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. 

Title:  Effective field theory, factorization and renormalization: neutrino interactions, the Fermi function and more

Abstract:  Precisely calculating differences between muon- and electron-neutrino interactions is difficult, but is vital for correctly interpreting neutrino oscillation experiments. I describe recent progress with powerful quantum field theory techniques to precisely determine the ratio of νe and νµ cross sections.  The beta decay Fermi function describes electron propagation in a nuclear Coulomb field, and accounts for QED radiative corrections that are enhanced at small electron velocity β or large nuclear charge Z.  Such enhanced corrections impact a variety of processes in and beyond the Standard Model, ranging from nuclear beta decay to dark matter annihilation signals.  I present the field theory factorization formula for the Fermi function and discuss implications for precision measurements. 

Organic light emitting diodes (OLEDs) were introduced into commercial application in the early 2000's and are now used regularly in phone screens and high-end TVs, while organic photovoltaics and transistors have not found the same technological success. In this talk, I will cover some of the fundamental performance and physical reasons that these devices are hindered while OLEDs flourish. Looking into the future beyond traditional electronics, I will discuss the unique physical properties of organic semiconductors that make them suitable for the development of spin-based devices and room temperature excitonic devices.

Host: Joe Brill

Burning fossil fuels is increasing the CO_2 content of the atmosphere.  This may lead to climate change.  How hard will it be to convert to a renewable energy economy?  I'll discuss the issues that are involved.

Host: Kaul

BCS theory and its strong-coupling extension have been widely successful in accounting for the properties of conventional superconductors, where phonons mediate pairing. These theoretical frameworks, however, neglect critical physical processes like the formation of competing phases or lattice polarons. This aspect has left several crucial questions in conventional superconductivity unanswered. For example, is there a maximum Tc for conventional superconductors? In this talk, I will discuss recent attempts to answer this question using numerically exact quantum Monte Carlo methods applied to the Holstein hamiltonian. I will show that many parameter regimes considered in the current literature violate the assumptions underlying this canonical toy model. These results remind us that we must be careful when using model hamiltonians to derive general conclusions about materials; however, they also direct us towards a wide-open are of research beyond BCS theory.

Host: Ambrose Seo

The Standard Model of Particle Physics (SM), our most fundamental mathematical description of physics reality, is the great triumph and the great frustration of modern physics.  The triumph is that the SM has survived all laboratory tests.  The frustration is that the SM is not consistent with basic features of the universe.  Tabletop measurements of electron dipole moments provide some of the most stringent tests of the SM and beyond. 

Host: Gardner and Korsch

We will introduce exotic particles like anyons and Majorana modes and describe

how they obey unfamiliar statistics which are neither like bosons nor like fermions. We will then discuss where one can expect to find such excitations. Finally, we will briefly explain our idea on how to distinguish Majorana modes from impostors.