Growing black-hole hair in nonminimally coupled biscalar gravity

Black holes offer a unique laboratory for fundamental physics and are crucial for probing theories beyond Einstein's theory of General Relativity. In this paper, we consider 4D effective field theories with scalar fields. We focus on axi-dilaton gravity, a quadratic gravity theory with two kinetically coupled scalar fields, an axion and a dilaton. To evolve these fields around black holes, we introduce Canuda-AxiDil, the first open-source, parameterized numerical relativity code for quadratic and bi-scalar gravity. Using this code, we perform single black hole simulations to show the dynamical formation of axion and dilaton hairs. Through these simulations, we measure the impact of black-hole spin and curvature coupling strength on the axion and dilaton, and show that a kinetic coupling between the fields increases the observed deviations from General Relativity. Furthermore, we simulate the axion and dilaton fields around a binary black hole coalescence demonstrating the growth of axion hair during the inspiral and the production of radiative modes for both fields.