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Yang–Mills mass gap

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Summary

Pure four-dimensional confining quantum Yang–Mills theory is expected to have a positive energy gap above its vacuum. Constructing the continuum theory rigorously and proving the gap remains an open Clay Millennium problem. This must not be conflated with exact chiral-limit QCD, whose spontaneously broken chiral symmetry gives massless Nambu–Goldstone modes.

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Distinction from QCD

Physical QCD with nonzero light-quark masses has a gapped hadron spectrum. In the exact massless-light-quark limit, however, spontaneous chiral-symmetry breaking produces massless pions, so the full theory does not have a positive spectral gap. The Clay problem concerns pure Yang–Mills theory.

Mathematical status

A satisfactory solution must construct a nontrivial four-dimensional quantum Yang–Mills theory with appropriate axiomatic properties and prove a strictly positive mass gap.

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Yang–Mills theoryYang–Mills mass gap

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Authored explanation

Pure confining four-dimensional quantum Yang–Mills theory is expected to have a positive spectral gap above the vacuum; rigorous continuum construction and proof remain open.

How to interpret this relation type

The target is a collective, organizational, or scale-dependent phenomenon of the source constituents and interactions; it is not a mere list or classical sum of constituents.

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