“…In this paper, we have shown that the FRG approach is also a very powerful method, notably at zero temperature, to determine the physical properties of the massive sine-Gordon model, which is the bosonized version of the massive Schwinger model. More generally, the FRG is very useful to study quantum systems in d = 1 + 1 dimensions where soliton-like excitations may play a crucial role [45] By computing the critical exponents, we confirm that the phase transition ocurring in the massive Schwinger model for a value θ = π of the vacuum angle belongs to the two-dimensional Ising universality class, as had been shown in several previous studies [13,15]. Though the precision of the critical exponents is typical of a secondorder derivative expansion (DE), more accurate results could be obtained by pushing the expansion to higher orders [46,47].…”