Topic: |
Shine a light! When matter shatters |
Date: |
08.06.20 |
Time: |
16:15 |
Place: |
cyberspace |
Guest: |
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TU Darmstadt |
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Abstract: |
The microscopic properties of strong-interaction matter under extreme conditions of temperature and density is a topic of great interest. Matter in equilibrium radiates photons with a thermal spectrum revealing its temperature in the slope of the energy distribution. This is generalized for virtual photons, which materialize after a short time by creation of a pair of charged leptons (dileptons), for which their invariant mass takes the role of the energy as observable. In contrast to the case of photons, their spectral distribution is not affected by a blue (or red) shift. Moreover, dileptons offer the unique opportunity also to directly monitor in-medium electromagnetic spectral functions. Hence, dilepton spectra from strong-interaction medium reflect not only its temperature but also are sensitive to possible effects of a restoration of the spontaneously broken chiral symmetry. This talk will discuss important experimental results obtained so far at various facilities and the latest theoretical developments on emissivity of matter. |
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Topic: |
The problem of latency in estimating the Covid-19 replication number |
Date: |
08.05.20 |
Time: |
16:15 |
Place: |
ZOOM/Konferenzschaltung |
Guest: |
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University of Texas at Austin |
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Abstract: |
Figuring out how to restart the world's economy without a resurgence of disease depends on understanding how contagious Covid-19 really is. However, estimates of the basic replication number $R_0$ vary greatly, with well-respected groups publishing estimates whose 95% confidence intervals don't even overlap. In this talk I'll go over the basic SIR and SEIR models of disease spread and present several different ways to treat the latency period between being exposed and becoming infectious. Simple SEIR models are unstable; working with a fixed set of data, small changes to the model can result in large changes to the estimated value of $R_0$. More realistic models are more complicated and are even less stable. The upshot is that we know much less about $R_0$ than is generally believed, and the error bars on the high side are particularly large. Containing the outbreak for an extended period may be a lot harder than our leaders think. |
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Topic: |
Fermion bag inspired Hamiltonian lattice field theory |
Date: |
14.07.20 |
Time: |
14:15 |
Place: |
cyberspace |
Guest: |
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Perimeter Institute |
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Abstract: |
Motivated by the fermion bag approach, we construct a new class of Hamiltonian lattice field theories that can help us to study fermionic quantum critical points, particularly those with four-fermion interactions. Although these theories are constructed in discrete time with a finite temporal lattice spacing $\epsilon$, when \$epsilon$ goes to zero, conventional continuous-time Hamiltonian lattice field theories are recovered. The fermion bag algorithms run relatively faster when $\epsilon = 1$ as compared to $\epsilon$ going to zero, but still allow us to compute universal quantities near the quantum critical point even at such a large value of $\epsilon$. As an example of this new approach, here we study the $N_f = 1$ Gross-Neveu chiral Ising universality class in $2+1$ dimensions by calculating the critical scaling of the staggered mass order parameter. We show that we are able to study lattice sizes up to $100^2$ sites when $\epsilon = 1$. while with comparable resources we can only reach lattice sizes of up to $64^2$ when $\epsilon$ goes to zero. The critical exponents obtained in both these studies match within errors. |
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Topic: |
22-05-2020-14.15 hrs - D5-153 - Construction of tight binding models from ab initio calculations using maximally localized Wannier functions |
Date: |
22.05.20 |
Time: |
14:15 |
Place: |
D5-153 |
Guest: |
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Universtität Bielefeld |
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Abstract: |
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Contact person: |
Topic: |
Statistics of Extremes in Eigenvalue-counting Staircases |
Date: |
04.06.20 |
Time: |
16:00 |
Place: |
ZOOM / Konferenzschaltung |
Guest: |
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King's College London |
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Abstract: |
We consider the counting function (“spectral staircase”) for eigenvalues of a random unitary matrix, drawn from the corresponding beta-ensemble. Our goal is to characterize the statistics of maximum deviation of this staircase from its mean slope in a fixed interval, when size of the matrix N >>1. We will show that one-sided extremes can be addressed by exploiting a mapping onto the statistical mechanics of log-correlated random processes and using an extended Fisher-Hartwig conjecture. The resulting statistics exhibits combined features of counting statistics of Fermions with Sutherland-type interaction and extremal statistics of the fractional Brownian motion with Hurst index H = 0. Some of the features are expected to be universal. The talk is based on the paper Fyodorov-Le Doussal arXiv:2001.04135. |
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Topic: |
>>> Tuesday/Dienstag <<< Combinatorial maps for the GUE, LUE, and something in between |
Date: |
07.07.20 |
Time: |
09:00 |
Place: |
ZOOM / Konferenzschaltung |
Guest: |
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University of Melbourne |
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Abstract: |
This talk will start with a relaxed introduction to the combinatorics of the Gaussian and Laguerre unitary ensembles’ spectral moments. In particular, ribbon graph and combinatorial map interpretations will be given, along with their connection to ramified coverings of the Riemann sphere. Then, we will briefly look at the spectral moments of a matrix ensemble that behaves like a fusion of the GUE and LUE. One of the motivations for studying this “fusion” is that it serves as a matrix realisation of the Hermite Muttalib-Borodin ensemble in the large separation regime. |
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