Thema: 
Destabilization of the Antarctic Ice Sheet and extreme sealevel rise  Europe's greatest threat from global warming 
Datum: 
16.12.19 
Uhrzeit: 
16:15 
Ort: 
H6 
Vortragender: 

University of Southampton 

Inhalt: 
Even if the transformation to sustainable energy would occur tomorrow, and after tomorrow global temperature would not rise any further, sealevel will continue to increase, and not by a small amount, but by many meters. With the Paris Agreement fully implemented it would imply 412 m sea level rise in response to 2 degrees warming. Because this equilibrium response was always thought to occur on timescales of a few thousands of years, politicians and media were not interested. However, in recent years it became clear that the ocean is quickly melting floating iceshelves surrounding the large ice caps of Greenland and WestAntarctica. Especially around the western part of the Antarctic ice cap the process progresses disturbingly fast. Moreover, ice shelves feature deepening cracks and rifts and crumble into pieces, as has been witnesses for 2 ice shelves of the size of half of North Rhine Westphalia, Larsen A and Larsen B. This year Larsen C lost a large fragment of 300 by 50 km. While melting ice shelves do not raise the sea level (like melting ice cubes in a glass of soft drink), their indirect effect on sea level is large. The large ice caps are unstable and need the ice shelves to prevent them from collapsing under their own weight. A recent study (DeConto and Pollard, 2016) accounting for these processes suggested that sea level could rise by 6 meters in 200 years from now, with a very fast acceleration in sealevel rise by the end of this century. The outcome of this study is still debated but cannot be neglected or put aside. In this colloquium I will review processes that give rise to such fast and extreme sealevel rise, what recent observations tell us about these processes and to what extent we can attribute observed changes, what the stateofart is in icesheet modelling and how we can make future projections with numerical models, what these models tell us and whether we should be worried about the results. 
Ansprechpartner: 
Thema: 
Thimble regularisation of quantum field theories 
Datum: 
29.11.19 
Uhrzeit: 
16:15 
Ort: 
V3201 
Vortragender: 

Università di Parma 

Inhalt: 
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. 
Ansprechpartner: 
Thema: 
QCD at nonzero isospin asymmetry: signatures of the BCS phase from the Dirac spectrum 
Datum: 
14.01.20 
Uhrzeit: 
14:15 
Ort: 
D6135 
Vortragender: 

Frankfurt 

Inhalt: 
We investigate the complex spectrum of the Dirac operator in 2+1flavor QCD, at nonzero temperature and isospin chemical potential, using the extension of the BanksCasher relation to the case of Complex Dirac eigenvalues (derived for the zerotemperature, highdensity limits of QCD at nonzero isospin chemical potential), as a prescription to obtain information on the BCS gap from the 2d density of the complex Dirac eigenvalues. Such study is motivated by the prediction, from perturbation theory, of a superfluid state of $u$ and $\bar{d}$ Cooper pairs (BCS phase) at asymptotically high isospin densities, plausibly connected via an analytical crossover to the a phase with BoseEinstein condensation of charged pions at $\mu_I>=m_\pi/2$. Further motivation comes from recent lattice observations (renormalized Polyakov loop measurements) that indicate a decrease of the deconfinement transition temperature as a function of $\mu_I$, suggesting that the deconfinement crossover smoothly penetrates into the pion condensation phase and thus favoring a scenario where the deconfinement transition connects continuously to the BECBCS crossover in the $(T,\mu_I)$ phase diagram. 
Ansprechpartner: 
Thema: 
tba 
Datum: 
30.01.20 
Uhrzeit: 
14:15 
Ort: 
D5153 
Vortragender: 
Stefano Bo 
MPI for the Physics of Complex Systems 

Inhalt: 

Ansprechpartner: 
Thema: 
Critical behaviour and characteristic polynomials of nonHermitian random matrices 
Datum: 
23.05.19 
Uhrzeit: 
16:15 
Ort: 
D5153 
Vortragender: 

University of Sussex 

Inhalt: 
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). 
Ansprechpartner: 
Thema: 
Dimensional reduction for elliptic SPDE's: integrable structures and large deviations 
Datum: 
18.12.19 
Uhrzeit: 
16:15 
Ort: 
V3201 
Vortragender: 

University of Warwick 

Inhalt: 
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 
Ansprechpartner: 