Events at Physics
Events on Thursday, April 26th, 2018
- R. G. Herb Condensed Matter Seminar
- Quantum field theory of nematic transitions in spin orbit coupled spin-1 polar bosons
- Time: 10:00 am
- Place: 5310 Chamberlin Hall
- Speaker: Elio König , Rutgers University
- Abstract: We theoretically study an ultra-cold gas of spin-1 polar bosons in one spatial dimension which are subject to a quadratic Zeeman field and a Raman induced spin-orbit coupling. Concentrating on the regime in which the background fields can be treated perturbatively we analytically solve the model in its low-energy sector, i.e. we characterize the relevant phases and the quantum phase transitions between them. Depending on the sign of the effective quadratic Zeeman field ε, two superfluid phases with distinct nematic order appear. In addition, we uncover a spin-disordered superfluid phase at strong coupling. We employ a combination of renormalization group calculations and duality transformations to access the nature of the phase transitions. At ε = 0, a line of spin-charge separated pairs of Luttinger liquids divides the two nematic phases and the transition to the spin disordered state at strong coupling is of the Berezinskii-Kosterlitz-Thouless type. In contrast, at ε ≠ 0, the quantum critical theory separating nematic and strong coupling spin disordered phases contains a Luttinger liquid in the charge sector that is coupled to a Majorana fermion in the spin sector (i.e. the critical theory at finite ε maps to a quantum critical Ising model that is coupled to the charge Luttinger liquid). Due to an emergent Lorentz symmetry, both have the same, logarithmically diverging velocity. We discuss the experimental signatures of our findings that are relevant to ongoing experiments in ultra-cold atomic gases of 23Na.
- Host: Alex Levchenko
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- Observation of the highest-energy gamma rays with the HAWC Observatory
- Time: 2:30 pm
- Place: 5310 Chamberlin Hall
- Speaker: Kelly Malone, Penn State
- Abstract: Galactic sources that accelerate particles to PeV energies (“PeVatrons”) are expected to exist, but to date only the Galactic Center has been identified as such. One of the signatures of a PeVatron is a hard gamma-ray spectrum that extends without any apparent spectral cutoff to at least tens of TeV. High-energy (> 50 TeV) gamma-ray observations are therefore essential in identifying PeVatron candidates. The High Altitude Water Cherenkov Observatory (HAWC) has sensitivity to gamma rays in this previously largely unexplored energy regime. HAWC is well suited to performing all-sky surveys due to its large instantaneous field of view (~2 sr) and high duty cycle (> 95%). I will discuss candidate sources seen above 50 TeV in the first 1000 days of HAWC data and discuss potential connections to the IceCube neutrinos. I will also briefly discuss the energy estimation method used by HAWC.
- Host: Stefan Westerhoff
- Astronomy Colloquium
- Star formation, polarization, and magnetic fields in the ALMA era
- Time: 3:30 pm - 5:00 pm
- Place: 4421 Sterling Hall, Coffee and cookies 3:30 PM. Talk starts at 3:45 PM
- Speaker: Chat Hull, NAOJ Fellow, NAO of Japan, NAPJ Chile Observatory, Joint Alma Observatory
The results from the ALMA polarization system have begun both to expand and to confound our understanding of the role of the magnetic field in low-mass star formation. After a brief motivation via CARMA, SMA, and other polarization observations from the prior decade, I will discuss new ALMA results, including the highest resolution and highest sensitivity polarization images made to date of two very young, Class 0 protostellar sources. These new observations achieve ~140 AU resolution, allowing us to probe polarization -- and thus magnetic field orientation -- in the innermost regions surrounding the protostars. First is a Class 0 protostellar source in Serpens known as Ser-emb 8, where a comparison with cutting-edge, moving-mesh AREPO simulations suggests that cloud-scale turbulence -- not a large-scale magnetic field preserved from the source's natal cloud -- is dictating the magnetic field morphology immediately surrounding the protostar. In contrast, in the second source, known as Serpens SMM1, the magnetic field has clearly been shaped by the bipolar outflow emanating from the central source -- a situation that is quite different from the turbulence-dominated morphology of Ser-emb 8. Finally, I will show recent observations of polarization toward IM Lup, a much more evolved, Class II protoplanetary disk. In the case of IM Lup, consistent with previous observations of other disks, the polarization appears to be due to scattering by dust grains, thus complicating the search for magnetic fields in disks, but opening up a new window into dust grain growth and evolution.
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- Can a tool developed for hurricane prediction be taken to predict neutrino flavor evolution?
- Time: 4:00 pm
- Place: 5280 Chamberlin Hall
- Speaker: Eve Armstrong, University of Pennsylvania
- Abstract: We assess the utility of an optimization-based data assimilation (D.A.)
technique for treating the problem of nonlinear neutrino flavor
transformation in core-collapse supernovae. D.A. was invented for
numerical weather prediction, and it shares some features of machine
learning for the purposes of predictive power. Within the D.A. framework,
one uses measurements obtained from a physical system to estimate the
state variable evolution and parameter values of the associated model.
Formulated as an optimization procedure, D.A. can offer an
integration-blind approach to predicting model evolution, which offers an
advantage for models that thwart solution via traditional numerical
integration techniques. Further, D.A. performs most optimally for models
whose equations of motion are nonlinearly coupled. In this exploratory
work, we consider a simple steady-state model with two mono-energetic
neutrino beams coherently interacting with each other and a background
medium. As this model can be solved via numerical integration, we have an
independent consistency check for D.A. solutions.
We find that the procedure can capture key features of flavor evolution
over the entire trajectory, even given measurements of neutrino flavor
only at the endpoint, and with an assumed known initial flavor
distribution. Further, the procedure permits an examination of the
sensitivity of flavor evolution to estimates of unknown model parameters,
locates degeneracies in parameter space, and can identify the specific
measurements required to break those degeneracies.
- Host: Baha Balantekin
- WIPAC at Women in STEM outreach event
- Time: 4:30 pm - 5:30 pm
- Place: Kromery Middle School, 7009 Donna Dr. Middleton, WI 53562
- Speaker: WIPAC researchers, WIPAC
- Abstract: WIPAC will have a outreach booth at the Women in STEM night hosted by 2 Middleton high school clubs. The purpose of the event is to encourage young girls to pursue STEM and show them real-life examples of what they can do with their knowledge. We will be bringing our cosmic messenger bean bag toss game, DOM, South Pole gear and talk to the participants about IceCube Neutrino Observatory.