Colloquia are held Monday afternoons in 265 Phillips Hall at 4:00pm. Tea and coffee are served at the speaker’s reception in the Chapman meeting area, 3:30pm. The student Questions and Answers will be held in Phillips 277 after the colloquium from 5:15-6:00pm.

Jan 30
“Nuclear physics for beyond-the-Standard-Model searches”

Martin Hoferichter, University of Washington

Precision measurements of low-energy observables can provide constraints on physics beyond the Standard Model that are complementary to direct searches at the energy frontier, often extending the sensitivity to scales not directly accessible at high-energy colliders. However, in order to unambiguously establish anomalies that signal departures from the Standard Model or at least extract limits on the New-Physics parameter space, calculations of the relevant low-energy nuclear physics with controlled uncertainties, in terms of hadronic corrections or nuclear matrix elements, are becoming increasingly important. In the talk, this interplay between nuclear and particle physics will be discussed in the context of the anomalous magnetic moment of the muon, direct-detection searches for dark matter, and lepton flavor violation.

Feb 6
“Energy Landscape Theory: From Folding Proteins to Folding Chromosomes”

Peter Wolynes, Rice University

The statistical mechanics of energy landscapes has resolved the paradoxes of how information-bearing matter can assemble itself spontaneously. I will explain how our current understanding of protein folding landscapes not only leads to successful schemes for predicting protein structure from sequence but also has given quantitative insight into how folding and function shape molecular evolution. While protein folding is, in the main, thermodynamically controlled and not kinetically limited, longer structures in the cell can assemble in a kinetically controlled, non-equilibrium fashion. Nevertheless, I will show how energy landscape theory provides tools for extracting from low resolution experimental structural methods and kinetic information about the structure and cooperative dynamics of chromosomes.

Feb 13
“Fundamental Physics with Electroweak Probes of Nuclei”

Saori Pastore, LANL

The past decade has witnessed tremendous progress in the theoretical and computational tools that produce our understanding of nuclei. A number of microscopic calculations of nuclear structure and reactions with photons, electrons and neutrinos have successfully explained experimental data, yielding a complex picture of the way nuclei interact with those particles. This achievement is of great interest from the pure nuclear-physics point of view. But it is of much broader interest too, because the level of accuracy and confidence reached by these calculations opens up the concrete possibility of using nuclei to address open questions in other sub-fields of physics, such as, understanding the fundamental properties of neutrinos, or the particle nature of dark matter. In this talk, I will review recent progress in microscopic calculations of electromagnetic and weak-interaction properties of nuclei, including electromagnetic moments and transitions between low-lying nuclear states, along with studies of single- and double-beta decay rates. I will illustrate the key features required to explain the available experimental data, and present a novel framework to calculate neutrino-nucleus cross sections.

Feb 20
“Exploring beyond the Standard Model with Lattice QCD”

Amy Nicholson, Berkeley

While the Standard Model (SM) of particle physics has been enormously successful in describing the world around us, there still remain many important and unanswered questions requiring Beyond the SM (BSM) physics. One way to experimentally test the fundamental symmetries of the SM in searches for potential violations is to utilize properties of atomic nuclei which enhance these rare events. Connecting experimental signals from nuclear environments to a particular BSM model requires the numerical solution of Quantum Chromodynamics (QCD), a cornerstone of the SM which governs nuclear interactions. In this talk I will discuss the use of Lattice QCD as a tool for numerically calculating matrix elements relevant for experimental BSM searches. I will use neutrinoless double beta decay, which, if observed, offers an explanation for the observed matter-antimatter asymmetry of the universe, as a key example.

Feb 27
“Dark Matter Annihilation in the Gamma-Ray Sky”

Dan Hooper, Fermi Lab

In many models, dark matter particles can undergo self-annihilation, generating gamma-rays and other high-energy particles. One of the missions of the Fermi Gamma-Ray Space Telescope is to search for these annihilation products. Over the past several years, Fermi’s data has been shown to contain a spatially extended excess of ~1-3 GeV gamma rays from the region surrounding the Galactic Center, consistent with the signal expected from annihilating dark matter. Recent improvements in the analysis techniques have found this excess to be robust and highly statistically significant, with a spectrum, angular distribution, and overall normalization that is in good agreement with that predicted by simple annihilating dark matter models. I will discuss the characteristics of this signal, and ways to test its origin. In particular, the dwarf galaxies recently discovered by DES provide a potently important tool to test a dark matter origin of the Galactic Center excess.

Mar 6
“Topological Carbon: a New Perspective”

Shengbai Zhang, Rensselaer Polytechnic Institute

Topological physics in solids began with carbon but drifted away due to its exceedingly small spin-orbit coupling (SOC), which is thought to be essential for any observable effect. Here, I will discuss a different topological classification within carbon that is above the spin degree of freedom, has no need for the SOC, and hence can be observed at any reasonable temperature. It is based on the orbital symmetry, namely, that of the p_z orbital of a sp^2 carbon. Both spin and orbit are angular degrees of freedoms and both effects may be viewed as a result of (spherical)-symmetry breaking – in the former by SOC whereas in the latter by the formation of sp^2-bonded carbon network. First-principles calculations reveal similarities in their silent and often exotic physical properties.
The simplest topological carbon is polyacetylene. It is a one-dimensional (1D) p_z-orbital (semimetal) chain with a Dirac point right at the Fermi level. Starting with the polyacetylene, one can construct a whole family of topological matter. Graphene is a celebrated example, where parallel placement of the chains in 2D gives rise to Dirac cones. In 3D carbon, one may have two or three such p_z-chain sets intercepting with each other. Various arrangements of the two p_z-chain sets give rise to highly-stable Weyl semimetals with symmetry-protected loops [1] and surfaces [2] as their Fermi surfaces, which can be reduced to Weyl points with characteristic Fermi arcs for the surface states. Upon breaking the topological protection, e.g., by a biaxial strain, one may also obtain 3D Kagome lattice as a remarkable direct-gap, blue optoelectronic semiconductor [3]. The recent experimentally-realized carbon honeycombs (CHCs) [4] may be the first experimental realization of 3D topological carbon. It belongs to the family of three p_z-chain set. Besides its interesting but “trivial” applications such as gas storage that have excited the condensed matter physics community, CHC processes 3D Dirac cone that intercepts with a third flat band [5]. Such a triple-point topological metal [6] has becoming a forefront in today’s topological physics.
[1] Y.-P. Chen, et al., Nano Lett. 15, 6974 (2015).
[2] C. Zhong, et al., Nanoscale 8, 7232 (2016).
[3] Y.-P. Chen Y, et al., Phys. Rev. Lett. 113, 085501 (2014).
[4] N. V. Krainyukova and E. N. Zubarev, Phys. Rev. Lett. 116, 055501 (2016).
[5] Y. Gao, et al., Nanoscale 8, 12863 (2016).
[6] Chang, et al., arXiv:1605.06831v1.

Mar 20
“What is quantum mechanics? A minimal formulation”

Pierre Hohenberg, NYU

This talk asks why the interpretation of quantum mechanics, in contrast to classical mechanics, remains a subject of controversy. I shall present a ‘minimal formulation’ modeled on the formulation of classical mechanics. In both cases the formulation is ‘microscopic’, by which I mean applicable to any closed system with an arbitrary number of degrees of freedom. Starting from the sole assumption of a ‘Hilbert space ontology’, it is argued that all the usual features of quantum mechanics follow essentially inevitably, thus providing the sought after minimal formulation. The so-called ‘measurement problem’ is briefly discussed and claimed to be resolved. The usual questions and controversies over ‘interpretations’ of quantum mechanics can then be treated by ‘macroscopic quantum mechanics’ as an application of the more general microscopic theory and not part of the foundational formulation.

Mar 27
“Nanoscale Magnetic Imaging using Quantum Defects in Diamond”

Ronald Walsworth, Harvard University

Nitrogen vacancy (NV) color centers are quantum defects in diamond that provide an unpar-alleled combination of magnetic field sensitivity and spatial resolution in a room-temperature solid, with wide-ranging applications in both the physical and life sciences. NV centers can be brought into few nanometer proximity of magnetic field sources of interest while maintain-ing long NV electronic spin coherence times, a large (~Bohr magneton) Zeeman shift of the NV spin states, and optical preparation and readout of the NV spin. Recent applications of NV-diamond magnetometry include magnetic imaging of living cells, single proton MRI, sin-gle protein NMR, mapping magnetic signatures in >4 billion-year-old meteorites and early-Earth rocks, magnetic sensing of single neuron action potentials, and characterizing advanced materials such as spin torque oscillators. I will provide an overview of this technology and its applications.

Apr 3
“Benchmarking quantum control and developing semiconductor-based quantum devices”

Jonathan Baugh, University of Waterloo

The development of robust, scalable quantum information processors is both an extraordinary opportunity and an extraordinary challenge. Over the past 20 years, significant progress has been made in theory and experiment, including quantum control techniques, understanding realistic noise processes, refining error correction schemes, and advancing physical implementations. The first part of the talk will describe our recent experiments applying randomized benchmarking (RB) techniques to a solid-state qubit in the context of conventional electron spin resonance [1]. While standard RB measures only the average gate fidelity, our modified RB allows for distinguishing between coherent and incoherent noise processes. This is very useful since different strategies are employed to reduce the two kinds of error. The experiments also demonstrate a sophisticated use of control waveforms derived from optimal control theory. The second part of the talk will focus on semiconductor nanoelectronic devices as a basis for electron spin qubits and potentially for topologically protected qubits. Using a silicon MOS-type device, we recently implemented a novel ‘quantum memristor’ based on two capacitively coupled quantum dots [2].

1. G. Feng, J. J. Wallman, B. Buonacorsi, F. H. Cho, D. K. Park, T. Xin, D. Lu, J. Baugh, and R. Laflamme, Phys. Rev. Lett. 117, 260501 (2016).
2. Y. Li, G. W. Holloway, S. C. Benjamin, G. A. D. Briggs, J. Baugh, and J. A. Mol, arXiv:1612.08409 (2016).

Apr 10
“A research-validated approach to transforming upper-division physics courses.”

Steve Pollock, University of Colorado Boulder

At most universities, upper-division physics courses are taught using a traditional lecture approach that does not make use of many of the instructional techniques that have been found to improve student learning at the introductory level. At CU, we are transforming upper-division courses (E&M, Quantum, and Classical Mechanics) using principles of active engagement and learning theory, guided by the results of observations, interviews, and analysis of student work. I will outline these reforms including consensus learning goals, clicker questions, tutorials, modified homeworks, and more, as an example of what a transformed upper-division course can look like, and as a tool to offer insights into student difficulties in advanced undergraduate topics. We have examined the effectiveness of these reforms relative to traditional courses, based on grades, interviews, and attitudinal and conceptual surveys. Our results suggest that it is valuable to further investigate how physics is taught at the upper-division, and how education research may be applied in this context.

Apr 17
“Breaking Through Exoplanetary Atmospheres”

Mercedes Lopez-Morales, CFA, Harvard

In the past two decades we have gone from only knowing about the planets in our own Solar System to discovering thousands of planets orbiting around other stars. We have not only discovered that planets abound, but also that most planetary systems do not resemble our own. One of the next steps in the field of exoplanets is to study their atmospheres and to answer questions such as: do the physical properties of gas giant exoplanets resemble those of the Solar System
giants? Are there exoplanets with atmospheres similar to Earth? In this talk I will describe the state of the art techniques we are using and developing to characterize exoplanetary atmospheres, the main results so far, and our future plans to detect biomarkers.

April 24
“Modeling tumor-microenvironment interactions for development of biophysics-informed strategies in cancer therapeutics”

Jonathan Celli, University of Massachusetts, Boston

Cancer progression is regulated not only by the molecular biology and genetics of the disease but also by the physical properties of the tumor and surrounding tissues. In particular, the development of mechanically rigid fibrous stroma is a defining feature of many solid tumors and has been shown to play complex roles both promoting and constraining malignant growth behavior. It remains poorly understood however, how this altered mechanical landscape, which is dynamically remodeled during tumor progression and invasion, regulates susceptibilities to cancer therapeutics. Several projects in our group examine how biophysical interactions with the tumor microenvironment impact upon phenotypic changes which determine therapeutic response. This work is enabled by the use of in vitro 3D tumor models with tunable and rheologically-characterized extracellular matrix (ECM). Combined with imaging-based analyses of phenotype and in situ microrheology measurements of dynamic matrix remodeling, this platform provides a means to co-register rigidity-dependent cell shape, mechanics, and motility with response to therapeutic intervention. In this context we specifically contrast classical chemotherapy agents with photodynamic therapy (PDT), in which light activation of a photosensitizing agent leads to cell death by local generation of reactive oxygen species. Interestingly, our recent results show that while modulation of ECM composition to promote increased cell motility imparts resistance to chemotherapy, the same chemoresistant populations exhibit increased sensitivity to PDT. These and other emergent findings will be discussed in the broader context of connecting cancer biophysics with cancer therapeutics.