Seminars take place on Wednesdays 12:00-1:00pm in Randall Laboratory #3481
Organizer: Bibhushan Shakya, Uri Kol, John Golden
Matthijs Hogervorst (Stonybrook): "Logarithmic CFTs through the lens of the bootstrap" | Sep 14
Conformal field theories (CFTs) describe critical behaviour in a wide range of d-dimensional systems. In recent years much has been learned about unitary CFTs through the conformal bootstrap. However, many relevant systems are described by non-unitary CFTs with logarithmic correlation functions, known as logarithmic CFTs. In this talk I will discuss some examples of these models (e.g. critical percolation) and their observables, and explain how the bootstrap paradigm extends to logarithmic CFTs.
Duccio Pappadapulo (NYU): "Cannibal dark matter" | Sep 21
A hidden sector with a mass gap undergoes an epoch of cannibalism if number changing interactions are active when the temperature drops below the mass of the lightest hidden particle. During cannibalism, the hidden sector temperature decreases only logarithmically with the scale factor. We consider the possibility that dark matter resides in a hidden sector that underwent cannibalism, and has relic density set by the freeze-out of two-to-two annihilations.
Andrea Tesi (UChicago): "Fundamental Composite Models at the TeV scale" | Sep 28
New confining gauge theories offer a rich and robust phenomenology, mostly dictated by their accidental symmetries. For example, they naturally contain accidentally stable ‘baryon’ and/or ‘pion’ Dark Matter candidates. Moreover, if the strong sector enjoys chiral and custodial symmetries, it can offer a framework for Composite Higgs models, where the Higgs doublet is a partially-composite light ’pion’. I will discuss how this can be made consistent with the flavor sector of the SM and comment on the main LHC phenomenology both in direct and indirect searches.
Ioannis Papadimitriou (SISSA): "AdS_2 holography and non-extremal black holes" | Oct 5
I will present aspects of AdS_2 holography for a specific Einstein-Maxwell-Dilaton model that is obtained by Kaluza-Klein reduction from pure AdS_3 gravity with negative cosmological constant. In particular, I will derive the one-dimensional holographic dual for both running and constant dilaton solutions, and I will discuss the connection with the Sachdev-Ye-Kitaev model. Finally, I will explain how this dilaton gravity model provides a holographic description for four-dimensional asymptotically conformally AdS_2 x S^2 non-extremal black holes. The talk is based on the recent paper https://arxiv.org/abs/1608.07018
Steven Avery (MSU): "Soft Black Hole Absorption Rates as Ward Identities" | Oct 19
Recently, a number of exciting connections have been made between large gauge transformations (eg. BMS) and infrared physics (eg. Weinberg's soft graviton theorem). One of the more exciting explorations in this vein was Hawking-Perry-Strominger's (HPS) investigation of the consequences of these new symmetries for black hole physics. I will show very concretely that the Ward identity for the BMS-like large U(1) gauge transformations discussed by HPS fixes the low energy black hole absorption rate for photons. Time permitting, I will discuss broader implications and future extensions.
Ahmed Ismail (ANL): "Lighting Up Collider Searches for Electroweak States" | Oct 26
Despite appearing in many extensions of the Standard Model, uncolored electroweak particles face limited collider search prospects. For nearly degenerate electroweak multiplets where the lightest state is electrically neutral, searches typically rely on pair production of the new states in association with visible radiation, e.g. the mono-X final state. We show that for such new particles, considering final state photon radiation can provide increased sensitivity. The additional kinematical information provided by including final state electromagnetic radiation more than compensates for the reduced statistics.
Ellis Ye Yuan (IAS): "Projective geometry of scattering amplitudes" | Nov 2
I will introduce a technique in studying the analytic structure of perturbative S-matrix of QFTs. The main idea is to associate Feynman integrals to objects in a projective space: polytopes and hypersurfaces. At one loop, this reveals a universal structure for all possible Feynman integrals (in particular, including one-loop amplitudes in any QFTs), and gives rise to two simple algebraic algorithms to determine the integrals (as functions of the kinematics). I will also comment on the generalization to higher loop levels.
Heeyeon Kim (Perimeter): "Supersymmetric Indices of 3d TQFTs on a Riemann surface" | Nov 9
I will discuss the partition function of three-dimensional supersymmetric topological field theories defined on a circle bundle over Riemann surface and correlation functions of half-BPS loop operators. From the result, I will derive the quantum algebra of Wilson loops in terms of the associated Bethe equation, and show how the Seiberg-like dualities act on the half-BPS Wilson loops.
Joshua Berger (Wisconsin): "Cosmological Constraints on Decoupled Dark Photons and Dark Higgs" | Nov 16
Any neutral boson such as a dark photon or dark Higgs that is part of a non-standard sector of particles can mix with its standard model counterpart. When very weakly mixed with the Standard Model, these particles are produced in the early Universe via the freeze-in mechanism and subsequently decay back to standard model particles. In this work, we place constraints on such mediator decays by considering bounds from Big Bang nucleosynthesis and the cosmic microwave background radiation. We find both nucleosynthesis and CMB can constrain dark photons with a kinetic mixing parameter between log epsilon ~ -10 to -17 for masses between 1 MeV and 100 GeV. Similarly, the dark Higgs mixing angle epsilon with the Standard Model Higgs is constrained between log epsilon ~ -6 to -15. Dramatic improvement on the bounds from CMB spectral distortions can be achieved with proposed experiments such as PIXIE.
Nele Callebaut (Columbia): "Emergent de Sitter spaces from CFTs." | Nov 30
I will discuss the equivalence of two proposals for constructing an emergent de Sitter space on a 2D CFT, referred to as 'kinematic space'. Special attention is given to the case of a thermal CFT with a BTZ dual. This is based on 1604.02687. The key observation behind the equivalence is the fact that the entanglement entropy behaves as a Liouville field. I will point out how the two proposals have recently converged (in 1604.03110 and 1606.03307) into a generalized notion of kinematic space as the space of pairs of CFT points.
Lorenzo Di Pietro (Weizmann Institute of Science): "Cardy Formula for SUSY Theories and Localization" | Dec 7
I will consider 4d =1 supersymmetric theories on a compact Euclidean manifold of the form S1×3. Taking the limit of shrinking S1, I will present a general formula for the limit of the localization integrand, derived by simple effective theory considerations. The limit is given in terms of an effective potential for the holonomies around the S1, whose minima determine the asymptotic behavior of the partition function. If the potential is minimized in the origin, where it vanishes, the partition function has a Cardy-like behavior fixed by Tr(R), while a nontrivial minimum gives a shift in the coefficient. I will also discuss the generalization to 6d N=(1,0) indices and an application to Schur indices.
Cindy Keeler (Niels Bohr International Academy): "Spacetime Reconstruction in Applied Holography" | Dec 14
After a brief review of holographic techniques derived from the AdS-CFT correspondence, we specialize to a class of spacetimes proposed as duals to non-relativistic systems. We highlight classical and quantum features of these "Lifshitz spacetimes" which limit the reconstructability of bulk spacetime information from boundary data. We additionally discuss the fate of various spacetime reconstruction procedures in Lifshitz spacetimes. We close by examining the limitations placed on entropy-based spacetime reconstruction due to holographic screens.
