Introduction to the discussion section

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Real and imaginary-time quarkonium correlators in a hot plasma

The possibility of describing the behavior of a $Q\overline{Q}$ pair in a hot plasma in terms of an effective potential is investigated. It is shown that as long as medium effects can be embodied in a gaussian action, like in the QED case, the $Q\overline{Q}$ propagator obeys a closed temporal evolution equation whose large-time behavior is governed by an effective potential. The latter, beside screening, displays also an imaginary part related to collisions.Comment: Talk given at the 8-th Conference "Quark Confinement and the Hadron Spectrum", Mainz, Germany, 1-6 September 200

Renormalization flow for unrooted forests on a triangular lattice

We compute in small temperature expansion the two-loop renormalization constants and the three-loop coefficient of the beta-function, that is the first non-universal term, for the sigma-model with O(N) invariance on the triangular lattice at N=-1. The partition function of the corresponding Grassmann theory is, for negative temperature, the generating function of unrooted forests on such a lattice, where the temperature acts as a chemical potential for the number of trees in the forest. To evaluate Feynman diagrams we extend the coordinate space method to the triangular lattice.Comment: 26 pages, 4 figure

What causes the fragmentation of debris streams in TDEs?

A tidal disruption event (TDE) occurs when a star passes too close to a supermassive black hole and gets torn apart by its gravitational tidal field. After the disruption, the stellar debris form an expanding gaseous stream. The morphology and evolution of this stream is particularly interesting as it ultimately determines the observational properties of the event itself. In this work we perform 3D hydrodynamical simulations of the TDE of a star modelled as a polytropic sphere of index {\gamma} = 5/3, and study the gravitational stability of the resulting gas stream. We provide an analytical solution for the evolution of the stream in the bound, unbound and marginally bound case, that allows us to describe the stream properties and analyse the time-scales of the physical processes involved, applying a formalism developed in star formation context. Our results are that, when fragmentation occurs, it is fueled by the failure of pressure in supporting the gas against its self-gravity. We also show that a stability criterion that includes also the stream gas pressure proves to be far more accurate than one that only considers the black hole tidal forces, giving analytical predictions of the time evolution of the various forces associated to the stream. Our results point out that fragmentation occurs on timescales longer compared with the observational windows of these events and is thus not expected to give rise to significant observational features.Comment: 12 pages,15 figures, accepted for publication in MNRA

The reform of IMF quotas: the way towards the 2008 resolution

Policy evaluation based on the estimation of dynamic stochastic general equilibrium models with aggregate macroeconomic time series rests on the assumption that a representative agent can be identified, whose behavioural parameters are independent of the policy rules. Building on earlier work by Geweke, the main goal of this paper is to show that the representative agent is in general not structural, in the sense that its estimated behavioural parameters are not policyindependent. The paper identifies two different sources of nonstructurality. The latter is shown to be a fairly general feature of optimizing representative agent rational expectations models estimated on macroeconomic data.International Relations, International Monetary Fund, governance Classification JEL: F53, F59