Yang–Mills existence and mass gap
In mathematical physics, the Yang–Mills existence and mass gap problem is an unsolved problem and one of the seven Millennium Prize Problems defined by the Clay Mathematics Institute which has offered a prize of US$1,000,000 to the one who solves it.
Contents
Official problem description
The problem is phrased as follows:^{[1]}
 Yang–Mills Existence and Mass Gap. Prove that for any compact simple gauge group G, a nontrivial quantum Yang–Mills theory exists on and has a mass gap Δ > 0. Existence includes establishing axiomatic properties at least as strong as those cited in Template:Harvtxt, Template:Harvtxt and Template:Harvtxt.
In this statement, Yang–Mills theory is the (nonAbelian) quantum field theory underlying the Standard Model of particle physics; is Euclidean 4space; the mass gap Δ is the mass of the least massive particle predicted by the theory.
Therefore, the winner must prove that:
 Yang–Mills theory exists and satisfies the standard of rigor that characterizes contemporary mathematical physics, in particular constructive quantum field theory,^{[2]}^{[3]} and
 The mass of the least massive particle of the force field predicted by the theory is strictly positive.
For example, in the case of G=SU(3)—the strong nuclear interaction—the winner must prove that glueballs have a lower mass bound, and thus cannot be arbitrarily light.
Background
“  [...] one does not yet have a mathematically complete example of a quantum gauge theory in fourdimensional spacetime, nor even a precise definition of quantum gauge theory in four dimensions. Will this change in the 21st century? We hope so!  ” 
— From the Clay Institute's official problem description by Arthur Jaffe and Edward Witten.

The problem requires the construction of a QFT satisfying the Wightman axioms and showing the existence of a mass gap. Both of these topics are described in sections below.
The Wightman axioms
{{#invoke:mainmain}} The Millenium problem requires the proposed YangMill theory to satisfy the Wightman axioms or similarly stringent axioms.^{[1]} There are four axioms:
 W0 (assumptions of relativistic quantum mechanics)
Quantum mechanics is described according to von Neumann; in particular, the pure states are given by the rays, i.e. the onedimensional subspaces, of some separable complex Hilbert space.
The Wightman axioms require that the Poincaré group acts unitarily on the Hilbert space. In other words, they have position dependent operators called quantum fields which form covariant representations of the Poincaré group.
The group of spacetime translations is commutative, and so the operators can be simultaneously diagonalised. The generators of these groups give us four selfadjoint operators, , j = 1, 2, 3, which transform under the homogeneous group as a fourvector, called the energymomentum fourvector.
The second part of the zeroth axiom of Wightman is that the representation U(a, A) fulfills the spectral condition  that the simultaneous spectrum of energymomentum is contained in the forward cone:
The third part of the axiom is that there is a unique state, represented by a ray in the Hilbert space, which is invariant under the action of the Poincaré group. It is called a vacuum.
 W1 (assumptions on the domain and continuity of the field)
For each test function f, there exists a set of operators which, together with their adjoints, are defined on a dense subset of the Hilbert state space, containing the vacuum. The fields A are operatorvalued tempered distributions. The Hilbert state space is spanned by the field polynomials acting on the vacuum (cyclicity condition).
 W2 (transformation law of the field)
The fields are covariant under the action of Poincaré group, and they transform according to some representation S of the Lorentz group, or SL(2,C) if the spin is not integer:
 W3 (local commutativity or microscopic causality)
If the supports of two fields are spacelike separated, then the fields either commute or anticommute.
Cyclicity of a vacuum, and uniqueness of a vacuum are sometimes considered separately. Also, there is property of asymptotic completeness  that Hilbert state space is spanned by the asymptotic spaces and , appearing in the collision S matrix. The other important property of field theory is mass gap which is not required by the axioms  that energymomentum spectrum has a gap between zero and some positive number.
Mass gap
{{#invoke:mainmain}} In quantum field theory, the mass gap is the difference in energy between the vacuum and the next lowest energy state. The energy of the vacuum is zero by definition, and assuming that all energy states can be thought of as particles in planewaves, the mass gap is the mass of the lightest particle.
For a given real field , we can say that the theory has a mass gap if the twopoint function has the property
with being the lowest energy value in the spectrum of the Hamiltonian and thus the mass gap. This quantity, easy to generalize to other fields, is what is generally measured in lattice computations. It was proved in this way that Yang–Mills theory develops a mass gap on a lattice.^{[4]}^{[5]}
Importance of YangMills theory
Most known and nontrivial (i.e. interacting) quantum field theories in 4 dimensions are effective field theories with a cutoff scale. Since the betafunction is positive for most models, it appears that most such models have a Landau pole as it is not at all clear whether or not they have nontrivial UV fixed points. This means that if such a QFT is welldefined at all scales, as it has to be to satisfy the axioms of axiomatic quantum field theory, it would have to be trivial (i.e. a free field theory).
Quantum YangMills theory with a nonabelian gauge group and no quarks is an exception, because asymptotic freedom characterizes this theory, meaning that it has a trivial UV fixed point. Hence it is the simplest nontrivial constructive QFT in 4 dimensions. (QCD is a more complicated theory because it involves quarks.)
Quark confinement
It has already been well proven—at least at the level of rigor of theoretical physics but not that of mathematical physics—that the quantum Yang–Mills theory for a nonabelian Lie group exhibits a property known as confinement. This property is covered in more detail in the relevant QCD articles (QCD, color confinement, lattice gauge theory, etc.), although not at the level of rigor of mathematical physics. A consequence of this property is that beyond a certain scale, known as the QCD scale (more properly, the confinement scale, as this theory is devoid of quarks), the color charges are connected by chromodynamic flux tubes leading to a linear potential between the charges. Hence free color charge and free gluons cannot exist. In the absence of confinement, we would expect to see massless gluons, but since they are confined, all we would see are colorneutral bound states of gluons, called glueballs. If glueballs exist, they are massive, which is why we expect a mass gap.
References
 ↑ ^{1.0} ^{1.1} Arthur Jaffe and Edward Witten "Quantum YangMills theory." Official problem description.
 ↑ R. Streater and A. Wightman, PCT, Spin and Statistics and all That, W. A. Benjamin, New York, 1964.
 ↑ K. Osterwalder and R. Schrader, Axioms for Euclidean Green’s functions, Comm. Math. Phys. 31 (1973), 83–112, and Comm. Math. Phys. 42 (1975), 281–305.
 ↑ {{#invoke:Citation/CS1citation CitationClass=journal }}.
 ↑ {{#invoke:Citation/CS1citation CitationClass=journal }}.
External links
 The Millennium Prize Problems: Yang–Mills and Mass Gap
 {{#invoke:citation/CS1citation
CitationClass=book }}
 {{#invoke:Citation/CS1citation
CitationClass=journal }}
 {{#invoke:Citation/CS1citation
CitationClass=journal }}
 {{#invoke:citation/CS1citation
CitationClass=book }}
 {{#invoke:citation/CS1citation
CitationClass=book }}
 {{#invoke:Citation/CS1citation
CitationClass=journal }}
 {{#invoke:Citation/CS1citation
CitationClass=journal }}
 {{#invoke:citation/CS1citation
CitationClass=book }}