The Yang-Mills existence and mass gap problem asks for a mathematically rigorous construction of a nontrivial quantum Yang-Mills
theory on with a positive mass gap, for
every compact simple gauge group
(Jaffe and Witten). Here
is a compact Lie group
with a simple Lie algebra. The construction
must satisfy the axiomatic requirements of quantum
field theory specified in the official problem description. A positive mass
gap requires the energy operator spectrum
above the vacuum to be bounded away from zero.
This is one of the Millennium Prize Problems and remains unsolved (Clay Mathematics Institute). The problem concerns the existence and properties of the quantum field theory, rather than merely finding solutions of the classical Yang-Mills equation.