Unless otherwise noted, Colloqium is held Wednesdays 3:45-4:45 in Galallee room 227.
Coffee and cookies are available starting at 3:30.
A list of past colloquia is also available.
|Title / Abstract
|Galaxy Formation in a Hierarchical Universe - Predictions and Observations
| Jeremy Bailin
University of Michigan
|Properties of Dark Matter from the Faintest Galaxies
| Louis Strigari
Stanford / Kavli Inst. for Particle Astrophysics and Cosmology
|The Fermi Bubbles: Gamma-ray Relics of the Milky Way's Supermassive Black Hole
| Fulai Guo
Lick Observatory, U.C. Santa Cruz
|A New Probe of the Distribution of Dark Matter in Galaxies
| Sukanya Chakrabarti
Florida Atlantic University
|Exoplanets and the Search for Another Earth
| Angelle Tanner
Mississippi State University
|Computing Nuclei and Reaction Rates from Scratch
| Ken Nollett
Argonne National Laboratory
|Shedding Light on Dark Matter
| Rupak Mahapatra
|New Twists in Spintronics: anomalous Hall effect, spin-helix transistors, and topological thermoelectrics
| Jairo Sinova
- Aug 31
Faculty Research Presentations
Searching for Physics Beyond the Standard Model at the Large Hadron Collider
University of Alabama
The LHC is colliding protons at the highest energies ever achieved in the laboratory. It is hoped that these high-energy collisions will uncover the elusive Higgs boson, and perhaps reveal even more exotic new physics, such as supersymmetry or extra dimensions of space. UA is a collaborating institution on the Compact Muon Solenoid (CMS) experiment at the LHC. I will describe the research activities of the UA group, which focus on searches for physics beyond our current Standard Model of elementary particles.
The IceCube Neutrino Detector
University of Alabama
IceCube is the largest neutrino detector in the world. Construction was completed in January 2011 and IceCube is now taking data with a full cubic kilometer of instruments. I will describe the role that the University of Alabama group plays in the IceCube collaboration.
Galaxy Morphology in the 21st Century
University of Alabama
Classical galaxy morphology and classification has moved from the exclusive domain of blue light photography to other wavelength domains and even to color imaging. I will say a little about my morphological work with 3.6 micron images obtained as part of the Spitzer Survey of Stellar Structure in Galaxies (S4G).
- Sep 7
Faculty Research Presentations
The High Altitude Water Cherenkov (HAWC) Gamma-ray Observatory
University of Alabama
HAWC is a new gamma-ray detector currently being constructed near the peak of Sierra Negra in Mexico. I will give a short introduction to the experimental technique, an overview of the science goals, and an update on the construction status.
Spin-Torque Magnetic Random Access Memory
STT-MRAM is one candidate to serve as a "universal memory" replacing SRAM, DRAM, Flash and the hard drive. I will outline the status of the technology and MINT Center research aimed at helping it succeed as a commercial product.
- Sep 21
Facing a new era of discoveries in particle physics: higher energies, higher precision, higher expectations
Florida State University
High Energy Physics is a mature field which is now facing a new age of discoveries, between the end of the Tevatron and the beginning of the Large Hadron Collider eras. Very solid theoretical and experimental foundations allow us to clearly identify the remaining unanswered questions. Fundamental among them is the existence and the nature of the Higgs boson, the missing link between the Electromagnetic and Weak forces, and its relation to the diversity of mass scales witnessed in nature. Equally important is the existence of a spectrum of new particles predicted by different theories of electroweak symmetry breaking. We all expect the Large Hadron Collider to break the spell and give us evidence of new physics that will close most doors and possibly open new ones. It is fair to say that for the first time in decades we are facing the unknown and special care will be needed to disentangle its evidence and measure its properties. In recent years, enormous theoretical effort has gone into providing the best tools to attack this problem and impressive results can already be shown.
- Oct 3 Monday colloquium
Modeling Instruction: A Foundation for Transforming FIU's Physics Department
Florida International University
Florida International University (FIU) implemented Modeling Instruction in introductory physics as part of a broader educational reform initiative. Modeling replicates the central activities of scientists as students build, validate, and deploy models in the classroom. The Modeling Instruction course operates as a collaborative learning environment, with 30 students in a studio-format class with integrated lab and lecture. Inquiry labs and activities focused on conceptual reasoning and problem solving are the primary vehicles through which models are built, validated, and extended.
An overview of Modeling Instruction as well as results from the courses will be presented. The broader educational transformation sparked by the Modeling reform, including new physics teacher preparation programs, an undergraduate Learning Assistant program, and a budding university commitment to STEM Education, will be provided. These reforms have contributed to a 1500% increase in the number of intended and declared physics majors, when comparing current three-year averages to the early 1990's.
- Oct 19
New adventures in testing Einstein's general relativity
University of Mississippi
Einstein's general relativity and the standard model are two pillars of modern physics, but their unification is still an open problem. For this reason it is important to study general relativity in strong-gravity, high-energy situations, in search of experimental signatures of a possible breakdown of the theory. I will talk about different physical systems where Einstein's theory may break down, including black hole binaries and compact stars. I will explain how Earth- and space-based gravitational-wave detectors could constrain alternative theories of gravity, and maybe even point the way to a new theory of gravity.
- Oct 26
Gravitational Waves and Supernova Explosions from Merging White Dwarfs
University of Oklahoma
The identity of Type Ia Supernovae (SNe) Progenitors is one of the key open questions in astrophysics. Mergers of binary white dwarf stars are one of the proposed channels for the formation of SNe Ia. Short period binary white dwarfs may merge within a Hubble time due to gravitational wave radiation. We have begun a targeted survey to find merging white dwarf systems, and our first results have tripled the number of known merger systems. Our sample includes tidally distorted and eclipsing systems with orbital periods as short as 12 minutes and with merger times less than 1 Myr. These systems are among the strongest known gravitational wave sources. I will discuss the characteristics of this sample and potential links to Type Ia and underluminous supernovae.
- Nov 2
Loop Quantum Gravity: status and recent results for cosmology and black holes
Louisiana State University
Loop quantum gravity is one of the approaches followed to apply the rules of quantum mechanics to the gravitational field. We give an introduction to the main ideas behind loop quantum gravity and discuss recent results concerning cosmology and black holes.
- Nov 16
Probing Electron Charge Density and Bonding with Advanced Quantitative Transmission Electron Microscopy
University of Pittsburgh
The intrinsic properties of crystals are intimately related to their electronic structure. Establishing robust, accurate and commonly accessible methods for probing electronic charge density (Δρ(r)) in crystalline solids remains a high impact goal of basic science. Motivated by recent considerable progress in understanding transition metal intermetallics properties from electronic structure calculations we use these chemically ordered phases as examples to investigate aspects of ρ(r) determination by convergent-beam electron diffraction (CBED) in a transmission electron microscope. CBED permits study of nano-scale volumes, thereby offering access to a broad range of crystalline phases. Our experimental studies focus on Δρ(r) as one of the quantum mechanical characteristics central for developing fundamental understanding of material properties and for validation of electronic structure theory in solids. Successes and challenges in development of a robust experimental CBED method to determine Δρ(r) will be described. CBED studies performed on cubic β-NiAl and the iso-structural tetragonal γ1-FePd and γ-TiAl intermetallic phases for temperatures ranging from 100K-300K will be discussed. Relative to solids comprised of light elements (e.g. Z≤14, where Z is the atomic number), experimental bonding studies in intermetallics based on d-electron transition metals is very challenging due to increasingly stringent requirements for the precision on structure factor measurements. Applying a newly developed CBED method we have been able to deliver first measurements of sufficient accuracy and precision for Δρ(r) determination in FePd. Comparative CBED of FePd and TiAl enables assessment of possible magnetism effects. Quantitative CBED experiments promise an additional and effective path towards validation of electronic structure theory and further progress on understanding electronic structure – properties relationship in intermetallics. This work is supported by the Office of Basic Energy Sciences, Division of Materials Science and Engineering (Grant No DE-FG02-08ER46545).
- Jan 25
Galaxy Formation in a Hierarchical Universe - Predictions and Observations
University of Michigan
In the standard Cold Dark Matter paradigm, galactic-scale structure grows hierarchically: small structures collapse first before merging into larger and larger structures. The outer regions of galaxies, i.e. their stellar halos, are a particularly rich source of information about the assembly history of galaxies. I will present the McMaster Unbiased Galaxy Simulations, a suite of simulated galaxies that I am using to understand the formation and structure of galaxies, and highlight some recent results regarding successes and failures of the model. I will also present ground-based observations of the stellar halos of several galaxies beyond the Local Group and discuss how I am using the observations and simulations to constrain our understanding of how galaxies form.
- Feb 1
Properties of Dark Matter from the Faintest Galaxies
Stanford / Kavli Institute for Particle Astrophysics and Cosmology
In recent years the number of known Milky Way satellite galaxies has more than doubled. Follow-up measurements indicate that among this population of objects are the nearest and most dark matter-dominated galaxies known in the Universe. In this talk I will focus on two particularly exciting implications of these observations. I will first show that gamma-ray observations of these objects by the Fermi-LAT are now able to, for the first time, test well-motivated models of thermal relic WIMP dark matter. I will then discuss how the number counts of these objects strongly constrain the small-scale power spectrum of cold dark matter. I will close by discussing how, in the next several years, a variety of surveys will improve and strengthen these results with the discovery of new faint galaxies.
- Feb 15
The Fermi Bubbles: Gamma-ray Relics of the Milky Way's Supermassive Black Hole
Lick Observatory The Fermi Gamma-ray Space Telescope is a major new astronomical observatory. During its first three years of observing, it has made one of its most important and unexpected discoveries: two enormous gamma-ray-emitting "Fermi bubbles", located north and south of the Galactic center. I will talk about gamma-ray emission mechanisms and possible scenarios on the bubbles' origin, arguing for an origin related to the 4-million-solar-mass black hole at the Galactic center. Our recent hydrodynamical simulations, including cosmic ray dynamics and transport, describe the dynamical evolution of the bubbles for the first time, showing that they could be produced by a pair of powerful jets from the black hole initiated 1-3 million years ago that was active for 0.1 - 0.5 Myr. When compared with observations, our work suggests important additional physical processes during the bubble evolution: magnetic draping, shear viscosity and cosmic ray diffusion. At the end, I will briefly talk about remaining mysteries (future work) and astronomical implications of the bubbles.
- Feb 22
A New Probe of the Distribution of Dark Matter in Galaxies
Florida Atlantic University
The cold dark matter (CDM) paradigm of structure formation is successful at recovering the basic skeletal structure of the universe -- the large-scale distribution of galaxies. However, the agreement between theory and observation is less secure when this model is applied to galactic (and sub-galactic) scales. The "missing satellites problem" refers to the excess of predicted CDM sub-structure relative to observed Local Group dwarf galaxies. Recent discoveries of dark-matter dominated dwarf galaxies, some fainter than some star clusters, makes one wonder whether there may be a population of faint dwarf galaxies, lurking just beyond our reach. The extended atomic hydrogen disks of galaxies provide an unique probe of galaxy evolution. They are ideal tracers of tidal interactions with satellites and the galactic gravitational potential well. We have recently developed a method whereby one can infer the mass, and relative position (in radius and azimuth) of satellites from analysis of the observed disturbances in outer gas disks, without requiring knowledge of their optical light. I will present the proof of principle of this method by applying it to galaxies with known optical companions. I will also present our earlier prediction for a dim companion of the Milky Way. I will end by presenting an extension of this method to characterize the density profile of the dark matter halo in spiral galaxies, and discuss future prospects for understanding galaxy evolution by studying the interplay of dark matter and baryons in galaxies.
- Mar 7
Exoplanets and the Search for Another Earth
Mississippi State University
We are on the verge of potentially the most significant scientific discovery of the past century – the discovery of a habitable planet around a star in our solar neighborhood. Alas, there are still a few observational hurdles to overcome before this accomplishment. My research addresses two issues impeding the discovery of Earth-mass planets. One aspect involves the detection of planetary-mass companions around M-dwarfs, the most diminutive and prolific stars in the galaxy. The other component involves studying the properties of the host stars to predict any measurement noise due to features on the surface of the target star. For planet detection, I use infrared radial velocity and optical astrometry measurements to survey nearby M-dwarfs for hot-Jupiters and long period planets. Both of these observing methods are in their infancy and improvements in measurement precisions are aiding in the detection of Earth-like planets in the habitable zones of nearby M-dwarfs. If our ultimate goal is to survey nearby Sun-like stars for habitable planets, then it will be essential for us to fully understand the intrinsic radial velocity jitter inherent to each star as well as the properties of stars which hint to whether planets would exist or are probably not there. To that end, I am studying the variability, multiplicity, debris disk properties and metallicity of nearby stars. Not only could this lead to the detection of habitable planets but it will also tell us how all planets form and evolve.
- Mar 28
Computing Nuclei and Reaction Rates from Scratch
Argonne National Laboratory
A hundred years have now passed since Rutherford discovered the nucleus (in the data of Geiger and Marsden) and eighty years since Chadwick discovered the neutron. Until the very end of the twentieth century, the interaction between nuclear constituents (nucleons) was poorly characterized and too complex for predictive ab initio calculations connecting it quantitatively to properties of nuclei. In the last two decades, the nuclear force problem has finally been solved, revealing a complex interaction that requires about 30 parameters for its accurate description and three-body as well as two-body terms. I will discuss these problems and describe calculations that start from the nuclear interaction and yield masses and other properties of nuclei as large as carbon. I will conclude by describing efforts to extend these calculations beyond static properties to describe nuclear scattering and reactions, with an eye particularly on astrophysical applications. Throughout, I will mainly focus on work using quantum Monte Carlo techniques.
- Apr 11
Shedding Light on Dark Matter
Texas A&M University
Dark Matter dominates the matter content in the Universe and is believed to be made up of Weakly Interacting Massive Particles (WIMP). Deciphering the nature of dark matter is one of the hottest quests in Physics now and is pursued by more than 30 experiments, spanning hundreds of Universities and labs in all major countries in the world. Search for this elusive particle with very rare interaction is extremely challenging, especially compounded by dominant radioactive background that can confuse any dark matter signal. It takes ingenious detector technology to make such a discovery possible. I will review current experiments, ranging from my own experiment utilizing cryogenic semiconductor detectors operating at almost absolute zero temperature to experiments utilizing room temperature bubble chamber detectors. I will summarize how they compare in their scientific reach to the $10 billion dollar Large Hadron Collider in Europe, which is also searching for evidence of Supersymmetry, a theory that could explain the existence of dark matter.
- Apr 18
New Twists in Spintronics: anomalous Hall effect, spin-helix transistors, and topological thermoelectrics
Texas A&M University
Understanding the origin and properties of the different phases of materials and how to control them is at the heart of condensed matter physics. One of the grand challenges of the field is controlling spin-dependent properties without using magnetic fields. To do so, one can resort to the spin-orbit coupling (SOC), which is one of the few echoes of special relativity in condensed-matter physics that connects the spin and charge of the electron. While the weak SOC regime is relatively well understood, the strong SOC regime remains one of the most theoretically challenging at a fundamental level. The study of this regime has given rise to new physical insights in established phenomena, such as the anomalous Hall effect, and new emerging frontier fields, such as topological insulators. We have learned how to exploit SOC to create new paradigms of spin control in complex materials and discover new unexpected links between seemingly disparate ideas such as topology, materials science, ferromagnetism, and thermoelectricity. I will broadly describe joint theoretical and experimental efforts on how we use SOC to control transport and other materials properties. I will also show in some detail new theory insights on the anomalous Hall effect the realization of a spin-transistor by exploiting the recent spin-helix state and a new proposal on how to increase thermoelectric efficiency originating from the topological properties of the band structure imparted by the strong SOC in topological insulators.
Spin pumping effects: magnetic metal and insulator interfaces
Simon Fraser University (Canada)
Discoveries of spin transfer, quantum spin hall and other spin dependent effects , invigorated interest in spin transport phenomena in recent years. Spin pumping is one of such effect which attracted significant attention due to its fundamental physics appeal and potential applications. In this presentation I discuss results of spin pumping studies with Fe/Au/Fe(001) and YIG/Au/Fe systems using ferromagnetic resonance (FMR). FMR measurements were performed at frequencies ranging from 27 to 41 GHz at temperatures between 88 to 295 K. First I describe how using the magneto-transport and spin diffusion equations we determine the spin mixing conductance at the Fe/Au interface and the spin flip relaxation time in Au as a function of temperature. We find that the spin flip relaxation time in Au is dominated by phonon interactions and is only weakly dependent on the interface diffuse scattering. I continue with recent experimental results for ferrimagnetic insulator (YIG)/normal metal (Au) interfaces. In the case of YIG/2.0nmAu/4.3nmFe we demonstrate that spin pumping mechanism originates from the time retarded interlayer exchange coupling. Spin mixing conductance is found to be 70% of that recently theoretically predicted using first principle calculations. We argue that YIG film can be excited to a large angle of precession with moderate microwave powers so that the efficient spin pumping could provide a large dc spin current suitable for spintronics applications.
The MiniCLEAN Dark Matter Experiment
Los Alamos National Laboratory
The MiniCLEAN dark matter experiment will exploit a single-phase liquid-argon detector instrumented with photomultiplier tubes submerged in the cryogen with nearly 4π coverage of a 500 kg (150 kg) target (fiducial) mass. The high light yield and unique properties of the scintillation time-profile provide effective defense against radioactive background through pulse-shape discrimination and event-position reconstruction. The detector is designed also for a liquid-neon target that allows for an independent verification of signal and background and a test of the expected dependence of the WIMP-nucleus interaction rate.