In physics, the cluster decomposition property is related to locality in quantum field theory. In a quantum field theory having this property, the vacuum expectation value of a product of many operators - each of them being either in region A or in region B where A and B are very separated - asymptotically equals the product of the expectation value of the product of the operators in A, times a similar factor from the region B. Consequently, sufficiently separated regions behave independently.
If A1, ..., An are n operators each localized in a bounded region and
where
provided a is a spacelike vector.
Expressed in terms of the connected correlation functions, it means if some of the arguments of the connected correlation function are shifted by large spacelike separations, the function goes to zero.
This property only holds if the vacuum is a pure state. If the vacuum is degenerate and we have a mixed state, the cluster decomposition property fails.
If the theory has a mass gap