Directed polymer for very heavy tailed random walks

Abstract

In the present work, we investigate the case of directed polymer in a random environment (DPRE), when the increments of the one-dimensional random walk are heavy-tailed with tail-exponent equal to zero ($\mathbf{P}[|{\mathit{X}}_1|\ge \mathit{n}]$ decays slower than any power of n). This case has not yet been studied in the context of directed polymers and presents key differences with the simple symmetric random walk case and the cases where the increments belong to the domain of attraction of an α-stable law, where $\mathit{\alpha }\in (0,2]$. We establish the absence of a very strong disorder regime—that is, the free energy equals zero at every temperature—for every disorder distribution. We also prove that a strong disorder regime (partition function converging to zero at low temperature) may exist or not depending on finer properties of the random walk: we establish nonmatching necessary and sufficient conditions for having a phase transition from weak to strong disorder. In particular our results imply that for this directed polymer model, very strong disorder is not equivalent to strong disorder, shedding a new light on a long standing conjecture concerning the original nearest-neighbor DPRE.