Physics and Astronomy Calendar

Week of Monday, October 22nd 2018

Monday, October 22nd 2018
12:15 pm:
Speaker: Wenlei Chen, UMN
Subject: Novel search for TeV-initiated pair cascades in the intergalactic medium

The observed magnetic fields in galaxies and galaxy clusters are believed to result from dynamo amplification of weak magnetic field seeds whose origin remains a long-standing open question. Beams of TeV gamma-rays from blazar jets can be used to infer the intensity, coherence length, and helicity of the intergalactic magnetic field. Intergalactic magnetic fields deflect the electron-positron pairs produced by TeV gamma-rays from blazars, resulting in broadened beams of secondary GeV gamma-rays known as pair halos. Such pair-cascades develop along the projected direction of the blazar jet, which is known from imaging radio observations. We searched for GeV pair halos in Fermi-LAT data around 12 high-synchrotron-peaked BL Lacs with well-determined jet orientation from VLBA radio observations. Our study exploits the expected asymmetry of blazar pair halos and uses advanced simulations of the pair cascades to improve the sensitivity of previous studies and increase the signal to noise. Although we find no significant detection, a 2-sigma hint for an extended pair halo along the direction of the jet appears in the stacked LAT data in the 30-300 GeV energy range, corresponding to an intergalactic magnetic field with strength of about 1E-15 Gauss. This magnetic field value is consistent with similar hints from independent studies using LAT data. We will present the results of our analysis and discuss the limitations of pair-halo searches due to astrophysical uncertainties. Finally, assuming that the apparent convergence on B ~ 1E-15 G is not coincidental, we will outline a clear path towards a positive detection of blazar pair halos with future space-borne and ground-based gamma-ray observatories.

Faculty Host: Patrick Kelly

Tuesday, October 23rd 2018
11:15 am:
Nuclear Physics Seminar in Tate 301-20
Speaker: Ermal Rrapaj, University of Minnesota & University of California, Berkeley
Subject: Supernovae and neutron stars: Constraining dark matter through nuclear interactions
1:25 pm:
Space Physics Seminar in Tate 301-20
Speaker: Julie Vievering, P.S. Athiray and Sophie Musset
Subject: Investigating high-energy solar phenomena with the FOXSI-3 sounding rocket

Wednesday, October 24th 2018
1:25 pm:
Speaker: Johnpierre Paglione, University of Maryland, College Park
Subject:  High-spin superconductivity in topological half-Heusler semimetals

In all known fermionic superfluids, Cooper pairs are composed of spin-1/2 quasi-particles that pair to form either spin-singlet or spin-triplet bound states. The "spin" of a Bloch electron, however, is fixed by the symmetries of the crystal and the atomic orbitals from which it is derived, and in some cases can behave as if it were a spin-3/2 particle. The superconducting state of such a system allows pairing states to form beyond triplet, with higher spin quasi-particles combining to form quintet or even septet pairs. After reviewing experimental evidence for high-spin pairing in the exotic superconducting state of the half-Heusler compound YPtBi, I will introduce our recent work elucidating the influence of spin-orbit coupling on both the normal and superconducting states of this system

Faculty Host: Vlad Pribiag
To be announced
4:40 pm:
CM Journal Club in Tate 201
Subject: Boltzmann Transport Equation

Thursday, October 25th 2018
10:10 am:
Biophysics Seminar in 120 PAN
Speaker: Oleg Krichevsky, Molecular Biophysics Lab, Ben-Gurion University
Subject:  T cell communication through cytokines follows a simple sink-diffusion model

Immune cells communicate by exchanging cytokines to achieve a context-appropriate response, but the distances over which such communication happens are not known. We used theoretical considerations and experimental models of immune responses in vitro and in vivo to quantify the spatial extent of cytokine communications in dense tissues. Using T cell exchange of IL-2 as a model system, we established that competition between cytokine diffusion and consumption generated spatial niches of high cytokine concentrations with sharp boundaries. The size of these self-assembled niches scaled with the density of cytokine-consuming cells, a parameter that gets tuned during immune responses. In vivo, we measured interactions on length scales of 80–120 um, which resulted in a high degree of cell-to-cell variance in cytokine exposure. Despite the complexity of the immune organs, the profiles of cytokine fields both in vitro and in vivo quantitatively follow the predictions of a simple model, essentially without any free parameters.

Ref. Oyler-Yaniv A, Oyler-Yaniv J, Whitlock B.M, Liu Z, Germain R.N, Huse M, Altan-Bonnet G. and O. Krichevsky (2017) , Immunity, 46, 609-620.

Faculty Host: Elias Puchner
Speaker: Nathan Eggen and Aliza Beverage, University of Minnesota
3:35 pm:
Speaker: Steven Gubser, Princeton
Subject: Number theory and spacetime
Refreshments in atrium after the Colloquium.

Our description of spacetime relies on the real numbers and hence is wedded to arbitrarily small intervals of length and time. But quantum theory hints at the existence of a smallest possible length, the Planck length. Number theory provides an alternative to the real numbers known as the p-adic numbers. Recent work has argued that quantum field theory defined over the p-adic numbers is holographically dual to a discrete spacetime. Constructions related to p-adic numbers also have a surprisingly prominent role in the early development of the renormalization group. I will explain what the p-adic numbers are and provide some intuition for what they are good for in string theory and beyond. The ultimate aim of using them to understand quantum gravity is ambitious indeed, but I will explain some first steps that give hope for the future.

Bio: Steve Gubser got his PhD from Princeton University in 1998, working with Igor Klebanov on what became the gauge-string duality. He did a post-doc at Harvard, then joined the faculty at Princeton. After a year at Caltech, he returned to Princeton and has been there ever since. He is now the Associate Chair for Undergraduates in Physics, and he is a recent recipient of a Simons Young Investigator award.

Faculty Host: Priscilla Cushman

Friday, October 26th 2018
12:20 pm:
Speaker: Yiming Wu
Subject: Special role of the first Matsubara frequency for superconductivity near a quantum-critical point --the non-linear gap equation below $T_c$ and spectral properties along the real frequency axis.
Speaker: Steven Gubser (Princeton)
Subject: From p-adic AdS/CFT to prospects in cold atoms

p-adic AdS/CFT is a version of the gauge-gravity duality where the boundary theory is defined over the p-adic numbers and the bulk is a discrete graph. A variant of p-adic field theories has emerged from recently proposed cold atom experiments based on sparse couplings. I will explain how a simplified version of these theories interpolates between an ordinary continuum field theory and p-adic field theory as a spectral exponent is dialed.

Speaker: Bin Chen,Center for Solar Terrestrial Research, New Jersey Institute of Technology

Known as one of the most capable radio telescopes for studying various astronomical objects in the universe, the Karl G. Jansky Very Large Array (JVLA) is also commissioned to observe the Sun in a wide range of radio wavelengths from <1 GHz to 18 GHz. Thanks to the Sun’s proximity, the JVLA can be configured as a powerful data machine when observing the Sun, capable of generating over one billion (1,000,000,000) radio images per hour of observing at an ultra-high time cadence (10 milliseconds), spectral resolution (~1%), and spatial resolution (~21”/f GHz). JVLA’s powerful dynamic spectroscopic imaging capability, combined with its high sensitivity, offer unique means for studying various radio emission from the Sun, particularly radio bursts emitted by energetic electrons accelerated in solar flares. We have recorded dozens of flare events with various sizes since 2011. The study of a subset of these flare events has yielded new insights on the magnetic energy release, particle acceleration and transport processes, which are not only important in solar flare physics, but also relevant in space physics and astrophysics contexts that involve magnetized plasma. I will discuss recent results based on these JVLA flare observations.

Speaker: Benjamin Goldberg , Department of Humanities and Cultural Studies, University of South Florida
Subject: Margaret Cavendish’s Medical Recipes: Medicine, Experience, and Natural Philosophy in Early Modern England
Refreshments served at 3:15 p.m.

In collaboration with Justin Begley (Helsinki), I am working on a transcript of Margaret Cavendish’s family book of medical recipes (often called ‘receipts’). This previously unpublished manuscript (MS Pw V 90, located at the University of Nottingham, UK) is a fascinating and rare document, written by a number of hands (including Margaret’s), and which provides some unique insights into Cavendish’s thought.

In this talk, I describe the contents of the MS, placing it in its proper context, namely, recent historiography on food, medicine, and cooking in early modern England. I focus, in particular, on how this work differs from most other recipe collections, both manuscript and printed, in, e.g., its exclusive medical focus and its inclusion of doctor’s reports. I then discuss how to interpret this work within the larger context of Cavendish’s natural philosophy, noting that we must be careful in how we interpret it, since it is not exclusively her writing, though there is evidence she was the one who compiled and organized it. With this proviso, I argue that this MS places some pressure on the received view of Cavendish’s conception of experience and experiment, seemingly undermining her anti-experimentalism and penchant for speculation. When this MS is read along with Cavendish’s extensive, if scattered and disorganized, discussions of medicine and food, however, we can resolve these apparent tensions by carefully attending to the various roles that empirical experience plays in Cavendish’s thought. Taken together with recent historiography on non-traditional aspects of the Scientific Revolution (women, kitchens, cooking, etc), this work can help us define some of the novel ways that experience was thought about in these various alternative contexts.

I conclude with some thoughts on our historical accounts of experience by scholars such as Steven Shapin and Peter Dear. I argue that these accounts, while not untrue, also do not exhaust the ways in which these ideas were understood in early modern England. Our accounts of experience and experiment are thus in need of revision and expansion so as to adequately account for the complex ways in which these ideas were used by various thinkers beyond the canonical philosophers and scientists of early modernity.

4:40 pm:
Speaker: Michael Zudov, Condensed Matter

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