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.

Refreshments will be served in CP 179 at 3:15 PM

 

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.

Refreshments will be served in CP 179 at 3:15 PM

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.

Refreshments will be served in CP 179 at 3:15 PM

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.

Refreshments are served in Chem-Phys 179 at 3:15 PM

 

 

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.

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.

Organic molecules such as tetracene crystallize into solids that can be semiconductors, metals, or even superconductors. Although they were first developed over half a century ago, it is only fairly recently that the considerable promise that organic semiconductors hold as materials for electroncs, display technologies, and solar cells has begun to be realized. Lightweight, flexible, and inexpensive, these materials offer an attractive balance between cost and performance, complemented by versatility, and functionally accomplished by means of molecular design. I will review the physics of organic semiconductors and describe how their electronic and optical properties can be utilized in a variety of applications.

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.

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.
 

Refreshments will be served in CP 179 at 3:15 PM

It has long been known that galaxies' internal structure is connected with their star formation activity in the nearby universe. Recent surveys have allowed us to study these correlations out to very large distances, allowing us for the first time to quantify these relationships over a significant span of cosmic time for statistically robust samples of objects. It has been known for several years that galaxies are growing in mass and radius, experiencing morphological transformation, and "downsizing" their star formation activity over cosmic time. Now, new observations are painting a picture in which the internal structure of galaxies (size and morphology) is intimately linked with their star formation activity and formation history. There are hints that the co-evolution of supermassive black holes with their host galaxies may be the driving force behind these correlations, but this remains controversial. While cosmological simulations set within the hierarchical formation scenario of Cold Dark Matter currently offer a plausible story for interpreting these observations, many puzzles remain. I will review recent insights gleaned from deep multi-wavelength surveys and state-of-the-art theoretical models and simulations, as well as highlight the open questions and challenges for the future.