Search for Additional Quasinormal Modes in Black-Hole Ringdowns

The General Theory of Relativity predicts that the ringdown phase of black holes is governed by a discrete spectrum of damped oscillations, known as quasinormal modes (QNMs), whose frequencies and damping times depend solely on the mass and spin of the final object. While current gravitational-wave observations have confirmed the dominant Kerr modes, present measurement precision does not yet allow a definitive assessment of the completeness of the observed spectrum. In this work, we formulate a strictly phenomenological framework to search for potential deviations from the classical Kerr quasinormal-mode spectrum. Without introducing theoretical assumptions about their origin, we consider the possibility of additional subdominant modes, frequency shifts, or modified damping rates in the ringdown signal. We propose a model-agnostic parametrization of such effects, identify robust observable signatures, and formulate clear falsification criteria. The approach is fully compatible with General Relativity and is directly testable with current and future gravitational-wave detectors, including LIGO, Virgo, KAGRA, and LISA.