Since 2002 Perimeter Institute has been recording seminars, conference talks, and public outreach events using video cameras installed in our lecture theatres. Perimeter now has 7 formal presentation spaces for its many scientific conferences, seminars, workshops and educational outreach activities, all with advanced audio-visual technical capabilities. Recordings of events in these areas are all available On-Demand from this Video Library and on Perimeter Institute Recorded Seminar Archive (PIRSA). PIRSA is a permanent, free, searchable, and citable archive of recorded seminars from relevant bodies in physics. This resource has been partially modelled after Cornell University's arXiv.org.
On-shell methods provide a powerful tool for the perturbative computation of scattering amplitudes in gauge theories, such as QCD. In these lectures I will focus on such methods which can be developed without invoking unitarity, but rather by setting one propagator by loop level on-shell ("single-cuts"). After summarizing the present status of applying these ideas to QCD, I will discuss their recent successful application to planar N=4 super Yang-Mills, leading to on-shell recursion relations for loop integrands.
A brief introduction to the notorious "cosmological constant problem" is given. Then, a particular approach is discussed, which has been developed by Volovik and the present speaker over the last years and which goes under the name of q-theory. This approach provides a possible solution of the main cosmological constant problem, why is |Lambda|^(1/4) negligible compared to the energy scales of the electroweak standard model (not to mention the Planck energy)?
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