Cluster decomposition Property

Lorentz covariant

asymptotic state: https://physics.stackexchange.com/questions/765258/proof-that-asymptotic-particle-states-are-free

https://en.wikipedia.org/wiki/S-matrix

$$ \hat \phi (x)=\int d\vec p\sum_\sigma(u(\vec p,x,\sigma)\hat a(\vec p ,\sigma)+v\hat a^\dagger) $$