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Past Events

Event Date Summary
Joachim Brod (University of Cincinnati) Tue. November 17th, 2020
11:30 am-12:30 pm

Precision Standard-Model Prediction of epsilon_K

The parameter epsilon_K describes CP violation in the neutral kaon
system and is one of the most sensitive probes of new physics. The large
uncertainties related to the charm-quark contribution to epsilon_K have
so far prevented a reliable standard-model prediction. In this talk, I
will review mixing in the neutral kaon system, and then show that CKM
unitarity suggests a unique form of the weak effective Hamiltonian in
which the short-distance theory uncertainty of the imaginary part is
dramatically reduced. The uncertainty related to the charm-quark
contribution is now at the percent level.

Continue reading… Joachim Brod (University of Cincinnati)

Kara Farnsworth (CWRU) Tue. November 10th, 2020
11:30 am-12:30 pm

The Newman-Penrose Map and the Classical Double Copy

Abstract: Double copy relations between gauge and gravitational theories, originally found in the context of string theory and scattering amplitudes, have recently been realized in a classical setting as maps between exact solutions of gauge theories and gravity. I will present a new map between a certain class of real, exact solutions of Einstein’s equations and self-dual solutions of the flat-space vacuum Maxwell equations. This map, which we call the Newman-Penrose map, is well-defined even for non-vacuum, non-stationary spacetimes, providing a systematic framework for exploring gravity solutions in the context of the double copy that have not been previously studied in this setting.

Continue reading… Kara Farnsworth (CWRU)

Ravi Sheth (University of Pennsylvania) Tue. November 3rd, 2020
11:30 am-12:30 pm

Energy as a guiding principle in nonlinear structure formation

Abstract: One goal of studies of large scale structure formation is to understand why the dense, virialized clumps which host galaxies form where they do.  In cold dark matter cosmologies, the late time field retains some memory of the initial conditions, which models of dark matter halo formation try to exploit.  The simplest models are motivated by a spherical collapse calculation which dates back to the early  1970s.  In the late 1980s, this approximation for the physics of collapse was coupled with the heuristic assumption that collapse occurs around regions that are maxima of the initial matter density fluctuation field. 

Continue reading… Ravi Sheth (University of Pennsylvania)

Zach Weiner (University of Illinois) Tue. October 27th, 2020
11:30 am-12:30 pm

Seeing the dark: gravitational relics of dark photon production

Axion-like particles are a recurrent feature of models of early Universe phenomena, spanning inflation, dark matter, and solutions to the Hubble tension. The nonperturbative decay of axions into beyond the Standard Model photons is a generic feature of these models. I will present the complex nonperturbative and nonlinear dynamics of axion–gauge-field couplings, studied via numerical simulation. These scenarios result in a significant stochastic background of gravitational waves, which provides various means to rule out and constrain models. In the two examples I will present, the (over)production of GHz gravitational waves at preheating imposes the tightest constraints on the inflaton’s axial coupling to gauge fields,

Continue reading… Zach Weiner (University of Illinois)

David Weinberg (Ohio State University and Institute for Advanced Study) Tue. October 20th, 2020
11:30 am-12:30 pm

Decoding Chemical Evolution and Nucleosynthesis

I will discuss insights from analytic and numerical models of galactic chemical evolution and observations of Milky Way elemental abundances from the Sloan Digital Sky Survey’s APOGEE project. Under generic model assumptions, abundances and abundance ratios approach an equilibrium in which element production from nucleosynthesis is balanced by element depletion from star formation and outflows. The efficiency of outflows required to reproduce observed abundances is strongly degenerate with the uncertain overall scale of supernova yields. APOGEE observations show that the distributions of stars in (magnesium,iron,age)-space change steadily across the Milky Way disk,

Continue reading… David Weinberg (Ohio State University and Institute for Advanced Study)

Benjamin Elder (University of Hawaii) Tue. October 13th, 2020
11:30 am-12:30 pm

Chameleon dark energy in the lab
The accelerated expansion of the universe hints at the existence of a new light degree of freedom in the gravitational sector.  Such a degree of freedom, generally taken to be a scalar, mediates a fifth force between matter particles.  This property is in tension with existing tests of gravity, unless the fifth force is screened, i.e. it dynamically weakens in certain environments.  A new generation of gravitational experiments, being performed in the laboratory, are designed to be sensitive to screened forces, and have made great headway towards detecting or ruling out screened forces over the past several years. 

Continue reading… Benjamin Elder (University of Hawaii)

Chunshan Lin (Warsaw) Tue. October 6th, 2020
11:30 am-12:30 pm

Is GR unique?

Not sure. I will present an iterative Hamiltonian approach, to build up a gravity theory with all constraints being first class and thus possesses only 2 local degrees of freedom. The results are conjectural, rather than conclusive. If it is true, however, it implies GR may not be unique in the 4-dimensional space-time. If time permits, I will also briefly discuss the recently proposed 4D Einstein-Gauss-Bonnet gravity, which was another attempt of mine, yet probably unsuccessful one, along the line.

Zoom meeting ID:  999 3023 4812
For the password to access the meeting please contact one of us:
Kurt Hinterbichler: kjh92
Alexis Plascencia: adp110
Ellen Rabe: exr223
Idit Zehavi: ixz6


Continue reading… Chunshan Lin (Warsaw)

Anson Hook (Maryland) Tue. September 29th, 2020
11:30 am-12:30 pm

A CMB Millikan Experiment with Cosmic Axiverse Strings

We study axion strings of hyperlight axions coupled to photons. These axions strings produce a distinct quantized polarization rotation of CMB photons which is O(1%).  As the CMB light passes many strings, this polarization rotation converts E-modes to B-modes and adds up like a random walk. Using numerical simulations we show that the expected size of the final result is well within the reach of current and future CMB experiments through the measurement of correlations of CMB B-modes with E- and T-modes. The quantized polarization rotation angle is topological in nature and its value depends only on the anomaly coefficient,

Continue reading… Anson Hook (Maryland)

Xiaoju Xu (CWRU) Tue. September 22nd, 2020
11:30 am-12:30 pm

Halo and galaxy assembly bias

Measuring galaxy clustering is an effective way to gain knowledge of galaxy formation and constraining cosmology. Cosmology determines dark matter halo population and clustering, and halo clustering and halo occupation determine the galaxy clustering. It is important to understand halo clustering and galaxy-halo connection to build halo occupation models. In N-body simulations, halo clustering is shown to depend not only on halo mass but also on secondary halo properties, which is called the halo assembly bias. However, traditional halo occupation models only consider halo mass dependence and ignore effects caused by secondary halo properties.

Continue reading… Xiaoju Xu (CWRU)

Gordan Krnjaic (Fermilab) Tue. September 15th, 2020
11:30 am-12:30 pm

A Dark Matter Interpretation of Excesses in Multiple Direct Detection Experiments

We present a novel unifying interpretation of excess event rates observed in several dark matter direct-detection experiments that utilize single-electron threshold semiconductor detectors. Despite their different locations, exposures, readout techniques, detector composition, and operating depths, these experiments all observe statistically significant excess event rates of ~10 Hz/kg. However, none of these persistent excesses has yet been reported as a dark matter signal because their common spectral shapes are inconsistent with dark matter particles scattering elastically off detector nuclei or electrons. We show that these results can be reconciled if the semiconductor detectors are seeing a collective inelastic process known as a plasmon.

Continue reading… Gordan Krnjaic (Fermilab)

Hooman Davoudiasl (Brookhaven) Tue. September 8th, 2020
11:30 am-12:30 pm

Ultralight Fermionic Dark Matter

Tremaine and Gunn argued long ago that fermionic dark matter lighter than a few hundred eV is not feasible, based on the Pauli exclusion principle. We highlight a simple way of evading this conclusion which can lead to various interesting consequences. In this scenario, a large number of fermionic species with quasi-degenerate masses and no couplings, other than gravitational, to the standard model are assumed. Nonetheless, we find that gravitational interactions can lead to constraints on the relevant parameter space, based on high energy data from the LHC and cosmic ray experiments,

Continue reading… Hooman Davoudiasl (Brookhaven)

Saurabh Kumar (CWRU) Tue. September 1st, 2020
11:30 am-12:30 pm

Radiating Macroscopic Dark Matter

Dark matter is believed to constitute about 5/6th of the matter in the universe, but its nature and interactions remain one of the great puzzles of fundamental physics. Despite extensive experimental efforts, there have been no widely believed detections of WIMPS, axions or any other physics Beyond the Standard Model (BSM) (except for neutrino oscillations, which are BSM principally by historical accident). The question then arises: could the Standard Model, the most accurate and extremely well-tested theory of all observed particles in nature, explain dark matter as well? Many models of exotic quark matter have been proposed,

Continue reading… Saurabh Kumar (CWRU)

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