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Abstract: We consider the particle spectrum and event shapes in large N gauge theoriesin different regimes of the short-distance -t Hooft coupling, lambda. Themesons in the small lambda limit should have a Regge spectrum in order to agreewith perturbation theory, while generically the large lambda theories withgravity duals produce spectra reminiscent of KK modes. We argue that theseKK-like states are qualitatively different from QCD modes: they are deeplybound states which are sensitive to short distance interactions rather than theflux tube-like states expected in asymptotically free, confining gaugetheories. In addition, we also find that the characteristic event shapes forthe large lambda theories with gravity duals are close to spherical, verydifferent from QCD-like small lambda, small N and Nambu-Goto-like smalllambda, large N theories which have jets. This observation is in agreementwith the conjecture of Strassler on event shapes in large -t Hooft couplingtheories, which was recently proved by Hofman and Maldacena for the conformalcase. This conclusion does not change even when considering soft-wallbackgrounds in the gravity dual. The picture that emerges is the following:theories with small and large lambda are qualitatively different, whiletheories with small and large N are qualitatively similar. Thus it seems thatit is the relative smallness of the -t Hooft coupling in QCD that prevents areliable AdS-QCD correspondence from emerging, and that reproducingcharacteristic QCD-like behavior will require genuine stringy dynamics to beincorporated into any putative dual theory.

Autor: Csaba Csaki, Matthew Reece, John Terning



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