"Indices, Twisted Partition Functions, H-Saddles, and Gauge Theories on Compact Spacetimes"
In recent years, varieties of index-like quantities have been computed by exact path integrals, a.k.a. the localization. However, the interpretation of the results, which should be really called the twisted partition functions rather than the indices, requires even more care.
As a prototype, we start with supersymmetric Yang-Mills quantum mechanics. Exploiting the problematic rational structure of the twisted partition functions, we propose a universal tool for extracting the truly enumerative and integral index for theories with noncompact Coulombic directions, such as quiver theories. For pure Yang-Mills, this solves an old problem, dating back to 1990's, and along the way also resolves a critical conflict between Kac/Smilga and Staudacher/Pestun, circa 1999~2002.
The latter brings us to the new notion of H-saddles which proves to be a universal feature of twisted partition functions on a compact spacetime in any dimensions. We close with its ramifications on some recent claims on Casimir energies and Cardy exponents in even dimensions.
“The HAWC Observatory: Detecting the Highest Energy Gamma-Rays”
The High Altitude Water Cherenkov (HAWC) is a continuously operating (>95% on-time), wide field-of-view (~2 sr) observatory located at 14000’ above sea level in Puebla, Mexico. HAWC observes ~2/3 of the sky each day and has produced a map from the first year and a half of operations with ~ 40 sources of which about one quarter were previously unknown. Several of these sources have emission > 50 TeV. Most of the sources are within the Galactic plane; however, the two active galactic nuclei Mrk 421 and Mrk 501 are also strongly detected and are observed to be variable. Within the region surveyed by HAWC are many dark matter rich objects, such as dwarf spheroidal galaxies, and these HAWC data place the strongest constraints to date on annihilating or decaying dark matter with masses >10 TeV. The Crab nebula is detected in this map at > 100 σ and is each day ~ 5σ. The HAWC data are searched in real time for transient sources. HAWC monitors the same sky as gamma-ray satellites (Fermi), gravity-wave (LIGO) detectors and neutrino observatories (IceCube) allowing for multi-wavelength and multi-messenger observations.
About the speaker
Dr. Brenda Dingus is the Principal Investigator of the High Altitude Water Cherenkov (HAWC) TeV gamma-ray observatory for the DOE High Energy Physics program. She was the first US Spokesperson of HAWC from 2010-2014 and was the Deputy Project Manager during construction from 2011-2015. She is now the Operations manager. HAWC is the most sensitive, wide field of view,TeV gamma-ray and cosmic-ray detector and the science goals of HAWC include searching for evidence of new physics in the TeV sky, such as gamma-rays from the annihilation or decay of dark matter.
Dr. Dingus is a Fellow at the Lab and a Fellow of the American Physical Society (APS) and was a winner of the Presidential Early Career Award for Scientists and Engineers (PECASE) in 2000. Prior to coming to Los Alamos in 2002 she was a tenured professor first at the University of Utah and then at the University of Wisconsin. She has served on various advisory committees, such as the NASA Astrophysics Subcommittee, and has been elected as a member of the executive committees of the Division of Astrophysics of the APS and the High Energy Astrophysics Division of the American Astronomical Society.
Galaxy clusters represent excellent laboratories to search for Axion-Like Particles (ALPs). They contain magnetic fields which can induce quasi-sinusoidal oscillations in the X-ray spectra of AGNs situated in or behind them. Ultra-deep Chandra observations of the Perseus cluster contain over 5 x 10^5 counts from the central NGC1275 AGN, and represent an extraordinary dataset for ALP searches. In this talk I will describe how we used these to search for spectral irregularities from the AGN. No irregularities were found at the ~30% level, allowing us to place leading constraints on the ALP-photon mixing parameter gaγγ <1.5 × 10−12GeV−1 for m_a < 10^-12 eV. I will move on to discuss the upcoming Athena X-ray Observatory, due for launch in 2028. The X-ray Integral Field Unit (X-IFU) instrument onboard will be far better able to constrain ALPs than Chandra, due to its excellent energy resolution. Using the SIXTE simulation software, we estimate that non-observation of spectral modulations for a 200ks observation of NGC1275 will constrain gaγγ <1.5 × 10−13GeV−1 , an order of magnitude improvement over that derived from Chandra data.
In this talk, I will revisit the computation of scattering amplitudes in two-dimensional string theory on a black hole background. I will present recent results on duality symmetries between non-compact conformal field theories which amount to describe the corresponding worldsheet theory in terms of seemingly different integrable models, which turn out to be solvable. I will review the so-called Generalized Fateev-Zamolodchikov-Zamolodchikov (GFZZ) conjecture, recently proposed by Giveon, Itzhaki, and Kutasov.
The existence for dark matter, the invisible energy that holds galaxies together, has become an inescapable part of our understanding of modern cosmology. Discovering its particle nature has thus become a central question for particle physics. I will review how people think about a models of dark matter, and how searches for dark matter have unfolded. I will discuss how recent results have forced people to confront their priors on what dark matter is, and how as we have wreaked those, a broad set of new possibilities have appeared. This has resulted in an explosion of new ideas of how to find it.
About the speaker
Neal Weiner received his undergraduate degree in physics and math from Carleton College in 1996, and his PhD in physics at UC Berkeley with Lawrence Hall his advisor. He was a postdoc at the University of Washigton from 2000 to 2004, joining the CCPP at NYU in the fall of 2004. He has broad interests in particle physics and cosmology. His focus is generally on physics beyond the standard model. In this broad field, his work has included studies of extra dimensional theories (large, small, warped and flat), supersymmetry, grand unification, flavor, neutrino mass, dark matter, inflation and dark energy, as well as relationships between the different subjects.
The reactor antineutrino anomaly (RAA) has been puzzling reactor neutrino physics community since 2011. The RAA refers to the deficit of electron antineutrinos detected by reactor neutrino experiments compared with the number of electron antineutrinos predicted by state of the art reactor models. The Daya Bay experiment has utilized eight functionally identical underground detectors to sample reactor antineutrino fluxes from three pairs of nuclear reactors in South China, accruing the largest reactor antineutrino sample to date. This talk will summarize Daya Bay’s most recent result, which presents observations of correlations between reactor core fuel evolution and changes in the detected reactor antineutrino flux and energy spectrum. A 10σ variation in IBD yield was found to be energy-dependent, rejecting the hypothesis of a constant antineutrino energy spectrum at 5.1 standard deviations. While measurements of the linear variation with respect to the fuel content in the IBD spectrum show general agreement with predictions from recent reactor models, the measured linear variation with respect to the fuel content in the total IBD yield disagrees with recent predictions. This discrepancy indicates that an overall deficit in measured flux with respect to predictions does not result from equal fractional deficits from the primary fission isotopes 235U, 239Pu, 238U, and 241Pu. A 7.8% discrepancy between the observed and predicted 235U yield suggests that this isotope may be the primary contributor to the reactor antineutrino anomaly.
The last years have seen remarkable progress in understanding the scattering amplitudes of massless particles in arbitrary dimensions. They not only exhibit a simplicity completely obscured by the Feynman Diagram approach, but also a structure reminiscent of the worldsheet models describing string theory. After reviewing the key developments, I will discuss these underlying worldsheet models - so-called ambitwistor strings - and derive simple formulas for the one-loop integrands in gauge theory and gravity. The models manifest the BCJ colour-kinematics duality, and I will end my talk by deriving a compact representation for pure Yang-Mills BCJ numerators.
The 1D Ising model is a classical model of great historical significance for both classical and quantum statistical mechanics. Developments in the understanding of the Ising model have fundamentally impacted our knowledge of thermodynamics, critical phenomena, magnetism, conformal quantum field theories, particle physics, and emergence in many-body systems. Despite the theoretical impact of the Ising model there have been very few good 1D realizations of it in actual real material systems. However, it has been pointed out recently, that the material CoNb2O6, has a number of features that may make it the most ideal realization we have of the Ising model in one dimension. In this talk I will discuss the surprisingly complex physics resulting in this simple model and review the history of "Ising’s model” from both a scientific and human perspective. In the modern context I will review recent experiments by my group and others on CoNb2O6. In particular I will show how low frequency light in the THz range gives unique insight into the tremendous zoo of phenomena arising in this simple material system.
About the speaker
N. Peter Armitage has been at Johns Hopkins University since 2006. He received his B.S. in Physics from Rutgers University in 1994 and his Ph.D. from Stanford University in 2002. He is a physicist whose research centers on material systems which exhibit coherent quantum effects at low temperatures, like superconductors and "quantum" magnetism. Dr. Armitage's principal scientific interest is understanding how is it that large ensembles of strongly interacting, but fundamentally simple particles like electrons in solids act collectively to exhibit complex emergent quantum phenomena. He is exploiting (and developing) recent technical breakthroughs using very low frequency microwave and THz range radiation to probe these systems at their natural frequency scales. The material systems of interest require novel measurement techniques as their relevant frequencies typically fall between the range of usual optical and electronic methods.
He has been the recipient of a DARPA Young Faculty Award, an NSF Career Award, a Sloan Research Fellowship, was a three time Kavli Frontiers Fellow, the Spicer Award from the Stanford Synchrotron Radiation Laboratory, the McMillan Award from the University of Illinois and 2016 Genzel Prize. He was also the co-chair of the 2014 Gordon Research Conference in Correlated Electron Systems.
In this talk, we study the asymptotic symmetries in the near-horizon region of 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 asymptotic diffeomorphisms. When evaluated on the stationary solutions, the only non-vanishing charges are the zero-modes and one of them is shown to reproduce the Bekenstein-Hawking entropy of Kerr black holes. The limit in which the black hole becomes extremal will also be discussed.
Enrico Fermi was one of the most significant figures of 20th century physics, with major contributions across a wide range of sub-disciplines. How did Fermi become Fermi? Drawing on research undertaken in preparation for his forthcoming biography of Fermi, “The Last Man Who Knew Everything: The Life and Times of Enrico Fermi, Father of the Nuclear Age” (Basic Books, December 5, 2017) David N. Schwartz will discuss the development of Fermi as a physicist; the role of nature, nurture, and historical circumstance in his career; and the characteristics behind both his strengths and his weaknesses. Are great physicists born, do they make themselves, or do others make them? How does the accident of one’s birth influence a career like Fermi’s? What was it that enabled Fermi to continue to contribute to the field well beyond the age when many great physicists are content to rest on their previous achievements?
About the speaker
David N. Schwartz holds a PhD in political science from MIT and is the author of two previous books. He has worked at the State Department Bureau of Politico-Military Affairs, and at Goldman Sachs in a variety of roles in both London and New York. He lives in New York with his wife, Susan. His father, Melvin Schwartz, was on the Columbia physics faculty from 1958 to 1966, and returned as the first I.I. Rabi professor of physics in 1994.