Study of bound states in a thermal gas using the S-matrix formalism
We study how to include bound states in a thermal gas in the context of quantum field theory (QFT). We have used a scalar QFT with a $\phi^4$ interaction where the field $\phi$ represents a particle with mass $m$. We observe the creation of bound states when the coupling constant is negative and its modulus is larger than a certain critical value. Then we investigate the contribution of this bound state to the pressure of the thermal gas of the system by using the $S$-matrix formalism.