Random fields, large deviations and triviality in quantum field theory. Part I
The issue of the existence and possible triviality of the Euclidean quantum scalar field in dimension 4 is investigated by using some large deviations techniques. As usual, the field $φ_{d}^{4}$ is obtained as a limit of regularized fields $φ_{k}^{4}$ associated with a probability measures $μ_{k,V}$, where $k, V$ represent ultraviolet and volume cutoffs. The result obtained is that in a fixed volume, the almost sure limit (as $k \rightarrow \infty$) of the density of $μ_{k,V}$, with respect to the Gaussian free field measure, exists and is equal to $0$, when the coupling constant is not vanishing. This implies that $μ_{k,V}$ can not have a strong limit as the ultraviolet cutoff is removed. Furthermore, the normalization sequence $Z_{k,V}=E e^{-{\cal A}_{k,V}}$ is divergent as $k \rightarrow \infty$ for dimensions $d\geq4$ when the vacuum renormalization is lower than some threshold, which leads to the non ultraviolet stability of the field in this case. These assertions are also valid for vector fields and can be extended to polynomial Lagrangians.