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The QCD Lagrangian permits a CP-violating topological term proportional to the parameter theta. In the standard picture, this makes theta physical and leads to the strong CP problem. Recently, however, this view has been challenged by proposals claiming that a refined treatment of topology, gauge invariance, or the infinite-volume limit removes the physical role of theta. In this seminar I review the standard arguments for CP violation in QCD, from pure Yang-Mills theory to QCD with one and several light quark flavors, emphasizing the role of topology, vacuum energy, topological susceptibility, and lattice results. Along with this review, I discuss two recent challenges due to Ai and collaborators. In pure Yang-Mills theory, they argue that a constrained definition of the physical trace eliminates the theta-dependence of observables. In QCD with one light quark flavor, they claim that if the infinite-volume limit is taken before summing over topological sectors, the phase of the quark mass and the theta-parameter cancel. I explain where these proposals depart from the conventional framework and present complementary arguments, based on local source functionals and on the structure of the vacuum energy, which support the standard conclusion that theta remains a physical parameter. The aim is to clarify what is at stake in the current debate and why the strong CP problem continues to be a central conceptual issue in QCD.

Bio: Dr. Andreas Ringwald is working on a broad class of theoretical and phenomenological questions. Among other things, he is one of the initiators of the ALPS and WISPDMX experiments at DESY and of the SHIPS helioscope at Hamburg Observatory. One focus of his work is on the theory and cosmology of axions and other weakly interaction slim particles beyond the Standard Model.

Flyer: http://dfweb.phys.unisa.it/seminars/AvvisoSeminari.asp?1025