Exceptional Seminar Philip Argyres (University of Cincinnati)
at: 11:00 room G.O. Jones 610  abstract:

Regular Seminar Edoardo Lauria (University of Durham)
at: 13:15 room S2.49  abstract: A fourdimensional abelian gauge theory can be coupled to a 3d CFT with a U(1) symmetry living on a boundary. This coupling gives rise to a continuous family of boundary conformal field theories (BCFTs) parametrized by the gauge coupling \tau and by the choice of the CFT in the decoupling limit. Upon performing an ElectricMagnetic duality in the bulk and going to the decoupling limit in the new frame, one finds a different 3d CFT on the boundary, related to the original one by Witten's SL(2, Z) action. In particular the cusps on the real \tau axis correspond to the 3d gauging of the original CFT. We study general properties of this family of BCFTs. We show how to express bulk one and twopoint functions, and the hemisphere freeenergy, in terms of the twopoint functions of the boundary electric and magnetic currents. Finally, upon assuming particlevortex duality (and its fermionic version), we show how to turn this machinery into a powerful computational tool to study 3d gauge theories. 
Polygon Seminar Congkao Wen (QMUL)
at: 15:00 room A130  abstract: Scattering amplitudes are one of most important class of physical observables in quantum field theories. Over the last decade or so, there has been a lot of activities regarding the computation of scattering amplitudes in a wide range of interesting theories, where extremely powerful new frameworks for studying scattering amplitudes have emerged, known as the modern Smatrix program. In this talk I will review some of these powerful techniques, and discuss their applications. The applications will mostly focus on effective field theories, that include twistorlike formulas for scattering amplitudes of worldvolume theories of probe Dbrane and M5 brane, amplitude constraints on effective field theories, such as supersymmetric nonrenormalization theorems, unitarity bounds on lowenergy spectra. 
Regular Seminar Massimo Bianchi (Roma Tor Vergata)
at: 14:00 room G O Jones 610  abstract: Ultraplanckian collisions represent a fertile arena where to test quantum theories of gravity such as string theory. Glimpses of black hole formation and evaporation can be taken. For suitably defined infraredsafe observables, we show that the `classicalization' approach to highmultiplicity processes agrees with the ACV approach based on the resummation of ladder diagrams. Since a significant fraction of energy is lost in gravitational bremsstrahlung, we rederived the zerofrequency limit (ZFL) of the GW flux using soft graviton theorems at leading order and compute the corrections at sub and subsubleading order. For massless twoparticle elastic collisions the former is shown to vanish, while the latter takes an explicit expression which is checked against a simple treelevel process.However, if the treelevel form of the soft theorems is used at subsubleading order even when the elastic amplitude needs resummation, an unphysical IR singularity occurs due to the infinite Coulomb phase. We briefly discuss a recent proposal as how to deal with these divergences and find agreement with the eikonal approach. 