Physics and Astronomy Colloquium

semester, 2016


Thursday, January 28th 2016
3:35 pm:
Speaker: Kathy Levin, University of Chicago
Subject: Atomic Gases as a Proxy-Laboratory for Condensed (& Other) Matter
Refreshments to be served outside 101 Fraser after the colloquium.

In this talk I summarize how experiments in atomic gases can inform about condensed matter and also quark-gluon matter. The difficulties of performing these atomic physics experiments are evident. One can't insert thermometers or put current leads into trapped atom gases. But very important are their unique capabilities which provide windows into physics not otherwise accessible. We focus on 3 successful encounters between theory and experiment in atomic Fermi superfluids which take advantage of these unique windows. The ultimate motivation behind these examples is, of course, the pursuit of various holy grails in condensed matter physics (e.g.,understanding high temperature superconductors and topological trapped superfluidity).

Faculty Host: Oriol T. Valls

Thursday, February 4th 2016
3:35 pm:
Speaker: Eva Halkiadakis, Rutgers University
Subject: Exploring the Energy Frontier at the Large Hadron Collider
Refreshments to be served outside 101 Fraser after the colloquium.

The Large Hadron Collider (LHC) at the CERN laboratory is the world’s most powerful particle accelerator. The start of the proton collider program at the LHC brought the dawn of the exploration of a new energy frontier. The LHC has had a successful and highly productive Run 1 (2010-2012), colliding protons with a center-of-mass energy up to 8 TeV, and in 2012 the observation of a new Higgs-like boson was announced to the world by the CMS and ATLAS collaborations. The year 2015 marked the beginning of Run 2 of the LHC and we entered a new era of even higher proton collisions at 13 TeV. The LHC experiments have an extensive program of searches for physics beyond the Standard Model, exploring uncharted territory at the energy frontier. I will present the status of the LHC and the first experimental results using the latest 13 TeV data in Run 2, with a special focus on the CMS experiment.

Faculty Host: Jeremiah Mans

Thursday, February 11th 2016
3:35 pm:
Speaker: Scott Dodelson, Fermilab
Subject: Myth-Busters: Dark Energy Survey and South Pole Telescope
Refreshments to be served outside 101 Fraser after the colloquium.

The standard cosmological lore is that galaxy surveys and cosmic microwave background (CMB) experiments open up a window on fundamental physics such as dark energy and dark matter. The four distinct probes enabled by surveys -- Baryon Acoustic Oscillations, Clusters, Gravitational Lensing, and Supernovae -- together with a pristine view of the early universe via the CMB allow us to determine cosmological parameters such as the equation of state of dark energy. I argue that this paradigm is breaking down, as we come to realize that the four probes are all correlated with one another and with the CMB itself. How to proceed is the subject of a raging debate, with many possible routes and assumptions. I give a few examples and recent results from the Dark Energy Survey and the South Pole Telescope

Faculty Host: Marco Peloso

Thursday, February 18th 2016
3:35 pm:
Speaker: Karl Gebhardt, University of Texas, Austin
Subject: Black Holes and Dark Energy through the Eyes of Texas
Refreshments to be served outside 101 Fraser after the colloquium.

Both the central black hole and the dark matter profile play essential roles for understanding the dominant evolutionary processes in galaxies. I will overview recent observations which demonstrate significant differences compared to previous analysis, focusing on black holes at the extreme mass ranges in galaxies. I will also present expectations from future advances in each area, specifically coming from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX).

Faculty Host: Evan Skillman

Thursday, February 25th 2016
3:35 pm:
Speaker: John Wilkerson, University of North Carolina
Subject:  The Neutrino — Beyond the Standard Model
Refreshments to be served outside 101 Fraser after the colloquium.

Today we know that neutrinos are not the “dull”, massless particles originally encompassed in the standard model of fundamental interactions. Instead, we have discovered — as recognized by the 2015 Nobel Prize in Physics —that neutrinos oscillate as they travel through space and matter, and have tiny, yet to be understood masses. Because neutrinos are amongst the most abundant particles in the universe, these revolutionary discoveries about neutrinos have far reaching consequences for cosmology, astrophysics, and our basic understanding of fundamental interactions. Yet there remain intriguing and unanswered questions about neutrino properties: Is lepton number a conserved quantity? Are neutrinos their own anti-particle (Majorana as opposed to Dirac)? How much do neutrinos weigh? The search for a postulated rare decay process known as neutrinoless double beta decay potentially holds the key to answering these questions and to further elucidating the properties of these elusive subatomic particles.

Faculty Host: Marvin Marshak

Thursday, March 3rd 2016
3:35 pm:
Speaker: Leo Radzihovsky, University of Colorado
Subject: Critical matter, chiral symmetry breaking and emergent Higgs mechanism
Refreshments to be served outside 101 Fraser after the colloquium.

The upshot of extensive studies of fluctuations is that their qualitative importance typically confined to isolated critical points of continuous transitions between phases of condensed matter. This conventional wisdom also predicts the number of low energy Goldstone modes based on the pattern of symmetry breaking. I will discuss condensed matter systems, that violate this standard paradigm via an emergent Higgs mechanism. Even more spectacularly, these systems exhibit “critical” ordered phases, with universal power-law properties reminiscent of continuous phase transition, but without any fine-tuning and extending throughout the ordered phase. The most interesting recent example is the twist-bend nematic liquid crystal —— a homogeneous liquid that spontaneous breaks chiral symmetry.

Faculty Host: Natalia Perkins

Thursday, March 10th 2016
3:35 pm:
Speaker: David Brain, University Colorado at Boulder
Subject: Has the Martian Atmosphere Disappeared Over Time?
Refreshments to be served outside 101 Fraser after the colloquium.

The Martian surface has abundant evidence for the presence of stable liquid surface water over long periods of time in its history, a situation that cannot be accommodated by the present thin atmosphere. One potential inference is that Martian atmosphere was thicker long ago, and atmospheric particles have since been removed. Escape of the atmosphere to space is one possibility, but the amount of escape that has occurred over time is highly uncertain.

I will present results from the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft mission to evaluate the importance of atmospheric escape for the climate history of Mars. MAVEN has orbited Mars since September 2014, and is currently in its first mission extension. I will highlight results that teach us about the chain of events that lead to atmospheric escape - including the drivers of escape from the Sun and solar wind, the atmospheric particle reservoirs for escape, and the escape processes and rates. I’ll then describe how lessons from MAVEN can teach us about planetary atmospheric evolution everywhere.

Faculty Host: Cynthia Cattell

Thursday, March 17th 2016
Speaker: There will be no colloquium this week--SPRING BREAK

Thursday, March 24th 2016
3:35 pm:
No colloquium due to late cancellation

Thursday, March 31st 2016
3:35 pm:
Speaker: Abhay Pasupathy, Columbia University
Subject: Experiments in Flatland
Refreshments to be served outside 101 Fraser after the colloquium.

In the past decade, scientists have figured out how to create and probe crystalline materials that are a single or few atomic layers in thickness. These two-dimensional (2D) materials have a host of new physical, chemical and mechanical properties. The most well-known member of this family is graphene, but in the past few years we have been able to synthesize a whole host of 2D materials including semiconductors, superconductors, magnets, charge density waves, and so on. In this talk, I will present some of my research group's experimental exploration of these materials. In particular, I will discuss the following topics (a) how one can induce spatially ordered phases in graphene (b) what happens to a crystalline superconductor when we take it to the 2D limit (c) ways in which complex phases like charge density waves can be tuned in 2D samples.

Faculty Host: Rafael Fernandes

Thursday, April 7th 2016
3:35 pm:
Speaker: Adam Lyon, Fermilab
Subject:  The New Muon g-2 Experiment at Fermilab
Refreshments to be served outside 101 Fraser after the colloquium

The anomalous magnetic moment of the muon is a direct window to new physics and has been the subject of nuclear and particle physics experiments for the past 60 years. It drives both theory and experiment to new levels of precision and sensitivity. The latest result in this enterprise is from the 1999-2001 run of the Brookhaven E821 experiment. With 0.54 parts per million (ppm) precision, it differs from the Standard Model prediction by more than 3 sigma – enough to be extremely intriguing, but not conclusive. By moving the main apparatus, a 50 foot diameter superconducting storage ring, from Brookhaven to Fermilab, the experiment will be repeated with a more intense and cleaner beam and all new detectors. The outcome will be a factor of four improvement in precision. If the difference seen at E821 between experiment and the Standard Model persists, the discrepancy will greater than 5 sigma and possibly be greater than 7 sigma if the theoretical prediction improves as expected. Such a result would be an undeniable signal of new physics. I will present the past, present, and future aspects of Muon g-2 measurement. With first muons only a year away, I will focus on the progress at the Fermilab experiment.

Faculty Host: Dan Cronin-Hennessy

Thursday, April 14th 2016
3:35 pm:
Speaker: Wendy Freedman, University of Chicago
Subject: Measurement of Cosmological Parameters
Refreshments to be served outside 101 Fraser after the colloquium.

Over the past few decades, cosmologists have for the first time identified the major constituents of the universe. Surprisingly, the universe hardly resembles what we thought only a few decades ago. The universe is filled with dark matter that is not visible and energy that permeates all of space, causing its expansion to speed up with time. Accurate distances remain central to a number of fundamental problems in astrophysics and cosmology. They are critical for measurements of the acceleration of the universe using supernovae. A more accurate measurement of the Hubble constant is critical for providing independent constraints on dark energy, the geometry, and matter density of the universe. The increased precision of cosmic microwave background fluctuations (most recently with the Planck satellite) make these direct comparisons even more critical, given the physical degeneracies amongst different cosmological parameters, and the apparent tension with the direct measurements of the Hubble constant. There has been fundamental progress over the last couple of decades in measuring extragalactic distances. The upcoming decade promises robust distances and a measurement of the Hubble constant to a few percent accuracy.

Faculty Host: Clement Pryke

Thursday, April 21st 2016
3:35 pm:
Physics and Astronomy Colloquium in 230 STSS/Bruininks Hall
There will be no colloquium this week due to the Kaufmanis lecture
Speaker: Nobel Laureate Dr. Adam Riess, Johns Hopkins University's Krieger School of Arts and Sciences
Subject: Supernovae and the Discovery of the Accelerating Universe.
For more information refer to: http://www.astro.umn.edu/seminars/kaufmanis/

Thursday, April 28th 2016
3:35 pm:
Speaker: Maxim Pospelov, University of Victoria and Perimeter Institute
Subject: Neutrino portal
Refreshments to be served outside 101 Fraser after the colloquium.

Neutrino masses and mixing point to the existence of new physics beyond the Standard Model. The simplest realization of neutrino mass generation is related to new fermionic massive states, the right-handed neutrinos, not charged under any of the Standard Model gauge groups. The mass scale of these new states is almost arbitrary, while their couplings are most likely related to their masses via the so-called seesaw relation. In my talk, I will describe how one can search for right handed neutrinos should their masses fall within energy reach of modern colliders and beam dump experiments, such as the LHC and new planned SHiP facility. I will also discuss modifications to the SM with neutrinos carrying new gauge interactions, and show that this increases our chances to discover right handed neutrinos within the range of couplings suggested by the seesaw relation.

Faculty Host: Tony Gherghetta

Thursday, May 5th 2016
3:35 pm:
Speaker: Josh Klein, University of Pennsylvania
Subject: Neutrinos, Antineutrinos, and No Neutrinos
Refreshments to be served outside 101 Fraser after the colloquium. Student Awards will be presented at the start of Colloquium.

Neutrinos have always been the black sheep of the particle physics world: they
make up nothing, they hardly ever interact with normal matter, and they are so
light that, for the most part, they can be completely ignored in any reaction
in which they do participate. But we now know that neutrinos influence an
enormous range of physics. They tell us how the Sun works, they allow us to
search for new forces of Nature, and, because they carry no electric charge,
they are the only fundamental particle of matter that can be both particle and
antiparticle. The talk will discuss new areas of physics that can be explored
with neutrinos, and focus in particular on the SNO+ experiment whose goals
include a test of whether the neutrino and antineutrino are really the same
thing.

Faculty Host: Marvin Marshak

Thursday, September 8th 2016
3:35 pm:
There will be no colloquium this week.

Thursday, September 15th 2016
3:35 pm:
Speaker: Vuk Mandic
Subject: First Direct Detections of Gravitational Waves
Refreshments to be served outside Keller 3-210 after the colloquium.

During their first observation run the Advanced LIGO gravitational-wave detectors recorded signatures of mergers of binary black hole systems. These events mark the beginning of gravitational-wave astrophysics, enabling a new approach to studying various astrophysical phenomena. I will describe the LIGO detectors and the events they recorded, and I will discuss the implications of these observations as well as our expectations for future observations.


Thursday, September 22nd 2016
3:35 pm:
Speaker: Lucy Fortson
Subject: Blazar Observations with VERITAS: expanding the cosmic gamma-ray horizon
Refreshments to be served outside Keller 3-210 after the colloquium.

Blazars are Active Galactic Nuclei (AGNs) that launch collimated jets of relativistic particles towards the Earth. They emit radiation that spans the entire observable electromagnetic spectrum from radio to gamma-ray energies and which varies in intensity over a wide range of timescales, from minutes to years. For the past decade, the VERITAS gamma-ray observatory has been recording very-high-energy (VHE ≥ 100 GeV) photons, yielding a catalog of over 175 VHE gamma-ray sources of which almost 40% are blazars. Pair production interactions between VHE and lower-energy photons strongly attenuate the flux of gamma-rays as they propagate to Earth through a cosmological photon field known as the extragalactic background light (EBL). This is expected to impose a gamma-ray horizon at redshifts well below unity, beyond which VHE gamma-ray sources are rendered undetectable to the current generation of instruments. I will describe recent results from the VERITAS blazar observing program including the exciting and unexpected detection of a particularly high redshift (z~1) blazar. I will then describe efforts to use blazar gamma-ray spectra to constrain the EBL and as a probe for the presence of Ultra-High Energy Cosmic Ray emission from blazars.


Thursday, September 29th 2016
3:35 pm:
Speaker: Greg Pawloski
Subject: Looking at the weird building blocks of the universe with the NOvA detectors
Refreshments to be served outside Keller 3-210 after the colloquium.

Neutrinos are the odd balls of the elementary particle menagerie that
forms the building blocks of the universe. Because of their odd
properties they can potentially be involved in interesting processes
that are responsible for our universe not annihilating itself. There
is currently great interest in the particle physics community to study
the properties of these particles. However, because of their odd
nature, their detection is challenging and extraordinary detectors are
needed to study them. In this talk, we will look at the role of the
NOvA detectors (the world’s largest plastic structures) in studying
the properties of neutrinos by shooting a beam of neutrinos through
the Earth from Illinois to northern Minnesota.


Thursday, October 6th 2016
3:35 pm:
Speaker: Tommaso Treu, UCLA
Subject: The other 95%: insights from strong gravitational lensing
Refreshments to be served outside Keller 3-210 after the colloquium.

In the standard cosmological model ninetyfive percent of the energy content of the universe consists of dark energy and dark matter. Even though their abundance seems well determined, very little is known about their fundamental nature. I will describe how we can learn about the physics of the dark sector by studying in detail its gravitational effect on the trajectories of photons as they travel across the universe, a phenomenon known as strong gravitational lensing. In the first part of the talk, I will use strong gravitational lenses with a time variable background source to measure the expansion rate of the universe (Hubble constant) to 3.8% precision. This result is completely independent of the local distance ladder and the cosmic microwave background, and thus provides a new opportunity to understand whether the tension between the two arises from systematic uncertainties or may be indicative of new physics. In the second part of the talk I will describe how strong lensing can be used to detect the presence of dark subhalos independent of their stellar content. This measurement tests a fundamental prediction of the cold dark matter model, i.e. that galaxies should be surrounded by large numbers of dark satellite subhalos. Proof that such satellites do not exist would force a revision of the model in favor of more exotic alternatives like warm dark matter. I will conclude by discussing the exciting future of strong lensing as a tool for cosmology, in light of the huge numbers of strong lenses that are on the verge of being discovered in the current generation of wide field astronomical surveys.

Faculty Host: M. Claudia Scarlata

Thursday, October 13th 2016
3:35 pm:
Speaker: Rafael Fernandes
Subject: High-temperature superconductivity near a quantum critical point
Refreshments to be served outside Keller 3-210 after the colloquium.

The elucidation of high-temperature superconductivity remains a fundamental challenge in quantum many-body physics, with potential implications for the development of unique technological applications. In contrast to conventional superconductors, the attractive interaction between the electrons mediated by the lattice cannot explain the formation of this quantum state of matter. Experimentally, however, high-temperature superconductivity is often observed in the vicinities of a putative zero-temperature phase transition, also known as a quantum critical point. In this talk, I will present new theoretical results for the superconducting instability of a very general model in which electrons interact by exchanging fluctuations associated with a magnetic quantum critical point. By combining numerical and analytical approaches, I will discuss the microscopic mechanisms that promote the emergence of a high-temperature superconducting state, and examine which of the many system’s parameters may be tuned to help achieving even higher transition temperatures.


Thursday, October 20th 2016
Speaker: John Preskill, California Institute of Technology
Subject: Quantum Information and Spacetime
Refreshments to be served outside Keller 3-210 after the colloquium.

Aside from enabling revolutionary future technologies, quantum information science is providing powerful new tools for attacking deep problems in fundamental physical science. In particular, the recent convergence of quantum information and quantum gravity is sparking exciting progress on some old and very hard questions.


Thursday, October 27th 2016
3:35 pm:
Speaker: Amir Yacoby, Harvard University
Subject: A New Spin on Superconductivity
Refreshments to be served outside Keller 3-210 after the colloquium.

Nearly a hundred years after its discovery, superconductivity remains one of the most intriguing phases of matter. In 1957 Bardeen, Cooper and Schrieffer (BCS) presented their theory of superconductivity describing this state in terms of pairs of electrons arranged in a spatially isotropic wave function with no net momentum and a spin singlet configuration. Immediately thereafter, a search began to find materials with unconventional superconductivity where pairing deviates from conventional BCS theory. One particular class of unconventional superconductors involves pairs arranged in triplet rather than singlet configurations. Such superconductors may enable dissipationless transport of spin and may also give rise to elementary excitations that do not obey the conventional Fermi or Bose statistics but rather have non-Abelian statistics where the exchange of two particles transforms the state of the system into a new quantum mechanical state. In this talk I will describe some of our recent experiments that explore the proximity effect between a conventional superconductor and a semiconductor with strong spin-orbit interaction. Using supercurrent interference, we show that we can tune the induced superconductivity continuously from conventional to unconventional that is from singlet to triplet. Our results open up new possibilities for exploring unconventional superconductivity as well as new ways for detecting unconventional pairing in known materials.

Faculty Host: Vlad Pribiag

Thursday, November 3rd 2016
3:35 pm:
Speaker: Vincent Noireaux
Subject: DNA programmed dynamical systems outside cells: from gene circuits to self-assembly
Refreshments to be served outside Keller 3-210 after the colloquium.

The analysis and characterization of complex dynamical interactions involved in gene regulation is a major theme in post-genomic research. Researchers from many different backgrounds have developed novel approaches to dissect gene network and to determine their basic principles.

In this talk I will present a unique cell-free platform that my lab has developed to construct complex information-based dynamical systems by executing gene circuits in vitro. The original goal was to prototype and to understand elementary regulatory gene networks in test tube reactions. Recently, we showed that our experimental platform is capable of recapitulating larger biochemical systems such as the complete synthesis of viruses that we use as a model of information to self-assembly process. Finally, I will show how we can execute gene circuits in synthetic liposomes to emulate biological functions in a synthetic cell analog.


Thursday, November 10th 2016
3:35 pm:
Speaker: Michel Devoret, Yale University
Subject: Fighting Decoherence with Dissipation
Refreshments to be served outside Keller 3-210 after the colloquium.

State superposition and entanglement are basic quantum phenomena that are usually observed in systems sufficiently decoupled from a lossy environment. Somewhat paradoxically, a specially engineered interaction with the environment can become a resource for the generation and protection of quantum states. Moreover, this notion can be generalized to a manifold of quantum states that consists of all coherent superpositions of several stable, dissipative steady states. In particular, it has now become practically feasible to confine the state of an harmonic oscillator to the quantum manifold spanned by two coherent states of opposite phases. In a recent experiment, we have observed a superposition of two such coherent states, also known as a Schrodinger cat state, spontaneously squeeze out of vacuum, before decaying into a classical mixture. The dynamical protection of logical qubits built from Schrodinger cat states is based on an engineered driven-dissipative process in which photon pairs are exchanged rather than single photons. The recently-developed class of experiments in which quantum information is encoded and manipulated using cat states opens a new avenue in quantum information processing with superconducting circuits.

Faculty Host: Vlad Pribiag

Thursday, November 17th 2016
3:35 pm:
Speaker: Fai Mak, Penn State University
Subject: Superconductivity in single-layer van der Waals’ metals
Refreshments to be served outside Keller 3-210 after the colloquium.

The discovery of graphene has stimulated not only the field of carbon nanoelectronics, but also studies of novel electronic phenomena in a wide range of atomically thin van der Waals’ materials. In this talk, I will discuss our recent effort in the isolation of a single layer of niobium diselenide (NbSe2), a new non-centrosymmetric superconductor. As a result of the crystal symmetry and the strong spin-orbit interactions, the electron spin in single-layer NbSe2 becomes Ising-like (i.e. spins locked to the out-of-plane direction), giving rise to unique magnetic properties in the superconducting state. I will present measurements of the material’s upper critical field, which significantly exceeds the conventional Pauli paramagnetic limit, supporting the Ising pairing mechanism. I will also discuss our measurements of the superconducting gap by tunneling experiments that further provide evidence for finite spin susceptibility arisen from spin-triplet pairing correlations. Our studies pave the way for the search for topological superconductivity in two-dimensional materials.

Faculty Host: Boris Shklovskii

Thursday, November 24th 2016
Speaker: There will be no colloquium this week due to Thanksgiving

Thursday, December 1st 2016
3:35 pm:
Speaker: David Kohlstedt
Subject: From Reaction-driven to Stress-driven Melt Segregation – Formation of High-permeability Paths through Earth’s Mantle
Refreshments to be served outside Keller 3-210 after the colloquium.

Separation of small amounts of melt from residual solid and migration of that melt from deep beneath a mid-ocean ridge to its eruption at Earth’s surface require a transition from porous to channelized flow in order to preserve chemical and radiogenic disequilibrium. Chemically isolated, high-permeability melt conduits in Earth’s mantle develop as a consequence of instabilities in the deformable and dissolvable porous media. Models for the formation of such flow instabilities include stress-driven and reaction-driven melt channelization.

Melt rising from depth through mantle rocks, which are composed primarily of the minerals olivine and pyroxene, becomes under saturated in pyroxene with respect to the surrounding upper mantle. Thus, pyroxene preferentially dissolves into the melt as it migrates toward the surface. Tabular rocks rich in olivine and depleted in pyroxene found in peridotite massifs serve as channels for rapid melt extraction from partially molten regions of the mantle. Formation of such dunite channels involves dissolution-precipitation reactions between mantle rock and percolating reactive melt. Dunite channels also coincide with shear zones, indicating that deformation together with reaction plays an important role during melt channelization.

My talk focuses on results from laboratory investigations of the formation and evolution of melt-enriched channels in mantle rocks. The first part examines the formation of stress-driven, melt-enriched channels predicted by theory, while the second part considers the development of reaction infiltration instabilities in mantle rocks. These channelization processes significantly increase permeability and, hence, the flux of melt through the partially molten rocks.


Thursday, December 8th 2016
3:35 pm:
Speaker: David Poland, Yale University
Subject: Critical Phenomena and the Conformal Bootstrap
Refreshments to be served outside Keller 3-210 after the colloquium.

From critical phenomena to quantum gravity, conformal field theories describe the universal scale-invariant structures that lie at the heart of theoretical physics. The conformal bootstrap is the powerful idea, dating back to the 70’s, that one can use fundamental consistency conditions to constrain, solve, and map out the space of conformal field theories. In this talk I will explain how one can use the conformal bootstrap to perform rigorous and precise calculations in strongly-interacting theories without reference to a microscopic Lagrangian, summarizing recent progress at learning about the 3D Ising and O(N) vector models, which are the universal structures describing phase transitions in fluids, magnets, superconductors, and numerous other systems in nature.

Faculty Host: Tony Gherghetta

Thursday, December 15th 2016
3:35 pm:
There is no colloquium this week.

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