Fall 2013
The discovery of a Higgs boson at the Large Hadron Collider
In summer 2012 the CMS and ATLAS experiments at the Large Hadron Collider (LHC) found a new particle which could be the long sought Higgs boson. This boson was already predicted to exist in 1964, and is thought to be responsible for making the Universe work the way we know it today. Scientists have been searching for this particle without success for the last few decades, at ever more powerful accelerators. Such a Higgs-like particle was finally found last year. In this lecture we will show a brief history of the discovery of the particle, highlight the importance of this discovery, and discuss the very latest results from the LHC, using mostly the CMS data as an example. In particular, new results obtained this year with the full 2011-2012 collisions data set show that this new particle has indeed all the properties that we can measure, for it to be called a Higgs boson. This Higgs boson is a brand new elementary particle, unlike any other elementary particle we have discovered so far.
How Globular Clusters Acquire Their Heavy Elements
I will present a theoretical model to describe how globular clusters self-enrich in heavy elements (“metals”). This model assumes that the highest mass stars formed, evolved, and exploded as supernovae while the lower-mass stars visible today were still forming, seeding the protocluster cloud with metals while also gravitationally unbinding some of the gas. The predictions of this model provide a good match to the “blue tilt” mass-metallicity relation of globular clusters around massive elliptical galaxies, and also predict a milder “red tilt”.
Explosive Thermonuclear Runaways in Helium Layers on White Dwarf Stars
Some binary star systems containing a white dwarf star star (WD) made of helium and another WD, made of carbon and oxygen or oxygen and neon, form in a close enough orbit that they will transfer helium from one star to the other. When enough helium accumulates, fusion is re-ignited in this surface layer and a runaway, thermonuclear-driven outburst takes place. Some of these outbursts can become vigorous enough that the local hydrostatic balance of the envelope is overcome by the heat input, possibly leading to the detonation of the helium shell. This detonation may form radioactive nickel and create an event bright enough to be observed as a supernova-like stellar transient. I will briefly discuss computations of how the dynamics of the detonation propagating through the thin surface layer, as well as how the thickness of the helium layer can change whether the products are predominantly calcium, titanium, nickel, or other materials. I will also discuss how these explosions and their host star systems are related to potential progenitor star systems for the brighter Supernovae Type Ia.
Klingons and hobbits and stars – oh, my! Science outreach at DragonCon
DragonCon in Atlanta is an annual gathering of more than 50,000 fans of science fiction, fantasy, gaming, costume design, and niche music. Its 38 fan-designated programming tracks include several dedicated completely to science and space, affording unusual opportunities for outreach to enthusiastic and engaged audiences (at all hours). I describe a decade’s experience in such events as live astronomy with remote observatories, presentations on requested topics, and launch sessions for a webcomic and a more complex graphic anthology tied to Hubble research programs.
Computational investigations of static and dynamic magnetization properties of spintronic materials
Spintronics is a rapidly growing research field with many emerging technologies that aim to exploit the spin of the electron in addition to its electric charge. The design of new materials with specific properties optimized for a particular application is of paramount importance for widespread adoption of spintronic technologies. This requires an understanding of the fundamental mechanisms responsible for the magnetization relaxation, the anisotropy and the spin polarization and their interdependencies. In this short presentation I will give an overview regarding different research projects my group is involved with.
Electrical transport characterization at UA
Electrical transport measurements are critical to characterizing modern materials and devices. In this talk, I will present examples of more advanced transport characterization measurements that can be used to characterize the detailed microscopic physics of electron transport in novel materials and devices. In particular, I will point out the utility of impedance spectroscopy, inelastic tunneling spectroscopy, and noise spectroscopy measurements in spintronic and other systems by way of illustrative examples. The main focus will be on the ‘non-traditional’ measurements performed here at UA and how they may be applied to systems of interest. I will also talk about some recent work in my laboratory on commercializing a low-cost electromyography device we have developed.
Growing a physics program at Arkansas: Increasing student success
A more scientifically literate society benefits all STEM disciplines, as well as society as a whole. It is best realized by better serving all undergraduate STEM students. In better-serving all students, a STEM department also benefits. The University of Arkansas, Fayetteville physics department has seen a drastic change in number of majors, the number of students active in research and the number of graduates pursuing graduate work, while also increasing the number of majors who decide to teach. Prior to our involvement with the Physics Teacher Education Coalition, graduation rates had increased by more than a factor of 4 in 4 years. After the increased efforts when we became a part of PhysTEC (www.PTEC.org) our graduation numbers doubled again. Specific attention to class policy to impact student learning in our introductory courses and strong preparation of the graduate teaching assistants, and quality advising were our primary areas of emphasis. Based on what we learned from our efforts in PhysTEC, we have implemented an undergraduate STEM teacher preparation program, UAteach. What worked to build these numbers and strengthen these resources at Arkansas will be discussed.
Adventures in commercializing research
Recently, we have developed a portable electromyography (EMG) device in my laboratory. EMG is a method of measuring the electrical activity produced by skeletal muscles, and is typically used in medical and rehabilitation settings. We were surprised to find out that device comparable to ours does currently not exist in the market. We were further surprised to find that while EMG is an almost ideal tool for fitness and strength monitoring, the cost of typical EMG units has prohibited its use in this the fitness community. In this colloquium, we will outline the basics of what EMG is and does, the principles behind the device we constructed, the process of commercializing technology developed at UA, and the process of forming a company. We will end with a demonstration of the technology for those interested.
On the trail of the elements: Astrobiology from a stellar perspective
The potential habitability of an extrasolar planet depends on the properties of the entire stellar system to which it belongs. One of the principle variables that affects everything from the evolution of the star to planetary properties to the inventory of bioessential elements available to life is chemical composition. We find that the ratios of chemical elements vary substantially from star to star in the solar neighborhood and examine this important fact from two perspectives. Multidimensional supernova simulations show how material sampling unique sites of nucleosynthesis can be incorporated into forming planetary systems, resulting in large compositional variations in nearby stars from even a single supernova event. We also explore the effect of variable compositions on the evolution of stars and their habitable zones and the interior physics of terrestrial planets. For the range of oxygen to iron ratios seen in nearby stars the habitable lifetime of a 1AU orbit around a star otherwise identical to the sun can change by billions of years.
What low surface brightness galaxies tell us about dark matter
Dark matter plays an important role in our current understanding of the universe. Low surface brightness spiral galaxies are particularly excellent laboratories for probing the dark matter distribution and placing constraints on theoretical models. I will discuss the observational techniques used to study these galaxies and their dark matter halos. I will also discuss how well current dark matter models describe the observations and suggest how they might need to be updated.
Atmospheric Circulation Models of Hot Jupiters: Exploring Exotic Regimes and Predicting Observable Properties
Hot Jupiters are relatively rare compared to other types of exoplanets and yet they have received the most attention from observers and theorists alike. This is in part because they are the best targets for atmospheric characterization measurements and in part because they are so unlike anything in our solar system that they present an excellent opportunity to expand our understanding of atmospheric physics. I will review the current status of observations of hot Jupiter atmospheres, with an emphasis on those pertaining to the three-dimensional structure of the planet. I will also discuss the development of atmospheric circulation models for this unique regime, from basic scaling-law predictions to complex models that include exotic effects (such as magnetic drag and heating). Finally, I will comment upon the new types of measurements that will be enabled by future instruments and how we can work toward a holistic understanding of these planets.
Current-driven transfer of spin momentum between magnets: its genesis and prospects
Consider two nanoscopic mono-domain magnetic films connected by a spacer composed of a non-magnetic metal or a tunnel-barrier. Any externally applied electric current flowing through these three layers will transfer oriented electron spins that result in feeble pseudo-torques acting on both magnetic moments (J. S. 1989). Such a weak spin-transfer torque (STT) may counteract and overcome a comparably weak torque caused dynamically by viscous dissipation (L. Berger 1996; J. S. 1996). As a result, any initial motion (e. g. excited by ambient temperature) of one moment (or both), may grow in amplitude and culminate in a steady precession or a transient switching to a new direction of static equilibrium. At very small scales, this STT method of exciting a magnet is more efficient than that relying on an induced magnetic field. Consequently, world-wide exploratory developments of radio-frequency oscillators and memory arrays utilizing current-driven STT enjoy today a huge dollar commitment.
Such creation, by flow of electricity, of each half-unit of oriented electron-spin momentum h/4π requires the transfer of one unit of electric charge. This fact limits the device efficiency possible using current-driven STT. But, arguably STT could also arise from an externally-driven flow of heat between an insulating magnet, of spinel or garnet ferrite composition, and the metallic spacer (J. S. 2010). For, whenever the exchange interaction between a 3d electron of the magnet and a 4s conduction electron annihilates a hot magnon at the insulator/metal interface, it transfers one full unit h/2π of oriented spin momentum to the spacer. Conduction electrons within the spacer will convey this spin momentum to the second magnet without need of an electric current. In theory, such a thermagnonic method, modestly powered by a Joule-effect heater, can substantially increase the efficiency of STT. This assessment is based alternatively on: (1) an estimate (J. S. 2010), considering a spinel ferrite, of sd-exchange from data on spin relaxation in magnetically dilute (Cu,Ag,Au):Mn alloys, (2) a DFT computation (J. Xiao et al 2010), and (3) most persuasively on data from spin pumping driven across a YIG/Au interface by RF ferromagnetic resonance (B. Heinrich et al 2011; C. Burrowes et al 2012). It follows that the notion of thermally generated spin transfer is ripe for a needed experimental test.
Search for new Physics in the cosmic rays with AMS
The Alpha Magnetic Spectrometer is a detector installed on the International Space Station to accurately measure the Cosmic Ray fluxes. Since May 2011 AMS has uninterruptedly taken data, and will continue for the whole life of the ISS representing the most important cosmic ray observatory on orbit. The recent AMS results on the fluxes of the dominant and the rare components of the cosmic rays will be presented and discussed. Possible interpretations of the observed phenomena will be also discussed.
Neutrino physics with the SNO+ detector
The SNO+ experiment is the successor to the Sudbury Neutrino Observatory (SNO), in which SNO’s heavy water is replaced by approximately 780T of liquid scintillator (LAB). The combination of the 2km underground location, the use of ultra-clean materials and the high light-yield of the liquid scintillator means that a low background level and a low energy threshold can be achieved. This creates a new multipurpose neutrino detector with the potential to address a diverse set of physics goals, including the detection of reactor, solar, geo- and supernova neutrinos. A main physics goal of SNO+ is the search for neutrinoless double beta decay. By loading the liquid scinitillator with 0.3% of natural Tellurium, resulting in about 800kg of Tellurium-130, a competitive sensitivity to the effective neutrino mass can be reached. This talk will present the status of the SNO+ detector, and then discuss the plans and goals of the double beta decay phase.
X-ray Observations of Hot Gas in Galaxy Groups and Clusters with Chandra, XMM-Newton, and Suzaku
The standard model of cosmology, a universe dominated by cold dark matter, directly predicts that bounded systems on various scales — galaxies, groups of galaxies, and clusters of galaxies — evolved from a near-uniform Big Bang through hierarchical formation. Such a scenario naturally gives rise to questions such as how well can self-similarity hold for systems on different scales and what is the role played by baryon physics in their evolution. 90% of the baryons in galaxy groups and clusters are in the form of hot gas emitting in X-rays through bremsstrahlung radiation. Together, three ongoing major X-ray missions (Chandra, XMM-Newton and Suzaku), with different advantages to complement each other, enable us to study various physics and over a wide range of physical parameters. I will talk about recent studies of baryon physics in groups and clusters, such as gas fraction, gaseous clumpiness, AGN feedback, ram pressure, and metal enrichment.
Spring 2014
Classical and Sub-wavelength Spectroscopy in Carbon Nanotube Systems
Optical spectroscopy, such as Raman and Photoluminescence, has being a powerful experimental tool to assess information about electrons and phonons in materials. Particularly special, Raman spectroscopy it is a fast and non-invasive technique that brings, besides information about electrons and phonons, a lot of meaningful structural analysis of materials. In this talk, I will introduce and discuss some details of Raman spectroscopy and Near-Field Raman/Photoluminescence spectroscopy as well as their application to my past and recent work on carbon nanotubes anti-fuses and remodeling.
The Luminosity-Size Relation for Galaxies
Recent studies have shown that massive galaxies in the distant universe are surprisingly compact, with typical sizes about a factor of three smaller than equally massive galaxies in the nearby universe. It has been suggested that these massive galaxies grow into systems resembling nearby galaxies through a series of minor mergers. In this model the size growth of galaxies is an inherently stochastic process, and the resulting size-luminosity relationship is expected to have considerable environmentally dependent scatter. To test whether minor mergers can explain the size growth in massive galaxies, we have closely examined the scatter in the size-luminosity relation of nearby elliptical galaxies using a large new database of accurate visual galaxy classifications. We demonstrate that this scatter is much smaller than has been previously assumed, and may even be so small as to challenge the plausibility of the merger-driven hierarchical models for the formation of massive ellipticals.
Understanding Interacting Many-Particle Systems: Accurate Theories for Liquids and Electronic Excitations in Solids
Despite the simplicity of their microscopic constituents, electrons and nuclei, the theoretical modeling of condensed matter systems is very difficult. The origin of the problem is that a very large number of particles are interacting with each other. In my talk, I will discuss various theoretical approaches to deal with interacting many-particle systems. In the first part, I will introduce a new method to describe molecular liquids, such as water, which plays an important role in biophysics and electrochemical energy technology. In the second part, I will describe recent theories of electronic excitations in materials. Specifically, I will show how an accurate description of the interaction between electrons and plasmons, quantized charge density oscillations, can explain recent photoemission experiments in doped graphene and silicon.
From Topological Insulators to Majorana Fermions
The discovery of topological insulators has created a revolution in condensed matter science that has far ranging implications over coming decades. I will introduce a simplest way to understand various topological phases that can fit into an elegant periodic table, and apply these ideas to semimetals, insulators, and superconductors. In the case for a time-reversal-invariant topological superconductor, a Majorana Kramers pair may appear on the boundary and induces unprecedented fractional Josephson effects. These effects indicate the existence of a periodic building for topological phases, with the aforementioned table being its ground floor. Strong connections will be made to ongoing experiments and related topics.
Shedding Light on Two-Dimensional Electrons in Graphene
Graphene, a single layer of carbon atoms, has stimulated intense scientific interest due to its distinctive electronic and mechanical properties. Graphene also exhibits strong interactions with light over a broad spectral range. This enables us to examine its electronic and vibrational properties through optical spectroscopy. In addition to gaining understanding of the properties of single-layer graphene, we can also probe the behavior of electrons in few-layer graphene. This reveals the unique electronic and vibrational properties for graphene of each layer thickness and stacking order, as well as their distinct capability to induce an electrically tunable band gap. I will also highlight recent development of 2D materials beyond graphene.
A Theory of Everything at Strong Coupling
Over the past 20 years an exciting new research area has emerged in Physics. It brings together physicists studying string theory, heavy ion collisions, condensed matter systems, cold atoms, cosmology, general relativity, and many more. What unifies all of these subjects is the question: how do quantum systems behave at strong coupling and far from equilibrium? The connection between these different subjects is provided by a holographic correspondence: strongly coupled quantum systems on one side correspond to certain theories of gravity on the other side. In this colloquium I will provide an intuitive introduction to the fascinating concepts of this thriving research area, especially with an eye on heavy ion collisions.
The Higgs Boson, Jets, and the Next Discovery
Recently, a new particle was discovered at the Large Hadron Collider that is broadly consistent with the long-sought Higgs boson, apparently validating our basic picture of elementary particle physics. The discovery has profound implications for Nature, but also sharpens a deep puzzle: What dynamics allows the Higgs boson to exist? The LHC is in a prime position to address this puzzle head-on, through an extensive program of searches for new phenomena. The enormous rate and complexity of energetic proton collisions make this a challenging endeavor, one that demands a strong interplay between theory and experiment. In the past several years, theorists have been developing new approaches to interpreting the ubiquitous collimated sprays of debris from these collisions, called “jets,” unlocking search channels that were formerly thought to be impossible. These approaches are already bearing fruit at the LHC, and continue to grow in importance. I will provide an overview of the current situation, highlighting some promising avenues for the next discovery.
Particle Physics Beyond the Standard Model: The Search for the Shadow World
All currently observed collider phenomena are perfectly explained by the Standard Model (SM) of Particle Physics. However, the SM suffers from a naturalness problem, and any solution of this problem would require modi cations of the SM at the TeV scale. I will review the naturalness problem and discuss several possible solutions of this problem, including supersymmetry, and show the connection of these theoretical scenarios with the current searches at the LHC. In addition to theoretical motivation, we have experimental evidence that the SM is incomplete: we know that approximately a quarter of the Universe’s energy budget is composed from Dark Matter. I will review the evidence for DM and further concentrate on the highly motivated, but largely overlooked possibility of self-interacting DM. In particular a portion of the DM can be self-interacting and form a dark Galactic disk. I will discuss the constraints on this scenario and emphasize possible future signals.
Searching for Dark Matter, from Colliders to the Cosmos
Dark matter makes up about 85% of the total matter in the Universe, but all we know about it so far comes from its gravitational influence in galaxies and on larger scales. From a particle physics point of view, we would like to know what is the underlying theory of dark matter, what are its fundamental interactions, and how does it interface with our current understanding of particle physics, the Standard Model? In this talk, I summarize recent results in this on-going search for dark matter, from experiments deep underground and at the highest energies to observations of the entire cosmos. Since we don’t yet know what the theory of dark matter is, we should be open-minded bout how dark matter signals may manifest beyond the standard paradigms.
Applications of the AdS/CFT Duality in Elementary Particle Physics
The Anti-deSitter/Conformal Field Theory correspondence (AdS/CFT) is a powerful tool in studying strongly-coupled field theories using a weakly-coupled theory of gravity or string theory. AdS/CFT was successfully applied to the study of phenomena in QCD, the quark-gluon plasma produced in heavy-ion collisions, and condensed matter physics. In this talk I will present a selection of examples in each of these areas. Among other topics, I will present a holographic model for the so-called Chiral Magnetic Effect in heavy-ion collisions and discuss an interesting condensed matter toy model for strange metals.
Computational Nuclear Structure in the Eve of Exascale
The long-term vision of nuclear theory is to arrive at a comprehensive and unified description of nuclei and their reactions, grounded in the interactions between the constituent nucleons. Theorists seek to replace current phenomenological models of nuclear structure and reactions with a well-founded microscopic theory that delivers maximum predictive power with well-quantified uncertainties.
High performance computing provides answers to questions that neither experiment nor analytic theory can address; hence, it becomes a third leg supporting the field of nuclear physics. Today’s petascale computers, capable of a quadrillion operations per second, have helped us move closer to solving the nuclear puzzle. They will soon be replaced by exascale computers, which will be capable of a million trillion calculations per second! All of this vast computing power will provide an unprecedented opportunity for nuclear theory. In this talk, advances in theoretical studies of nuclei will be reviewed in the context of the main scientific questions.
Galaxy Evolution in Cluster Cores
Near the center of massive clusters, densely packed galaxies are orbiting around the immense gravitational potential. At the very core, is usually a large red galaxy, often the most luminous of the cluster. These Brightest Cluster Galaxies (BCGs) are bathed in the hot X-ray emitting cluster gas, and are surrounded by massive stellar halos – perhaps their own (in the case of a cD galaxy) or that of the cluster (the IntraCluster Light, ICL). In this talk I will discuss my observational studies of massive red cluster galaxies from the point of view of their retirement from beyond the red and dead. We find that cluster properties are paramount to the detection of emission lines in a BCG, that when there is line emission, both AGN and recent star formation are important, and that the role of close companions – especially potential minor mergers – can be an important component for the buildup of stellar mass at the cluster core.