Sonia El Hedri (SLAC): "Perturbative Unitarity Constraints on a Supersymmetric Higgs Portal" | Sep 10
We place perturbative unitarity constraints on both the dimensionful and dimensionless couplings in the Next-to-Minimal Supersymmetric Standard Model Higgs Sector. These constraints, plus the requirement that the singlino and/or Higgsino constitutes at least part of the observed dark matter relic abundance, generate upper bounds on the Higgs and neutralino/chargino mass spectrum. We obtain an upper bound of 12 TeV for the charginos and neutralinos and 20 TeV for the heavy Higgses outside defined fine-tuned regions. By using the NMSSM as a template, we describe a method which replaces naturalness arguments with more rigorous perturbative unitarity arguments to get a better understanding of when new physics will appear.
John Golden (Brown): "Scattering Amplitudes and Cluster Polylogarithms" | Sep 17
In this talk I will give an introduction to three beautifully interconnected topics: scattering amplitudes, polylogarithms, and cluster algebras. I will begin by reviewing recent advances in the calculation and understanding of amplitudes in N=4 Super-Yang-Mills theory. Much of this progress comes from powerful mathematical machinery associated with polylogarithm functions. I will then introduce cluster algebras, a relatively recent area of mathematics with increasing prevalence in physics. Amplitudes in N=4 SYM unexpectedly provide an amazing class of functions that elegantly conjoin ideas from polylogarithms and cluster algebras. As an example, I will describe an algorithm that exploits these connections to calculate two-loop MHV amplitudes for any number of particles.
Anindya Dey (UT Austin): "Domain Walls, Triples, and Acceleration" | Oct 1
We will review and present new results regarding “no-go” theorems for four-dimensional de Sitter solutions in the Heterotic string. As a concrete means of realizing accelerating solutions in string theory, we present a construction of domain walls in string theory. The domain walls can bridge both Minkowski and AdS string vacua. A key ingredient in the construction are novel classical Yang-Mills configurations, including instantons, which interpolate between toroidal Yang-Mills vacua. Our construction provides a concrete framework for the study of inflating metrics in string theory. In some cases, the accelerating space-time comes with a holographic description. The general form of the holographic dual is a field theory with parameters that vary over space-time.
Travis Maxfield (Chicago): "Maximally Natural Supersymmetry and its Natural New Signatures" | Oct 8
Supersymmetric models with SUSY breaking by boundary conditions in a flat TeV-scale extra dimension avoid many of the problems of conventional MSSM-like models. A natural spectrum is easily realized by locality in the TeV sized direction, and the electroweak scale remains completely natural with stops and other superpartners above current LHC limits. These models motivate unusual new signatures of supersymmetric naturalness due to the absence of light higgsinos and the possible presence of additional large supersymmetric bulk dimensions.
Kiel Howe (Stanford): "Maximally Natural Supersymmetry and its Natural New Signatures" | Oct 8
Supersymmetric models with SUSY breaking by boundary conditions in a flat TeV-scale extra dimension avoid many of the problems of conventional MSSM-like models. A natural spectrum is easily realized by locality in the TeV sized direction, and the electroweak scale remains completely natural with stops and other superpartners above current LHC limits. These models motivate unusual new signatures of supersymmetric naturalness due to the absence of light higgsinos and the possible presence of additional large supersymmetric bulk dimensions.
Eric Mintun (UC Santa Barbara): "S-Duality for Intersecting D3-Branes" | Oct 15
Although S-duality is an immensely useful tool in the study of quantum field theory, an explicit derivation of the duality does not exist for most non-trivial theories. With the goal of better understanding a path integral derivation of S-duality, we study the defect gauge theory that lives on two perpendicular D3-branes with a 1+1 dimensional intersection. We show that S-duality in this theory may be realized by the composition of two T-dualities with an electromagnetic duality that exchanges the electric and magnetic fields on the branes. The T-dual’ed circle, which is in the field space of the intersection scalars, does not conserve winding number and thus maps to a circle on which momentum is not conserved. The problem of deriving S-duality in this theory then reduces to the known problem of performing a T-duality on a circle that lacks a continuous isometry.
David Curtin (Maryland): "Testing Electroweak Baryogenesis with Future Colliders" | Oct 22
Electroweak Baryogenesis (EWBG) is a compelling scenario for explaining the matter-antimatter asymmetry in the universe. It is in principal testable, but doing so exhaustively seems difficult in practice due to the large amount of model building freedom in realizing this mechanism. I will outline our first steps in investigating a possible "no-loose" theorem for testing EWBG in future e+e- or hadron colliders. This involves focusing on a factorized picture of EWBG which separates the new physics requirements of strong phase transition and CP violation. We then construct a "nightmare scenario" that generates a strong phase transition, but is very difficult to test experimentally. We show that a 100 TeV hadron collider is both necessary and possibly sufficient for testing the parameter space of the nightmare scenario that is consistent with EWBG. I will also give a preview of some work in progress, in which we try to construct a consistent effective field theory framework for an important class of electroweak phase transitions.
Cyril Closset (Stony Brook): "Holomorphic Twist and Supersymmetric Partition Functions" | Oct 29
The study of supersymmetric quantum field theories on compact manifolds has led to a plethora of new exact results in recent years. One of the simplest observable in this context is the supersymmetric partition function on a manifold M, Z_M, which can often be computed exactly. In the case of theories with four flat-space supercharges, I will explain some important properties of Z_M which can be understood on general ground. This understanding explains many previous results and makes sharp predictions for future computations.
Michele Cicoli: "Sequestered string scenarios: global embedding, de Sitter modulistabilisation and soft-terms" | Nov 5
I will present promising type IIB string models which allow for inflation, low energy supersymmetry and gauge coupling unification with no cosmological moduli problem. I will first describe how to embed thesemodels in a compact Calabi-Yau with all closed string moduli stabilised in a de Sitter vacuum. I will then analyse soft supersymmetry breaking showing how the Standard Model can be sequestered from the supersymmetry breaking sources, resulting in soft terms hierarchically smaller than the gravitino mass.
Benjamin Safdi (MIT): "Renyi entropy, stationarity, and entanglement of the conformal scalar" | Nov 12
I will consider a perturbative expansion of the Renyi entropy, S_q, around q = 1 for a spherical entangling surface in a general CFT. Applied to conformal scalar fields in various spacetime dimensions, the results appear to conflict with the known conformal scalar Renyi entropies. On the other hand, the perturbative results agree with known Renyi entropies in a variety of other theories, including theories of free fermions and vector fields and theories with Einstein gravity duals. I will propose a resolution stemming from a careful consideration of boundary conditions near the entangling surface. This is equivalent to a proper treatment of total derivative terms in the definition of the modular Hamiltonian. As a corollary, I'll present an understanding of an outstanding puzzle in the literature regarding the Renyi entropy of N = 4 super-Yang-Mills near q = 1. A related puzzle regards the question of stationarity of the renormalized entanglement entropy (REE) across a circle for a (2+1)-dimensional massive scalar field. I will point out that the boundary contributions to the modular Hamiltonian shed light on the previously-observed non-stationarity.
Christoph Keller (Rutgers) : "3d Gravity, Universality and Poincare Series" | Nov 19
Modular invariance plays an important role in the AdS3/CFT2 correspondence. Using modular invariance, I discuss under what conditions a 2d CFT shows a Hawking-Page phase transition in the largec limit, and what this implies for the range of validity of the Cardy formula and the universality of its spectrum. I will also discuss partition functions obtained by summing over the modular group, how their properties are compatible with their gravity interpretation, and briefly touch on implications for the existence of pure gravity.
Tim Cohen (IAS/Princeton): "Soft Collinear Effective Theory for Heavy WIMP Annihilation" | Dec 3
In a large class of models for Weakly Interacting Massive Particles (WIMPs), the WIMP mass M lies far above the weak scale m_W. This work identifies universal Sudakov-type logarithms ~ alpha log^2 (2 M/m_W) that spoil the naive convergence of perturbation theory for annihilation processes. An effective field theory (EFT) framework is presented, allowing the systematic resummation of these logarithms. Another impact of the large separation of scales is that a long-distance wavefunction distortion from electroweak boson exchange leads to observable modifications of the cross section. Careful accounting of momentum regions in the EFT allows the rigorous disentanglement of this so-called Sommerfeld enhancement from the short-distance hard annihilation process. The WIMP is described as a heavy-particle field, while the electroweak gauge bosons are treated as soft and collinear fields. Hard matching coefficients are computed at renormalization scale mu ~ 2 M, then evolved down to mu ~ m_W, where electroweak symmetry breaking is incorporated and the matching onto the relevant quantum mechanical Hamiltonian is performed. The example of an SU(2)_W triplet scalar dark matter candidate annihilating to line photons is used for concreteness, allowing the numerical exploration of the impact of next-to-leading order corrections and log resummation. For M ~ 3 TeV, the resummed Sommerfeld enhanced cross section is reduced by a factor of ~ 3 with respect to the tree-level fixed order result.
