Elizabeth H. Simmons (UCSD): "Simplified Limits on Resonances at the LHC" | 1/10/18
When an excess appears in LHC data, we should compare the results with broad classes of models, to get an immediate sense of which kinds of BSM theories could conceivably be relevant. Often, the new physics is likely to be an s-channel resonance. In this case, a simplified model of the resonance can translate an estimated signal cross section into bounds on the product of the dominant production and decay branching ratios. This quickly reveals whether a given class of models could possibly produce a signal of the required size at the LHC. This talk will outline a general framework and show how it operates for resonances of varying widths and with different numbers of production and decay modes. It will also discuss applications to cases of experimental interest, including resonances decaying to di-bosons, di-leptons, or di-jets. If the LHC experiments start reporting searches for BSM resonances in terms of the simplified limits variable ζ defined here, the community will home in more quickly on the models most likely to explain any observed excess.
Prateek Agrawal (Harvard): "The strong CP problem and UV instantons" | 1/17/18
The absence of sizeable CP violation in the strong sector is a long standing puzzle. A class of solutions to this problem rely on a global U(1) symmetry that is anomalous with QCD. These solutions lead to robust low-energy predictions, for example a massless up quark or a light axion. I will present simple extensions to such solutions which can dramatically change these low-energy predictions. In our models, contributions from small instantons play a significant role in affecting the low-energy physics while preserving the solution to the strong CP problem.
Tim Linden (OSU): "Astrophysical Signatures of Dark Matter Accumulation in Neutron Stars" | 1/31/18
Over the past few decades, terrestrial experiments have placed increasingly strong limits on the dark matter-nucleon scattering cross-section. However, a significant portion of the standard dark matter parameter space remains beyond our reach. Due to their extreme density and huge gravitational fields, neutron stars stand as optimal targets to probe dark matter-neutron interactions. As an example, over the last few years, the existence of Gyr-age neutron stars has placed strong limits on models of asymmetric dark matter. In this talk, I will discuss novel methods which utilize neutron stars to potentially detect dark matter interactions by studying the galactic morphology of neutron stars, as well as electromagnetic signals which may be produced via neutron star collapse. Intriguingly, these observations can probe extremely generic dark matter models spanning from MeV - PeV energies, and including troublesome portions of parameter space such as pure-Higgsino dark matter.
Yvonne Geyer (IAS): "Loops from Nodes: Two-loop supergravity amplitudes from the ambitwistor string" | 2/7/18
The last years have seen remarkable progress in understanding the scattering amplitudes of massless particles in arbitrary dimensions.Underlying the simple formulae are chiral worldsheet models, known as Ambitwistor Strings. While correlators of these models admit a conventional genus expansion of the worldsheet, the amplitudes actually localize on the maximal non-separating degeneration. We explore this simplification at two loops for type II supergravity, concluding in several observations for generic massless field theories.
Yotam Soreq (MIT): "The quest for new physics: from atomic physics to the LHC" | 2/14/18
The Standard Model of particle physics well describes a vast number of observables up to the TeV scale. However, it cannot be a complete description of Nature as it cannot explain various experimental observations. For example, it lacks a viable dark matter candidate and can neither explain the observed matter/antimatter asymmetry of our Universe nor neutrino oscillations. Thus, physics beyond the Standard Model is well motivated. In this seminar, we explore different methods to probe new physics at multiple energy scales, from high energy colliders, such as the LHC, to precision low energy experiments. In particular, we focus on searches for new force carriers at the LHCb experiment and in precision atomic spectroscopy. We show that the inclusive search for dark photons at the LHCb experiment already probes new parameter space and can be easily interpreted for a large variety of new physics models. On the precision frontier, we explore the potential of isotope shift spectroscopy to probe new long range force carriers.
Alba Grassi (Stony Brook): “Quantum curves and q-deformed Painlevé equations” | 2/21/18
In this talk I will first review some aspects of Painlevé equations and their connection to four dimensional gauge theory; then I will generalise this construction to q-difference Painlevé equations and topological string theory. I will show that their tau-functions are Fredholm determinant of operators associated to quantum mirror curves on a corresponding geometry. As a consequence, the zeroes of these tau-functions compute the exact spectrum of the associated quantum integrable systems. I will focus on the particular example of q-Painlevé III_3 which is related to topological string on local P1xP1 and to relativistic Toda system."
Xinan Zhou (Stony Brook): "Holographic Mellin Amplitudes" | 3/7/18
Holographic four-point functions are known for their notorious computational difficulties. In the past two decades, only a handful of them have been explicitly calculated using the standard algorithm. In this talk I will introduce modern methods to compute holographic correlators efficiently, which are inspired by the bootstrap philosophy and the on-shell methods of scattering amplitudes in flat space. I will show that by translating the problem into Mellin space many difficulties encountered when applying the traditional method are avoided. I will argue that imposing symmetry constraints and general consistency conditions -- avoiding all details of the complicated effective Lagrangian -- leads to many novel results for holographic four-point functions in AdS5×S5, AdS7×S4 and AdS4×S7. I will conclude by outlining some interesting future directions of this program.
Lina Necib (Caltech): "Empirical Determination of the Dark Matter Velocity Distribution" | 3/14/18
Using the hydrodynamic simulation Eris, as well as various realizations of the Milky Way from the FIRE simulation, we found that the kinematics of dark matter follows closely the kinematics of old metal poor stars. We use this correspondence to obtain the first empirical measurement of the local velocity distribution of dark matter, by first analyzing the Gaia data release coupled with RAVE as well as the ninth release from the Sloan Digital Sky Survey and computing the velocity distribution of metal poor stars. We find that this velocity distribution is peaked at lower velocities than the generally assumed Maxwell Boltzmann distribution, leading to a weakening of direct detection limits at dark matter masses less than 10 GeV by a factor of a few. We also found a few kinematic outliers in the stellar data that might be hints of dark matter substructure.
Zuhair Khandker (Illinois): "Conformal truncation: A new method for studying strong-coupled QFTs" | 3/21/18
I will present a new numerical method for studying strongly-coupled QFTs. The method is formulated for continuum spacetime of any dimension, in real time and infinite volume, and is thus complementary to other numerical methods, such as the lattice. The method harnesses conformal symmetry, but in a manner applicable to general, non-conformal QFTs. Specifically, the input is information about the UV CFT from which the QFT originates. The output is the physical IR QFT spectrum, along with real-time, infinite-volume correlation functions. I will discuss applications to 2D phi^4 theory, where we have performed novel computations of correlation functions at any coupling, such as the Zamolodchikov c-function along the full RG-flow. The 2D Ising model provides a highly-nontrivial cross-check of our numerics.
Sungwoo Hong (Cornell): "A natural generalization of the standard Randall-Sundrum framework and its phenomenological implications" | 3/28/18
In the first part of the talk, I will introduce a very natural extension of well-motivated extra-dimensional framework of Randall-Sundrum type. Such a generalization is motivated by (null) results from both high energy (LHC) and low energy (flavor, CP, and electroweak precision) experiments. In particular, null results from the LHC led us to consider the possibility that little hierarchy may exist. In addition to the consistency with low energy bounds, our generalization can address the question of the form of TeV scale new physics we can expect. I will argue that such new physics appearing at the TeV scale is in the form of vector-like confinement with new states interacting with SM through mostly flavor universal couplings. In the second part of the talk, I will discuss several exciting signals probable at the LHC and in future colliders.
Laura Donnay (Harvard): "Supertranslations and Superrotations at the Black Hole Horizon" | 4/4/18
In this talk, we study the asymptotic symmetries in the near-horizon region of extremal and non-extremal black holes. By prescribing a physically sensible set of boundary conditions at the horizon, we derive the algebra of asymptotic Killing vectors, which is shown to be infinite-dimensional; it includes two sets of supertranslations and two mutually commuting copies of the Virasoro algebra. We define the surface charges associated to these large diffeomorphisms and evaluate them for different stationary black hole solutions. We finally discuss the relationship between these horizon charges and the Bondi-Metzner-Sachs (BMS) ones by computing the memory effect produced at the black hole horizon by a gravitational shock wave sent from null infinity.
Rebecca Leane (MIT): "GeV-Mass Thermal WIMPs: Not Even Slightly Dead" | 4/11/18
A leading dark matter candidate is a Weakly Interacting Massive Particle (WIMP). The observed dark matter abundance can be naturally obtained through freezeout of the thermal annihilation rate. The defining feature of a thermal WIMP is that its total annihilation cross section is specified through the rate ~ 3 x 10^-26 cm^3/s, inversely proportional to the dark matter density. Searches for dark matter annihilation products have set strong limits in certain cases, requiring that the dark matter mass be greater than around 100 GeV if annihilation proceed solely to b quarks (Fermi), τ leptons (Fermi), or electrons (AMS). We construct the first limits on the total annihilation cross section, showing that allowed combinations of the annihilation-channel branching ratios considerably weaken these limits. We will show that GeV-mass thermal WIMPs have not yet been adequately tested, and outline ways forward.
