22 research outputs found
On the Temperature Dependence of the Shear Viscosity and Holography
We examine the structure of the shear viscosity to entropy density ratio
eta/s in holographic theories of gravity coupled to a scalar field, in the
presence of higher derivative corrections. Thanks to a non-trivial scalar field
profile, eta/s in this setup generically runs as a function of temperature. In
particular, its temperature behavior is dictated by the shape of the scalar
potential and of the scalar couplings to the higher derivative terms. We
consider a number of dilatonic setups, but focus mostly on phenomenological
models that are QCD-like. We determine the geometric conditions needed to
identify local and global minima for eta/s as a function of temperature, which
translate to restrictions on the signs and ranges of the higher derivative
couplings. Finally, such restrictions lead to an holographic argument for the
existence of a global minimum for eta/s in these models, at or above the
deconfinement transition.Comment: references adde
Liquid-gas phase transition in nuclear multifragmentation
The equation of state of nuclear matter suggests that at suitable beam
energies the disassembling hot system formed in heavy ion collisions will pass
through a liquid-gas coexistence region. Searching for the signatures of the
phase transition has been a very important focal point of experimental
endeavours in heavy ion collisions, in the last fifteen years. Simultaneously
theoretical models have been developed to provide information about the
equation of state and reaction mechanisms consistent with the experimental
observables. This article is a review of this endeavour.Comment: 63 pages, 27 figures, submitted to Adv. Nucl. Phys. Some typos
corrected, minor text change
On the Strength of First Order Phase Transitions
Electroweak baryogenesis may solve one of the most fundamental questions we
can ask about the universe, that of the origin of matter. It has become clear
in the past few years that it also poses a multi-faceted challenge. In order to
compute the tiny primordial baryonic excess, we probably must invoke physics
beyond the standard model (an exciting prospect for most people), we must push
perturbation theory to its ``limits'' (or beyond), and we must deal with
nonequilibrium aspects of the phase transition. In this talk, I focus mainly on
the latter issue, that of nonequilibrium aspects of first order transitions. In
particular, I discuss the elusive question of ``weakness''. What does it mean
to have a weak first order transition, and how can we distinguish between weak
and strong? I argue that weak and strong transitions have very different
dynamics; while strong transitions proceed by the usual bubble nucleation
mechanism, weak transitions are characterized by a mixing of phases as the
system reaches the critical temperature from above. I show that it is possible
to clearly distinguish between the two, and discuss consequences for studies of
first order transitions in general. (Invited talk given at the ``Electroweak
Physics and the Early Universe'' workshop, Sintra, March 23-25, 1994.)Comment: 16 pages, 4 figures not included (can be obtained from
hep-ph/9403310, or by request) RevTeX, DART-HEP-94/0