What we (don't) know about black-hole formation in high-energy collisions

Higher-dimensional scenarios allow for the formation of mini black holes from TeV-scale particle collisions. The purpose of this letter is to review and compare different methods for the estimate of the total gravitational energy emitted in this process. To date, black-hole formation has mainly been studied using an apparent horizon search technique. This approach yields only an upper bound on the gravitational energy emitted during black-hole formation. Alternative calculations based on instantaneous collisions of point particles and black-hole perturbation theory suggest that the emitted gravitational energy may be smaller. New and more refined methods may be necessary to accurately describe black-hole formation in high-energy particle collisions.