Analytic thermal bootstrap in momentum space: From thermal OPE to QNMs

Abstract

We initiate a bootstrap program that relates ultraviolet data, encoded in the thermal OPE, to infrared observables, namely, the low-frequency behavior and quasinormal modes. Starting from KMS-symmetric completions of individual thermal OPE blocks, which play the role of thermal Polyakov blocks, we construct their Fourier transform, yielding an asymptotic expansion of retarded thermal correlators valid at any spatial momentum. We use these results to derive inversion formulae and connect thermal OPE data to the analytic structure of retarded correlators in the complex frequency plane. Under the assumption of meromorphicity, the inversion formulae express OPE coefficients in terms of the quasinormal-mode frequencies, leading to nontrivial sum rules, constraints on the quasinormal spectrum, and its asymptotics at large spatial momentum. We illustrate these results in free theories, two-dimensional CFTs, the large-$N$ limit and $\varepsilon$-expansion of the $\mathrm{O}(N)$ model, and the $R$-current correlator of strongly coupled $\mathcal N = 4$ SYM at zero spatial momentum. As a byproduct, we derive universal asymptotic formulae for thermal OPE coefficients of heavy operators, resolving their dependence on spin and extending previous results at zero spatial separation. We test these formulae in the three-dimensional Ising CFT, finding good agreement between the resulting truncated correlators and Monte Carlo data.