Theory predicts the existence of some peculiar phases of quantum condensed matter systems that have multiple degrees of freedom with very low energy, when localized “defects” are introduced. I shall focus on a class of these phases where each defect has half of a conventional degree of freedom, and the defects may be considered as sites for localized zero-energy states of a “Majorana fermion”. Such defects would also exhibit the intriguing property of “non-Abelian statistics” -- i.e., if various defects can be moved around each other, or if two identical defects can be interchanged, the result is a unitary transformation on the quantum mechanical state that depends on the order in which operations are performed but is insensitive to many other details. In my talk, I will try to explain these various concepts and discuss the attempts to realize them in condensed matter systems.
In this presentation, I will discuss the hypothesis proposed by Francis and Vavrus (2012) that links rapid Arctic warming (so-called Arctic amplification) to changes in the large-scale atmospheric circulation in the northern hemisphere that favors more persistent weather patterns and a higher likelihood of extreme weather events such as droughts, cold spells, flooding, heavy snows, and heat waves. This hypothesis has been a topic of considerable controversy in recent months, particularly regarding its relationship to the unusual weather conditions that persisted in the winter of 2013/2014. I will discuss various aspects of this linkage, what we know and don't know, and present new related research. Refreshments will be served in CP 173 at 3PM
Fundamental scientific research, as a majority federally funded initiative, is becoming more deeply embedded in politics. Since the end of the Space Race, funding of basic physical sciences research as a percent GDP has continuously declined, indicating that policy makers see funding scientific research as less of a priority than they once did. Indeed, a lack of understanding about both science and how science is done amongst members of Congress has led to both reduced prioritization and also to misguided attempts at regulation, such as making peer review a public process and considering Congressional oversight for specific grants. Here we will examine a few current issues in science policy and the need for physicists to effectively weigh in on such policy issues. We will also consider the positive or negative effects such public engagement may have on our scientific careers and ways in which you can get involved.
Neutrinos are the most numerous massive particles in the Universe. Their masses are very tiny, no larger than one millionth the mass of the electron. Are they like all the known massive fermions, being four component particles, or are they a new type of fermion never seen before, a two component fermion? Are there only only three neutrinos or are there more species of neutrinos? Of the three neutrinos we know of, we have determined part of the massing pattern but not the completely pattern. Also we have measured some of their mixing parameters with reasonable precision via neutrino oscillation experiments but not all. Do neutrinos violate CP in neutrino oscillations? Can neutrinos help explain the baryon-antibaryon asymmetry of the Universe? I will address many of the important questions about the neutrinos and how the future Fermilab program will address some of these questions.
The cosmic ray spectrum spans many orders of magnitude in energy. At the very end of the spectrum (E>10^18 eV) lie the Ultra High Energy cosmic rays (UHECRs). Their origin remains largely unknown and their study is made difficult in part by the very low flux impinging on Earth's atmosphere. The Pierre Auger Observatory, located in the Mendoza province of Argentina, is an array of detectors spread over 3000 km^2 specifically designed to study the properties of the extensive air showers induced by the UHECRs in the atmosphere. The Observatory is fully operational since 2008 and is operated by a collaboration of more than 500 scientists and engineers from 19 countries. In this colloquium, a selection of recent results obtained by the Observatory and the plan for the upcoming upgrade will be presented.
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.
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.
