Abstract
We consider the particle spectrum and event shapes in large N gauge theories in different regimes of the short-distance 't Hooft coupling, lambda. The mesons in the small lambda limit should have a Regge spectrum in order to agree with perturbation theory, while generically the large lambda theories with gravity duals produce spectra reminiscent of KK modes. We argue that these KK-like states are qualitatively different from QCD modes: they are deeply bound states which are sensitive to short distance interactions rather than the flux tube-like states expected in asymptotically free, confining gauge theories. In addition, we also find that the characteristic event shapes for the large lambda theories with gravity duals are close to spherical, very different from QCD-like (small lambda, small N) and Nambu-Goto-like (small lambda, large N) theories which have jets. This observation is in agreement with the conjecture of Strassler on event shapes in large 't Hooft coupling theories, which was recently proved by Hofman and Maldacena for the conformal case. This conclusion does not change even when considering soft-wall backgrounds in the gravity dual. The picture that emerges is the following: theories with small and large lambda are qualitatively different, while theories with small and large N are qualitatively similar. Thus it seems that it is the relative smallness of the 't Hooft coupling in QCD that prevents a reliable AdS/QCD correspondence from emerging, and that reproducing characteristic QCD-like behavior will require genuine stringy dynamics to be incorporated into any putative dual theory.
Highlights
Confining gauge theories play a central role in particle physics: they provide the real theory of strong interactions (QCD), and possible models of dynamical electroweak symmetry breaking (EWSB) and dynamical supersymmetry breaking
One recent development that suggests that AdS/QCD is not that similar to real QCD is the conjecture of Strassler that event shapes in theories at large ’t Hooft coupling are spherical [3]
We found that for small ’t Hooft coupling agreement with perturbation theory suggests that the mass spectrum should asymptotically show the characteristic Regge-type behavior rather than the KK-type scaling, that arises in the large λ limit
Summary
Confining gauge theories play a central role in particle physics: they provide the real theory of strong interactions (QCD), and possible models of dynamical electroweak symmetry breaking (EWSB) and dynamical supersymmetry breaking. The analysis of the spectrum continues, which presents a simple quantum-mechanical toy model that illustrates how the spectrum of light states can change continuously from flux tube-type to RS-type as a coupling constant is varied This suggests that the KK-like modes that are characteristic to AdS/QCD are not QCD states, but rather they are deeply bound mesons, that are more related to the short-distance Coulomb-like potential, and not to the long-distance confining linear potential. We study models of flux tube evolution and breaking (motivated by Schwinger pair-production in the chromoelectric field), and find that the resulting events have a characteristic “jetty” structure, with energetic particles moving in opposite directions and only relatively soft particles in between This is very similar to the well-known jet structure that arises from perturbative QCD and the parton shower, independently of any assumptions about hadronization. This suggests that if AdS/QCD is ever to be useful for truly QCD-like theories, we will need qualitatively new ideas and a better understanding of this transition region
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