Speaker: Geoff Greene

Professor Emeritus

University of Tennessee

Host: Crawford

Title: The Life and Death of the Free Neutron

Abstract:  The decay of the free neutron is the simplest example of nuclear beta decay and, as such, is the archetype for a wide variety of Weak Interaction processes. These include radioactivity, Big Bang Nucleosynthesis, and energy production in the sun. Additionally, The precise value of the free neutron lifetime, can, along with other data, be used to test the consistency of the Standard Model. Remarkably, the value of neutron lifetime can also help determine the atmospheric composition of Venus. Given the breadth of physics involved, it is  disconcerting to note that, at present, measurements of the neutron lifetime by different methods are inconsistent. In this talk, I will discuss the physics of neutron decay and will review the strategies for the experimental determination of the neutron lifetime. I will discuss some of the experimental challenges and will attempt to provide some illumination of the current discrepant situation. 

Title: Electrify Everything!

Abstract: Making everything run on electricity is a necessary step in the transition from fossil fuels.    Starting that process immediately  is also necessary, and  helpful both to the process and the environment.

Host: Korsch

Title: Trapped-ion optical clocks: Telling time and testing physics at the quantum limit

Abstract: Optical transitions in trapped, laser-cooled ions can provide an extremely well-controlled frequency reference for atomic clocks.  The most stable and accurate atomic clocks now make measurements with total uncertainty approaching 1×10^-18.  The Ion Storage Group at NIST develops optical clocks based on the ¹S₀-³P₀ resonance in ²⁷Al⁺.  To perform precision spectroscopy on this atomic system we use the basic building block of a quantum computer, the two-qubit gate, which transfers information from ²⁷Al⁺ to a second ion species held in the same trap.  I will introduce these systems and present recent frequency comparisons between them and other optical clocks at NIST.   These comparisons provide valuable data for international time/frequency standards and can test our fundamental theories including relativity and the Standard Model.  I will also describe quantum metrology techniques that have allowed us to approach the quantum limit for stability in a ²⁷Al⁺ single-ion clock.

Host: Murthy
Title:  Dynamics at the edge: charge fractionalization and near-stationary high energy state 

Abstract:  Ergodic many-body systems are expected to reach quasi-thermal equilibrium. Indeed, the energy distribution in quantum Hall edge modes with short-range interactions generally relaxes to a near-thermal asymptotic state that can be described in terms of charge fractionalization. Surprisingly, high-energy electrons injected into a single edge mode with finite range interactions can stabilize over a long time scale in a state far from a thermal one: the resulting many-body state consists of rapidly decaying transient components followed by a near-steady-state distribution with a peak near the injection energy. 

Title: Quantum Codes from Condensed Matter to Quantum Gravity

Abstract: I will explore the appearance of quantum codes in diverse contexts, from the toric code of condensed matter physics to holographic codes  in quantum gravity.   The contexts and implementations of these codes  vary widely, but their structures have much in common and suggest a deeper connection between them. 

Host: Su

Title: Combining Galaxy and CMB Surveys — all the science that “comes for free”

Abstract: The LCDM model has been extraordinarily successful. In the past 20 years, the cosmology community has worked hard to make ever more precise measurements of the LCDM parameters using large datasets from cosmic surveys. As error bars shrink and several tensions arise, we are eager to look for new and different ways of making robust statements of the LCDM paradigm. In this talk I like to focus on one particular direction where we can get new information to help this effort — by combining different cosmic surveys, in particular galaxy and CMB surveys. I will first describe the latest cosmological analysis using the Dark Energy Survey (DES) and CMB lensing from the South Pole Telescope (SPT) and Planck. Then I will talk about combining DES galaxies and the thermal Sunyaev Zel'dovich (tSZ) effect measured from SPT and Planck to learn about the baryonic feedback in our galaxies. These analyses will highlight the power of combining different datasets to tackle some of the most pressing issues in observational cosmology today. 

Host: Su

This colloquium will be remote over zoom.

Title: Cosmic Building Blocks: Forming Planets from Tiny Grains of Dust

Abstract: Planet formation takes place in disks of dust and gas around young stars, where the dust grains are the building blocks to form new planets.  Nevertheless, capturing the planet-formation process is challenging as disks are complex and dynamic environments.    Observational studies of both disks and planet formation are rapidly changing with the development of high resolution and multi-wavelength instrumentation.    With current capabilities, we can characterize structural features in disks, study their chemistry, and even detect young protoplanets embedded in these systems.    In this presentation, I will provide an overview of both current and future capabilities, highlighting stunning images and results.  I will also discuss some of the challenging open questions and how future research may tackle many of these questions.

Title: Cloudy - numerical simulations of non-equilibrium plasmas and their spectraHow Stars Form

Abstract:  Most of the quantitative information we have about the universe comes from spectroscopy, most often of emission lines or the continuous broad-band spectral energy distribution. Observations cover the electromagnetic spectrum from the radio to the gamma-ray.  The material producing the spectrum is so far from thermodynamic equilibrium that concepts like temperature do not apply.  Numerical simulations of the conditions in the plasma must be done from first principles, working back to atomic cross sections and rate coefficients.  A large system of balance equations is solved to determine the level populations within atoms or molecules and the abundance of each ion or molecule.  The cloud is optically thick, so the transfer of radiation out of the cloud and reaching our telescopes must be solved simultaneously. The result is a prediction of the full spectrum with a minimum of free parameters.  Cloudy is an open-source code that solves all these problems.  It was developed almost entirely here in Lexington and is widely used across the astronomical community. I developed Cloudy for my research, and I will describe some of its applications by my group and as well as by the astronomical community.

Host: Gardner

Title: The Heavyweight W boson - an Upset to the Standard Model of Particle Physics

Abstract: The Standard Model of particle physics has been a crowning achievement of fundamental physics, culminating in the discovery of the Higgs boson in 2012. As a quantum theory of the building blocks of matter and forces, it has been one of the most successful theories in science. The recent measurement of the mass of the W boson disagrees with the theory prediction. This upset to the Standard Model may point towards exciting new discoveries in particle physics in the coming years. We will discuss the Standard Model, the crucial role of the W boson, and how it has become the harbinger of new laws of nature. 

Host:  Murthy

Title: The Superconducting Insulator

Abstract: Many two-dimensional superconductors undergo a transition to an electric insulator as a function of different parameters such as thickness, disorder, magnetic field, chemical composition etc. This superconductor-insulator-transition has been an active area of research for the last few decades, nevertheless, some fundamental questions remain unsolved. In particular, the nature of the insulating phase, which shows unconventional transport properties, is unclear. One exciting scenario suggests that this phase incorporates superconducting fluctuating islands embedded in an insulating matrix, making it exceptional and unsimilar to any conventional superconductor or insulator.

In this colloquium I will present findings, obtained by our group as well by others, demonstrating the uniqueness of this “superconducting insulator”.  These include a disorder enhanced superconducting energy gap, a novel proximity effect, unusual vortex motion and excess superconducting specific heat in the insulator.  These results provide important insight into the physics governed by the interplay between disorder and two-dimensional superconductivity.