Higgs production in association with top quark pair at e+e- colliders in theories of higher dimensional gravity -: art. no. 055

Models with large extra compact dimensions predict exciting phenomenological consequences with gravitational interactions becoming strong at the TeV scale. Such theories can be tested at the existing and future colliders. In this paper, we study the contribution of virtual Kaluza-Klein excitations in the process e(+)e(-) --> ttH at a future linear collider. The exchange of virtual KK gravitons is found to modify the cross-section sigma(e(+)e(-) --> ttH) from its Standard Model value to a very significant extent. The consequent sensitivity to the effective string scale M-S depends (weakly) on the number of extra-dimensions n and (strongly) on the center-of-mass energy of the collider. For n = 3, M-S similar or equal to 920 GeV (8 TeV) could be probed at machines working at roots = 500 GeV (3 TeV). More importantly, this process can serve to distinguish such models from other sources of new physics.