for describing atomic nuclei. How does Yang-Mills theory give rise to the physics of nuclei and nuclear constituents?

(more unsolved problems in physics)

Yang-Mills theory is a gauge theory based on the SU(N) group. It was formulated by Yang and Mills in 1954 in an effort to extend the original concept of gauge theory for an Abelian group, as was quantum electrodynamics, to the case of a nonabelian group with the intention to get an explanation for strong interactions. This initial idea was not a success as the quanta of the Yang-Mills field must be massless in order to maintain gauge invariance but such massless particles should have had long range effects that are not seen in experiments. So, the idea was put aside till the start of 1960 when the concept of breaking of symmetry in massless theories, initially due to Jeffrey Goldstone, Yoichiro Nambu and Giovanni Jona-Lasinio, with particles acquiring mass in this way, was put forward.

This prompted a significant restart of Yang-Mills theory studies that proved successful in the formulation of both electroweak unification and quantum chromodynamics (QCD). The electroweak interaction is described by SU(2)xU(1) group while QCD is a SU(3) gauge theory. The electroweak theory was obtained combining SU(2) with U(1), being quantum electrodynamics described by U(1), in order to produce the photon field. The Standard Model combines strong, weak and electromagnetic interaction through the symmetry group SU(2)xU(1)xSU(3). Strong interactions are not currently unified with the other two but couplings have been shown to converge to a single value at higher energies in an experiment at LEP, assuming a higher order symmetry as supersymmetry holds.

Phenomenology at lower energies in quantum chromodynamics is not completely understood due to the difficulties of managing such a theory with a strong coupling. This is the reason confinement has not been theoretically proven, though it is a consistent experimental observation. Proof that QCD confines at low energy is a mathematical problem of great relevance, and an award has been proposed by the Clay Mathematics Institute for whoever is able to show that the Yang-Mills theory has a mass gap.

Mathematical overview

Yang-Mills theories are a special example of gauge theory with symmetry non-abelian group given by the Lagrangian

$\ {\mathcal {L}}_{\mathrm {gf} }=-{\frac {1}{4}}\operatorname {Tr} (F_{a}^{\mu \nu }F_{\mu \nu }^{a})$

with the generators of the Lie algebra corresponding to the F-quantities (the curvature or field-strength form) satisfying

$\ [T^{a},T^{b}]=if^{abc}T^{c}$

and the covariant derivative defined as

$\ D_{\mu }=\partial _{\mu }-igT^{a}A_{\mu }^{a}$

where $A_{\mu }^{a}$ is the vector potential, and $g$ is the coupling constant. For D=4, $g$ is a pure number and for a SU(N) group one has

$a,b,c=1\ldots N^{2}-1.$

The relation

$\ F_{\mu \nu }^{a}=\partial _{\mu }A_{\nu }^{a}-\partial _{\nu }A_{\mu }^{a}+gf^{abc}A_{\mu }^{b}A_{\nu }^{c}$

can be derived by the commutator

$\ [D_{\mu },D_{\nu }]=-igT^{a}F_{\mu \nu }^{a}.$

The field has the property of being self-interacting and equations of motion that one obtains are said semilinear, as nonlinearities are both with and without derivatives. This means that one can manage this theory only by perturbation theory, with small nonlinearities. The Yang-Mills equations of motion are

$\ D^{\mu }F_{\mu \nu }^{a}=0,$

with Bianchi identities

$\ D_{\mu }F_{\nu \kappa }^{a}+D_{\kappa }F_{\mu \nu }^{a}+D_{\nu }F_{\kappa \mu }^{a}=0.$

Note that the transition between "upper" ("contravariant") and "lower" ("covariant") vector or tensor components is trivial for a indexes (e.g. $\,f^{abc}=f_{abc}$), whereas for μ and ν it is nontrivial, corresponding e.g. to the usual Lorentz signature, $\eta _{\mu \nu }={\rm {diag}}\,(+---)$.

Quantization of Yang–Mills theory

The most appropriate method to quantize the Yang-Mills theory is by functional methods, i.e. path integrals. One introduces a generating functional for n-point functions as

$\ Z[j]=\int [dA]e^{-{\frac {i}{4}}\int d^{4}x\operatorname {Tr} (F^{\mu \nu }F_{\mu \nu })+i\int d^{4}xj_{\mu }^{a}(x)A^{a\mu }(x)}$

but this integral has no meaning as 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 as it agrees with Fermi-Dirac statistics but it is a complex scalar field, violating in this way the spin-statistics theorem. So, we can write the generating functional as

${\begin{aligned}Z[j,{\bar {\varepsilon }},\varepsilon ]&=\int [dA][d{\bar {c}}][dc]\,e^{\left[-{\frac {i}{4}}\int d^{4}x\operatorname {Tr} (F^{\mu \nu }F_{\mu \nu })-i\int d^{4}x({\bar {c}}^{a}\partial _{\mu }\partial ^{\mu }c^{a}+g{\bar {c}}^{a}f^{abc}\partial _{\mu }A^{b\mu }c^{c})-i\int d^{4}x{\frac {1}{2\alpha }}(\partial \cdot A)^{2}\right]}\times \\&{}\qquad e^{i\int d^{4}xj_{\mu }^{a}(x)A^{a\mu }(x)+i\int d^{4}x[{\bar {c}}^{a}(x)\varepsilon ^{a}(x)+{\bar {\varepsilon }}^{a}(x)c^{a}(x)]}\end{aligned}}$

that is the expression commonly used to derive Feynman's rules (see Feynman diagram). Here we have $c^{a}$ for the ghost field while $\alpha$ fixes the gauge's choice for the quantization. Feynman's rules obtained from this functional are the following

These rules for Feynman diagrams are easily obtained when we realize that the generating functional given above can be rewritten as

${\begin{aligned}Z[j,{\bar {\varepsilon }},\varepsilon ]&=e^{-ig\int d^{4}x\,{\frac {\delta }{i\delta {\bar {\varepsilon }}^{a}(x)}}f^{abc}\partial _{\mu }{\frac {i\delta }{\delta j_{\mu }^{b}(x)}}{\frac {i\delta }{\delta \varepsilon ^{c}(x)}}}e^{-ig\int d^{4}xf^{abc}\partial _{\mu }{\frac {i\delta }{\delta j_{\nu }^{a}(x)}}{\frac {i\delta }{\delta j_{\mu }^{b}(x)}}{\frac {i\delta }{\delta j^{c\nu }(x)}}}\times \\&{}\qquad e^{-i{\frac {g^{2}}{4}}\int d^{4}xf^{abc}f^{ars}{\frac {i\delta }{\delta j_{\mu }^{b}(x)}}{\frac {i\delta }{\delta j_{\nu }^{c}(x)}}{\frac {i\delta }{\delta j^{r\mu }(x)}}{\frac {i\delta }{\delta j^{s\nu }(x)}}}Z_{0}[j,{\bar {\varepsilon }},\varepsilon ]\end{aligned}}$

being

$\ Z_{0}[j,{\bar {\varepsilon }},\varepsilon ]=e^{-\int d^{4}xd^{4}y{\bar {\varepsilon }}^{a}(x)C^{ab}(x-y)\varepsilon ^{b}(y)}e^{{\frac {1}{2}}\int d^{4}xd^{4}yj_{\mu }^{a}(x)D^{ab\mu \nu }(x-y)j_{\nu }^{b}(y)}$

the generating functional of the free theory. Expanding in $g$ and computing the functional derivatives, we are able to obtain all the n-point functions with perturbation theory. Using LSZ reduction formula we get from the n-point functions the corresponding amplitudes for the given processes and cross sections and decay rates are promptly obtained. The theory is renormalizable and corrections are finite at any order of perturbation theory.

For quantum electrodynamics, being in this case abelian the gauge group (see abelian group), the ghost field decouples. This can be easily realized when we look at the coupling between the gauge field and the ghost field that is ${\bar {c}}^{a}f^{abc}\partial _{\mu }A^{b\mu }c^{c}$. For the Abelian case all the structure constants $f^{abc}$ 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 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 assuming the coupling constant $g$ as small (so small nonlinearities), as indeed happens to high energies, and applying perturbation theory. The relevance of this result is due to the fact that a Yang-Mills theory describes strong interactions and asymptotic freedom permits to treat properly experimental results coming from deep inelastic scattering.

In order to obtain the behavior at high energies of the Yang-Mills theory, and so to prove asymptotic freedom, one does perturbation theory assuming a small coupling. This is verified a posteriori in the ultraviolet limit. In the opposite limit, infrared limit, the situation is quite the opposite being the coupling too large for perturbation theory to be reliable. Indeed, most of the difficulties that current research meets is just managing the theory at low energies that is the interesting one 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). Then, 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 right limit of infinite volume (smaller lattice spacing) is hit. This is the limit the results have to be compared with. Smaller spacing and larger coupling are not independent each others and to accomplish both larger computational resources are demanded. As for 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 also in view of the experimental observation of such exotic states. Indeed, $\sigma$ resonance is not seen in any of such lattice computations and contrasting interpretations have been put forward. This is currently a hot debated matter.

Open problems

The Yang-Mills theories were generally acknowledged in the physics community after Gerardus 't Hooft, in 1972, could prove their renormalizability. This applies even if the gauge bosons described by this theory are massive, as in the electroweak theory. However, the mass is only an "acquired" one, namely, as suggested, by the famous Higgs mechanism.

Concerning the mathematics, it should be noted that presently, i.e. in 2009, the Yang-Mills theory is a very active field of research, yielding e.g. a classification of differentiable structures of four-dimensional manifolds by Simon Donaldson. Furthermore, the field of Yang-Mills theories was put by the Clay Mathematics Institute into its list of "Millennium Prize Problems". Here the prize-problem consists, especially, in a proof of the suggestion 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 suggestion, is a proof of the confinement property in the presence of additional Fermion particles.

In physics the survey of Yang-Mills theories does usually not start from perturbation analysis or analytical methods, but more recently from systematic application of numerical methods to lattice gauge theories.

See also

• Yang–Mills existence and mass gap
• Aharonov-Bohm effect
• Bell's theorem
• Coulomb gauge
• Electroweak theory
• Field theoretical formulation of the standard model
• Gauge covariant derivative
• Kaluza-Klein theory
• Lorenz gauge
• Quantum chromodynamics
• Quantum gauge theory
• Symmetry in physics
• Weyl gauge
• Yang–Mills–Higgs equations

Bibliography

• Paul Frampton, Gauge Field Theories, Third, Enlarged and Improved Edition, WILEY-VCH (2008).
• George Svetlichny, Preparation for Gauge Theory, an introduction to mathematical aspects
• David Gross, Gauge theory - Past, Present and Future, Conference notes
• Cheng, T., Li, L., Gauge Theory of Elementary Particle Physics, Oxford University Press, 1983, ISBN 0198519613

External links

• Yang-Mills theory on DispersiveWiki
• The Clay Mathematics Institute
• The Millennium Prize Problems