  We introduce an algorithm to uncover the activated particle rearrangements,
or excitations, regulating structural relaxation in glasses at much higher
energies than previously achieved. We use it to investigate the density and
geometric properties of excitations in a model system. We find that the density
of excitations behaves as a shifted power-law, and confirm that this shift
accounts for the increase in the activation energy controlling the relaxation
dynamics. Remarkably, we find that excitations comprise a core whose
properties, including the displacement of the particle moving the most, scale
as a power-law of their activation energy and do not depend on temperature.
Excitations also present an outer deformation field that depends on the
material stability and, hence, on temperature. Our analysis suggests that while
excitations suppress the transition of dynamical arrest predicted by mean-field
theories, they are strongly influenced by it.
