Topic: 
Unser Platz im Universum  Der Nobelpreis für Physik 2019 
Date: 
09.12.19 
Time: 
16:15 
Place: 
H6 
Guest: 

Universität Bielefeld 

Abstract: 
Der diesjährige Nobelpreis für Physik wird and James Peebles (Princeton University), für theoretische Entdeckungen in physikalischer Kosmologie, und Michel Mayor (Universit\'e de Gen\`eve) und Didier Queloz (Universit\'e de Gen\´eve, University of Cambridge), für die Entdeckung eines Exoplaneten mit Umlaufbahn um einen sonnenähnlichen Stern, vergeben. Das Kolloquium wird den physikalischen Hintergrund der gewürdigten Forschungerfolge darstellen, und die spezifischen Beiträge der Preisträger beleuchten, sowie deren Bedeutung im Lichte unseres Wissenstands über unseren Platz im Universum würdigen. 
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Topic: 
Thimble regularisation of quantum field theories 
Date: 
29.11.19 
Time: 
16:15 
Place: 
V3201 
Guest: 

Università di Parma 

Abstract: 
Lattice regularisation provides an effective framework for a nonperturbative definition of Quantum Field Theories. It also enables numerical computations: in the euclidean formulation, lattice QFT resembles a statistical physics problem, the functional integral defines a decent probability measure and Monte Carlo simulations are viable. Nevertheless, this is not always the case. When a complex action is in place, we have no probability measure to start with and there is no obvious way to set up a Monte Carlo scheme. This is known as the sign problem. Among other theories, QCD with a chemical potential is plagued by a sign problem and we have no effective way to tackle the investigation of its (supposedly rich) phase diagram. A few years ago a conceptually simple technique was proposed to tame (or at least mitigate) the sign problem. The idea is to choose an alternative domain of integration within a complexified extension of the path integral. Most noticeably, there is a perfect candidate for such an alternative domain of integration: Lefschetz thimbles. These manifolds are characterised by a constant imaginary part of the action and the only residual sign problem is the one tied to the integration measure. Thimble regularisation is not only worth investigating to look for a decent Monte Carlo scheme; it is stimulating per se, and as a matter of fact the first attempts at a thimble formulation of QFT did not have computational applications as a goal. I will present an introduction to the technique, trying to highlight the conceptual challenges we have to face. In particular, I will discuss the problems that arise when we stumble into socalled Stokes phenomena and when we try to define a thimble formulation for gauge theories. 
Contact person: 
Topic: 
Symmetries of light hadrons in high temperature QCD 
Date: 
12.12.19 
Time: 
14:15 
Place: 
D6135 
Guest: 

Osaka University 

Abstract: 
I present the latest results of the light hadron spectrum within the high temperature QCD project of the JLQCD collaboration. The simulation uses two flavors of chirally symmetric Domainwall fermions and covers temperatures between 220 MeV  1 GeV. The hadron spectrum shows effectively restored chiral and axial U(1) symmetries in this temperature range, which can be seen for various hadronic observables. However, the connection to deconfined, noninteracting quarks is less straightforward, and the onset of a perturbative regime unclear. I show how additional SU(2) chiral spin and SU(4) symmetries emerge in hot QCD matter and might help to understand the dynamics in this temperature range. 
Contact person: 
Topic: 
tba 
Date: 
30.01.20 
Time: 
14:15 
Place: 
D5153 
Guest: 
Stefano Bo 
MPI for the Physics of Complex Systems 

Abstract: 

Contact person: 
Topic: 
Critical behaviour and characteristic polynomials of nonHermitian random matrices 
Date: 
23.05.19 
Time: 
16:15 
Place: 
D5153 
Guest: 

University of Sussex 

Abstract: 
I will discuss some recent developments regarding the normal matrix model. In particular my interest will be in certain critical models where the limiting support of the eigenvalues can radically change its topology by slightly adjusting an external parameter. I will discuss how aspects of the model can be explicitly mapped to the study of expectations of characteristic polynomials of nonHermitian random matrices (e.g. Ginibre or truncated unitary). Many of these averages are related to Painlevé transcendents, and by exploiting this, a precise and nontrivial asymptotic expansion of partition functions can be calculated in the critical models. This is joint work with Alfredo Deaño (University of Kent). 
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Topic: 
Dimensional reduction for elliptic SPDE's: integrable structures and large deviations 
Date: 
18.12.19 
Time: 
16:15 
Place: 
V3201 
Guest: 

University of Warwick 

Abstract: 
I will review the phenomenon of dimensional reduction for elliptic stochastic PDE's in two and three dimensions due to hidden supersymmetry discovered by Parisi and Sourlas. I will use dimensional reduction to establish a link between matrixvalued elliptic SPDE's and determinantal point processes. I will show that the large deviations principle can be established for a class of equations without any reference to supersymmetry. The talk is based on joint work with Roger Tribe and David Elworthy 
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