Maximum of the characteristic polynomial of random unitary matrices

TL;DR

This paper confirms the leading order growth rate of the maximum of the characteristic polynomial of random unitary matrices, using branching random walk techniques and Toeplitz determinant analysis, and proves the freezing transition conjecture.

Contribution

It verifies the conjectured growth rate of the maximum of the characteristic polynomial and establishes the freezing transition for random unitary matrices.

Findings

Maximum of the characteristic polynomial is in [N^{1 - ε}, N^{1 + ε}] with high probability.

Confirmed the freezing of the free energy for random unitary matrices.

Developed methods linking Fourier decomposition to branching random walks.

Abstract

It was recently conjectured by Fyodorov, Hiary and Keating that the maximum of the characteristic polynomial on the unit circle of a $N\times N$ random unitary matrix sampled from the Haar measure grows like $CN/(\log N)^{3/4}$ for some random variable $C$. In this paper, we verify the leading order of this conjecture, that is, we prove that with high probability the maximum lies in the range $[N^{1 - \varepsilon},N^{1 + \varepsilon}]$, for arbitrarily small $\varepsilon$. The method is based on identifying an approximate branching random walk in the Fourier decomposition of the characteristic polynomial, and uses techniques developed to describe the extremes of branching random walks and of other log-correlated random fields. A key technical input is the asymptotic analysis of Toeplitz determinants with dimension-dependent symbols. The original argument for these asymptotics followed the general idea that the statistical mechanics of $1/f$-noise random energy models is governed by a freezing transition. We also prove the conjectured freezing of the free energy for random unitary matrices.