Exploring Nonperturbative Behaviour of Moments and Cumulants in Quantum Theories

Abstract

The dynamics of quantum fields become nonperturbative when their interactions are probed by a large number of particles. To explore this regime we study correlation functions which involve a large number of fields, focussing on massive scalar theories that feature arbitrary self-interactions, $\phi^{2p}$. Treating quantum fields as operator-valued distributions, we investigate $n$-point correlation functions at ultra-short distances and compute moments and cumulants of fields, using a semiclassical saddle point approximation in the double scaling limit of weak coupling, $\lambda \to 0$, large quantum number, $n \to \infty$, while keeping $\lambda n$ constant. Addressing the nonperturbative regime, where $\lambda n \gtrsim 1$, requires a resummation of the effective saddle point to all orders in $\lambda n$. We perform this resummation in zero and one dimensions, and show that the moments, corresponding to correlation functions including disconnected contributions, grow exponentially with $n$. This growth is significantly reduced for higher-order self-interactions, i.e. for larger $p$. On the other hand, we argue that the cumulants, which represent connected correlation functions, grow even more rapidly and are mostly independent of $p$.