Alexey Onufriev, Departments of Computer Science and Physics, Virginia Tech


The nucleosome: from structure to function through physics


The nucleosome, a complex of 147 base-pairs of DNA with eight histone proteins, must protect its DNA, but, at the same time, allow on-demand access to it when needed by the cell. The exact mechanism of the control remain unclear.

We consider a series of physics-based models of the nucleosome, from the highly coarse-grained cylinders model, to fully atomistic, to multi-state atomistic.



One key conclusion is that at physiological conditions the nucleosome complex is close to the phase boundary separating it from the "unwrapped" states where the DNA is more accessible. A small drop in the positive charge (e.g. through acetylation of a lysine) of the globular histone core can significantly lower the DNA affinity to the core, and thus increase DNA accessibility. The findings suggest that charge-altering post-translational modifications in the histone core might be utilized by the cell to modulate accessibility to its DNA at the nucleosome level.



The multi-state atomistic model explores virtually all possible charge-altering post translational modifications (PTMs) in the globular histone core. The model reveals a rich and nuanced picture: the effect of PTMs varies greatly depending on location, including counter-intuitive trends such as decrease of DNA accessibility for some lysine acetylations in the core. A detailed connection to transcription regulation in-vivo is made.

Dr. Alexander Khaetskii
Department of Physics, University at Buffalo, SUNY
and Department of Physics, UConn

Giant edge spin accumulation in a symmetric quantum well with two subbands

We have studied [1] the edge spin accumulation due to an electric current in a high mobility two-dimensional electron gas formed in a symmetric quantum well with two subbands. This study is strongly motivated by the recent experiment [2] which demonstrated the spin accumulation near the edges of a symmetric two-subband GaAs structure in contrast to no effect in a single-subband configuration. The intrinsic mechanism of the spin-orbit (SO) interaction we consider arises from the coupling between two subband states of opposite parities [3].

We have explained the experimental results. We obtain, in particular, a parametrically large magnitude of the edge spin density for a two-subband well as compared to the usual single-subband structure. Following the method developed in [4], we show that the presence of a gap in the system (i.e., the energy separation between the two subband bottoms) changes drastically the picture of the edge spin accumulation. The gap value governs the effective strength of the inter-subband SO interaction which provides a controllable crossover from the regime of weak spin accumulation to the regime of strong one by varying the Fermi energy (electron density) and/or energy gap. We estimate that by changing the gap from zero up to 1-2 K, the magnitude of the effect changes by three orders of magnitude. This opens up the possibility for the design of new spintronic devices.

[1]. A. Khaetskii and J. C. Egues, Europhysics Letters 118, 57006 (2017).

[2]. F. Hernandez et al., Phys. Rev. B 88, 161305(R) (2013).

[3]. E. Bernardes et al., Phys. Rev. Lett. 99, 076603 (2007).

[4]. A. Khaetskii, Phys. Rev. B 89, 195408 (2014).

Dr. Adrian Price-Whelan,
Department of Astrophysical Sciences, Princeton University

The Milky Way as a benchmark: using surveys of our Galaxy to inform fundamental astrophysics

The Milky Way is currently the only galaxy in which we can study the detailed kinematic and chemical properties of large numbers of stars. It therefore provides a unique laboratory to test predictions made by astrophysical models of dark matter and galaxy assembly. In this talk, I will focus on how the recent confluence of all-sky photometric, astrometric, and spectroscopic surveys of Milky Way stars has enabled new dynamical constraints on the nature of dark matter. However, these surveys have also presented challenges to many of the simplifying assumptions that have enabled these inferences, for example by revealing the imprints of time-dependence and disequilibrium in the kinematics of stars throughout the Galaxy. I will discuss ways in which we can generalize our dynamical models and improve our statistical inferences to make and test precise astrophysical predictions about dark matter and galaxy formation using stars in the Milky Way.

Dr. Sarah Ballard,
MIT Kavli Institute


Lifetimes of Planetary Systems around Small Stars


The Solar System furnishes our most familiar planetary architecture: many planets, orbiting nearly coplanar to one another. However, a typical system of planets in the Milky Way orbits a much smaller M dwarf star. Small stars present a very different blueprint in key ways, compared to the conditions that nourished evolution of life on Earth. Using ensemble studies of hundreds-to-thousands of exoplanets orbiting small stars, my research program investigates the links between planet formation from disks, orbital dynamics and stability of planetary systems, and the presence and observability of their atmospheres. These processes bear upon the potential for M dwarf systems to evolve and sustain life. Studies of exoplanets with the James Webb Space Telescope comprise the clear next step toward understanding the hospitability of the Milky Way to life. Our success hinges upon leveraging the many thousands of planet discoveries in hand to determine how to use this precious and limited resource.

Prof. Lee Roberts, Boston University


Searching for Physics Beyond the Standard Model with the World's Largest Penning Trap

Measurements of the magnetic moments of the electron and muon were intertwined with the development of the "modern physics" of the 20th century. For the muon, this trend continues, perhaps pointing to New Physics beyond the Standard Model (BSM). The magnetic moment is along the spin,

For leptons, the factor g is greater than the Dirac value of 2 because of radiative corrections, so g =2(1+ a), or equivalently a = (g-2)/2, where a is the magnetic anomaly. While the value of the muon anomaly is dominated by the lowest-order Schwinger term /2≃ 0.00116⋯, it is necessary to include higher order contributions from QED, as well as contributions from the strong interaction and electroweak gauge bosons, to compare with the experimental value. The SM value now has an uncertainty of 0.3 parts per million (ppm), and the experimental value has an uncertainty of ± 0.54 ppm. The experimental value is larger than the Standard Model value by ≃ 3.7 standard deviations, which could come from new, as yet undiscovered, BSM physics. To clarify this situation, a new experiment, E989 with the total error budget of ±0.14 ppm has been mounted at Fermilab and will soon begin the second data collection period. I will review the history of what we have already learned from previous (g-2) experiments, the physics reach of 989, and will report on its present status.

Friday, February 8, 2019
3:30 P.M.
Gant Science Complex, Physics, GW-38

Coffee and tea will be served prior to the talk, at 3:00 p.m., In Room P-103

Dr. Daniel Angles-Alcazar,
Center for Computational Astrophysics, Flatiron Institute

Multi-scale physical modeling in galaxy formation and evolution


While observations across the electromagnetic spectrum are providing an increasingly detailed view of galaxy evolution across cosmic time, theoretical models face the challenge of understanding the multi-scale physical processes that connect stars and supermassive black holes to galaxies and their cosmological environment. I will show that cosmological hydrodynamic simulations are a crucial tool to interpret observations and improve our understanding of galaxy evolution, highlighting recent results on (1) the role of supernovae, stellar winds, and radiation from massive stars regulating galaxy growth, (2) the large-scale gas flows that connect galaxies and their surrounding circumgalactic medium, and (3) the co-evolution of galaxies and their central supermassive black holes. Looking forward, I will discuss the need for leveraging major advances in a variety of simulation methodologies and I will present novel techniques that will enable a more detailed understanding of the dominant physical processes operating over the full range of scales involved in galaxy evolution.

Dr. Massimo Gaspari,
Department of Astrophysical Sciences, Princeton University

Raining on Galaxies and Black Holes: Unifying the Micro and Macro Properties of AGN Feeding and Feedback

Feeding and feedback tied to supermassive black holes (SMBHs) play central role in the cosmic evolution of galaxies, groups, and clusters of galaxies. The self-regulated active galactic nucleus (AGN) cycle is matter of intense debate. I review key results of our numerical campaign to unveil how SMBHs are tightly coupled to the multiphase gaseous halos, linking the inner gravitational radius to the large Mpc scale and vice versa. Massively parallel magnetohydrodynamic simulations show the turbulent plasma halo radiatively cools via a top-down multiphase condensation rain of warm filaments and molecular clouds. The multiphase precipitation inherits the hot halo kinematics and thermodynamics, ultimately establishing a 'cosmic weather'. In the nuclear region, the recurrent collisions between the clouds and filaments promote angular momentum cancellation and boost the SMBH accretion rate through a mechanism known as Chaotic Cold Accretion (CCA). The CCA rapid variability triggers powerful AGN outflows, which quench the macro cooling flow and star formation, while preserving the atmospheres of galaxies, groups, and clusters in global thermal equilibrium throughout cosmic time. I highlight the key imprints of AGN feedback and feeding, such as bubbles, shocks, turbulence, and condensed structures, with a critical eye toward observational concordance, including the X-ray plasma, optical filaments, and radio molecular clouds.

Ben Augenbraun,
Harvard University


Laser-cooled polyatomic molecules for improved precision measurements


Ultracold molecules are a promising platform for applications in myriad fields, ranging from quantum simulation and chemistry to precision tests of fundamental physics. Recent work on the direct laser cooling of molecules has brought a number of diatomic molecules to the ultracold regime. These molecules are representative of a large, diverse class of molecules amenable to laser cooling. In this talk, we will focus on how polyatomic molecules (with three or more atoms) offer qualitatively new and powerful features, and the interesting physics of laser cooling polyatomic molecules will be described. We will discuss experimental work in the Doyle group on laser cooling CaF and SrOH molecules, and introduce a new experiment aimed at laser cooling of polyatomic molecules with high sensitivity to Beyond-the-Standard-Model physics, including YbOH and YbOCH3.