i am trying to understand how to express contractions of field operators via propagators. we are talking about an interacting theory of 2 complex scalar fields, lets call them ψ1 and ψ2. the interaction term is: Lint=λ(ψ2)^3(ψ1) i have found the free propagator defined as: Df-i(x-y)=<0|T(ψi(x) ψi*(y))|0> i=1,2. what i am strugling with is that when considering the free parts this propagator is 0 for <0|T(ψi ψi)|0> (no complex conjugate=no anti-particle). and also it is 0 for <0|T(ψi ψj)|0> ,i≠j. meaning that a field can only contract with it's conjugate counter part in the free theory. so when i try to calculate an interaction corelator in 1st order of different forms i get 0, for instantce: <Ω|T(ψ1 ψ2*)|Ω> this term turns out 0 because there is no one to contract with all the none-conjugate fields in the interaction hamiltonian. am i miss understanding things here, or is the first corelator to be none-zero in this picture of the form: <Ω|T(ψ2* ψ2* ψ2* ψ1* )|Ω> ?? help will be much apriciated. thank you.