Physics & Astronomy Colloquium

Top Eigenvalue of a Random Matrix: A tale of tails

Dr. Satya Majumdar 

CNRS Paris

The statistical properties of the largest eigenvalue of a random matrix are of interest in diverse fields such as in the stability of large ecosystems, in disordered systems, in statistical data analysis and even in string theory. In this talk I'll discuss some recent developments in the theory of extremely rare fluctuations (large deviations) of the largest eigenvalue using a Coulomb gas method. Such rare fluctuations have also been measured in recent experiments in coupled laser systems. I'll also discuss recent applications of this Coulomb gas method in three different problems: entanglement in a bipartite system, conductance fluctuation through a mesoscopic cavity and the vicious random walkers problem. 

 

Your textbook is still wrong about the Milky Way galaxy

 

 

Dr. Heidi Newberg Rensselaer Polytechnic Institute Fifteen years ago, we modeled the distribution of stars in the Milky Way using three components: an exponential disk, a power law spheroid, and a bulge. Then, we discovered the distribution of stars in the spheroid was lumpy due to the accretion and tidal disruption of dwarf galaxies that ventured too close the the Galactic center. We now wonder whether the Milky Way has a classical bulge at all; likely the bulge-like feature we see is instead due to the Galactic bar. And most recently, we are discovering large scale departures from the standard exponential disk. New discoveries point to variations in the expected bulk velocities of stars in the Galactic disk, and oscillations in the spatial densities of disk stars. Some believe these observations point to a wave response to the passing of dwarf galaxies (or dark matter lumps) through the Milky Way's disk. These waves may also explain the observed rings of stars, 15-25 kpc from the Galactic center, which is farther out than we originally believed the disk to extend.

 

 

Explaining the Global Warming Theory

 

 

Dr. Joseph P. Straley University of Kentucky Explaining the implications of science to contemporary public issues is an important part of our job. As an example I will give an introduction to the global warming issue.

 

 

Designing energy and climate security in different regions of the world

 

 

Dr. Rajan Gupta Los Alamos National Labs Spectacular developments in technology and resource exploitation have provided 2-3 billion people with unprecedented lifestyles and opportunities in the twentieth century. On the energy front, this has largely been achieved using inexpensive fossil fuels-- coal, oil and natural gas. The real costs of burning fossil fuels, many of which are hidden and long-term, have been environmental. Today, all species and nature, are being stressed at unprecedented levels and face conditions that have an increasing probability of resulting in catastrophes. Providing the same opportunities to nine or ten billion people will require 2-3 times current energy resources even with business-as-usual anticipated gains in efficiency. There is little doubt that, globally, we have the resources (100 more years of fossil fuels) and the technology to use fossil-fuels ever more cleanly so that the impacts on the environment are smaller and localized. Unfortunately, the emissions of green house gases and their contributions to climate change mandate we transform from the existing successful fossil-fuel system to zero-carbon emission systems. This talk will examine energy resources in different regions of the world and address the issue of whether these resources can provide energy security for the next fourty years. I will next examine how countries with enough resources (fossil, nuclear, hydroelectric) can reduce their carbon footprint in the power sector. I will then discuss the conditions needed to integrate large-scale solar and wind resources to create sustainable systems. Finally, I will identify areas which lack adequate reserves of fossil fuels and how they can address the simultaneous challenges of energy and climate security.

 

 

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Dr. Geoff Greene University of Tennessee, Knoxville While neutrons within nuclei may be stable, the free neutron is unstable against beta decay and has a mean lifetime of ~15min. Free neutron beta decay is, perhaps, the simplest weak nuclear process as it is uncomplicated by many body effects that are present in the decay of nuclei. As a result, it can be directly understood in terms of rather simple fundamental weak interaction theory. Additionally, because free neutron decay is the "prototype" for all nuclear beta decays, the neutron lifetime is a fundamental parameter whose value is important not only in nuclear physics, but also in astrophysics, cosmology, and particle physics. I will give an introduction to the theory of weak nuclear decay and briefly discuss the importance of the neutron lifetime as a parameter in the Big Bang. A review of the experimental strategies for the measurement of the neutron lifetime will be given as well as a discussion of the puzzling discrepancy among the measurements with the lowest quoted uncertainty. Finally, I present a very new result recently obtained at the NIST Cold Neutron Research Facility in Gaithersburg Md.

 

 

Topological Phases in Correlated Materials

Dr. Yong-Baek Kim University of Toronto

Recently there have been significant theoretical and experimental efforts to understand and identify the so-called topological phases of matter in interacting electron systems. These topological phases may be characterized by different kinds of topological properties such as non-trivial edge/surface states and/or unusual elementary excitations in the bulk or surface. Notable examples include quantum spin liquids, topological insulators, and other closely related phases. One of the main challenges is to come up with theoretical criteria that can be used to identify or predict correlated materials that hold promise for the emergence of such topological phases. We discuss recent theoretical and experimental developments in this direction, along with a brief introduction to some of the proposed topological phases. In particular, we focus on correlated materials with strong spin-orbit coupling and/or near a metal-insulator transition.

 

 

New Ideas for Axion Dark Matter Detection

Dr. Peter Graham

SLAC 

The axion is a well-motivated dark matter candidate, but is challenging to search for. We propose a new way to search for QCD axion and axion-like-particle (ALP) dark matter. Nuclei that are interacting with the background axion dark matter acquire time-varying CP-odd nuclear moments such as an electric dipole moment. In analogy with nuclear magnetic resonance, these moments cause precession of nuclear spins in a material sample in the presence of a background electric field. This precession can be detected through high-precision magnetometry. With current techniques, this experiment has sensitivity to axion masses below 10^-9 eV, corresponding to theoretically well-motivated axion decay constants around the grand unification and Planck scales. With improved magnetometry, this experiment could ultimately cover the entire range of masses below 10^-6 eV, just beyond the region accessible to current axion searches. A discovery in such an experiment would not only reveal the nature of dark matter and confirm the axion as the solution of the strong CP problem, but would also provide a glimpse of physics at the highest energy scales, far beyond what can be directly probed in the laboratory.

 

 

The Proton's Weak Charge

Dr. David Armstrong College of William and Mary

The Proton's Weak Charge One of the highest priorities of present-day experimental particle and nuclear physics is to search for indications of physics which is not contained in the Standard Model. Precision measurements of quantities that are robustly predicted within the Standard Model are an important class of such searches. An example is a measurement of the proton's weak charge. The weak charge is the strength of the proton's vector coupling to the weak neutral current, and its value is a firm prediction of the Standard Model. Thus an experimental test of the prediction is well motivated as a search for new physics. A recently completed experiment at Jefferson Lab, Qweak, has the goal of making the first precision measurement of the weak charge, using parity-violating electron scattering from hydrogen at very low momentum transfer. The result from the first subset of data will be presented, as well as an overview of the data analysis for the full data set and prospects for the final result, which will provide a sensitivity to new physics at the multi-TeV scale.

Studying Neutrino Mass with the Enriched Xenon Observatory (EXO)

THE ABSTRACT Neutrinoless double beta decay (0νββ) is a beyond-the-standard-model physics process in which a nucleus (A,Z) decays to (A,Z+2) with the emission of two electrons (but no neutrinos). Experimental searches for 0νββ are motivated by the access this process gives to testing any Majorana nature of neutrinos and lepton number non-conservation. This process is also a sensitive probe of the absolute neutrino mass scale. EXO (Enriched Xenon Observatory) is an experimental program searching for 0νββ decay of 136Xe. The first phase of the program, EXO-200, uses 200 kg of Xenon enriched to 80% in 136Xe, liquefied in a Time Projection Chamber (TPC) with scintillation readout (100 kg active mass), allowing for event calorimetry and 3D localization of ionizing events. EXO-200 has found the standard two-neutrino decay mode 2νββ of 136Xe, and has made a precision measurement of the (2.172±0.017[stat]±0.060[sys])×1021yr half. The collection of both light and charge signals and the reconstruction of event positions for both single and multi-cluster events allow background discrimination on top of the already low environmental background regime, and the possibility of studying events with extended topologies. A 5-tonne next generation liquid xenon experiment, nEXO, based on teh EXO-200 concept while implementing some notable innovations, is currently being designed. It promises to improve the sensitivity to improve the sensitivity to 0νββ of 136Xe by ~2 orders of magnitude and fully access the inverted hierarchy neutrino mass scale. This talk will discuss the detector performance and recent results from EXO-200 and present the nEXO experiment.

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