# Adding new branches to the "Christmas tree" of the quasinormal spectrum of black branes

## TL;DR

This paper explores how additional branches of the quasinormal spectrum, resembling a 'Christmas tree', emerge in black branes within holography, especially when considering higher-derivative gravity corrections at finite coupling.

## Contribution

It demonstrates numerically that higher-derivative terms in the gravitational action lead to new spectral branches, aligning with weak coupling expectations and revealing constraints on these terms.

## Key findings

- Additional spectral branches emerge from higher-derivative gravity.
- The phenomenon is robust across different models.
- Constraints on higher-derivative coefficients are identified.

## Abstract

In holography, quasinormal spectra of black branes coincide with the poles of retarded finite-temperature correlation functions of a dual quantum field theory in the limit of infinite number of relevant degrees of freedom such as colours. For asymptotically anti-de Sitter backgrounds, the spectra form a characteristic pattern in the complex frequency plane, colloquially known as the "Christmas tree". At infinite coupling, the tree has only one pair of branches. At large but finite coupling, the branches become more dense and lift up towards the real axis, consistent with the expectation of forming a branch cut in the limit of zero coupling. However, it is known that at zero coupling, the corresponding correlators generically have not one but multiple branch cuts separated by intervals proportional to the Matsubara frequency. This suggests the existence of multiple branches of the "Christmas tree" spectrum in dual gravity. In this note, we show numerically how these additional branches of the spectrum can emerge from the dual gravitational action with higher-derivative terms. This phenomenon appears to be robust, yet, reproducing the expected weak coupling behaviour of the correlators quantitatively implies the existence of certain constraints on the coefficients of the higher-derivative terms of the dual gravity theory.