A meron or half-instanton is a Euclidean space-time

Euclidean space

In mathematics, Euclidean space is the Euclidean plane and three-dimensional space of Euclidean geometry, as well as the generalizations of these notions to higher dimensions...

 solution

Solution

In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...

 of the Yang-Mills field equations

Gauge theory

In physics, gauge invariance is the property of a field theory in which different configurations of the underlying fundamental but unobservable fields result in identical observable quantities. A theory with such a property is called a gauge theory...

. It is a singular non-self-dual solution of topological charge 1/2. The instanton

Instanton

An instanton is a notion appearing in theoretical and mathematical physics. Mathematically, a Yang–Mills instanton is a self-dual or anti-self-dual connection in a principal bundle over a four-dimensional Riemannian manifold that plays the role of physical space-time in non-abelian gauge theory...

 is believed to be composed of two merons.

A meron can be viewed as a tunneling event between two Gribov vacua

Gribov ambiguity

In gauge theory, especially in non-abelian gauge theories, we often encounter global problems when gauge fixing. Gauge fixing means choosing a representative from each gauge orbit. The space of representatives is a submanifold and represents the gauge fixing condition. Ideally, every gauge orbit...

. In that picture, the meron is an event which starts from vacuum, then a Wu-Yang monopole

Wu-Yang monopole

The Wu–Yang monopole was the first solution to the Yang-Mills field equations. It describes a magnetic monopole which is pointlike and has a potential which behaves like 1/r everywhere.-References:...

emerges, which then disappears again to leave the vacuum in another Gribov copy.