QCD and strongly coupled gauge theories : challenges and perspectives

We highlight the progress, current status, and open challenges of QCD-driven physics, in theory and in experiment. We discuss how the strong interaction is intimately connected to a broad sweep of physical problems, in settings ranging from astrophysics and cosmology to strongly coupled, complex systems in particle and condensed-matter physics, as well as to searches for physics beyond the Standard Model. We also discuss how success in describing the strong interaction impacts other fields, and, in turn, how such subjects can impact studies of the strong interaction. In the course of the work we offer a perspective on the many research streams which flow into and out of QCD, as well as a vision for future developments.Peer reviewe

Cyclotron Resonance of Composite Fermions

The introduction of suitable fictitious entities occasionally permits to cast otherwise difficult strongly interacting many-body systems in a single particle form. We can then take the customary physical approach, using concepts and representations which formerly could only be applied to systems with weak interactions, and yet still capture the essential physics. A most notable recent example occurs in the conduction properties of a two-dimensional electron system (2DES), when exposed to a strong perpendicular magnetic field B. They are governed by electron–electron interactions, that bring about the fractional quantum hall effect (FQHE). S. Das Sarma and A. Pinczuk (eds.), Perspectives on Quantum Hall Effects (Wiley, New York, 1996). Composite fermions, that do not experience the external magnetic field but a drastically reduced effective magnetic field B*, were identified as apposite quasi-particles that simplify our understanding of the FQHE. J. K. Jain, Phys. Today, 39–45 (2000). J. K. Jain, Phys. Rev. Lett. 63, 199–202 (1989). They precess, like electrons, along circular cyclotron orbits, with a diameter determined by B* rather than B. B. I. Halperin, P. A. Lee, and N. Read, Phys. Rev. B 47, 7312–7343 (1993). O. Heinonen, (ed.), Composite Fermions (World Scientific, Singapore, 1998). R. R. Du, H. L. Stormer, D. C. Tsui, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 70, 2944–2947 (1993). R. R. Du et al., Phys. Rev. Lett. 75, 3926–3929 (1995). R. L. Willett, R. R. Ruel, K. W. West, and L. N. Pfeiffer, Phys. Rev. Lett. 71, 3846–3849 (1993). V. J. Goldman, B. Su, and J. K. Jain, Phys. Rev. Lett. 72, 2065–2068 (1994). J. H. Smet, D. Weiss, R. H. Blick, G. Lütjering, and K. von Klitzing, Phys. Rev. Lett. 77, 2272–2275 (1996). The frequency of their cyclotron motion remained hitherto enigmatic, since the effective mass is no longer related to the band mass of the original electrons and is entirely generated from electron–electron interactions. Here, we demonstrate the enhanced absorption of a microwave field that resonates with the frequency of their circular motion. From this cyclotron resonance, we derive a composite fermion effective mass that varies from 0.7 to 1.2 times the electron mass in vacuum as their density is tuned from 0.6× 1011/cm2 to 1.2× 1011/cm2