30,592 research outputs found
Absorption and J/psi Suppression in Heavy Ion Collisions
We discuss the J/psi suppression in the framework of multiple collision
models. From the analysis of the Pb-Pb NA50 data we conclude that the strength
of the absorption has increased, but we find no clear evidence for the
formation of the quark-gluon plasma.Comment: 8 pages, 4 figure
Pair creation of particles and black holes in external fields
It is well known that massive black holes may form through the gravitational
collapse of a massive astrophysical body. Less known is the fact that a black
hole can be produced by the quantum process of pair creation in external
fields. These black holes may have a mass much lower than their astrophysical
counterparts. This mass can be of the order of Planck mass so that quantum
effects may be important. This pair creation process can be investigated
semiclassically using non-perturbative instanton methods, thus it may be used
as a theoretical laboratory to obtain clues for a quantum gravity theory. In
this work, we review briefly the history of pair creation of particles and
black holes in external fields. In order to present some features of the
euclidean instanton method which is used to calculate pair creation rates, we
study a simple model of a scalar field and propose an effective one-loop action
for a two-dimensional soliton pair creation problem. This action is built from
the soliton field itself and the soliton charge is no longer treated as a
topological charge but as a Noether charge. The results are also valid
straightforwardly to the problem of pair creation rate of domain walls in
dimensions greater than 2.Comment: LaTeX file (World Scientific macros), no figures, 9 pages, talk given
at Xth Portuguese Meeting on Astronomy and Astrophysics, (Lisbon, Portugal,
27-28 July 2000), to be published in Proc. Xth A & A meeting, edited by J. P.
S. Lemos, A. Mourao, L. Teodoro, R. Ugoccioni, (World Scientific, 2001
Trapped waves between submerged obstacles
Free-surface flows past submerged obstacles in a channel are considered. The fluid is assumed to be inviscid and incompressible and the flow to be irrotational. In previous work involving a single obstacle (Dias & Vanden-Broeck 2002), new solutions called ‘generalized hydraulic falls’ were found. These solutions are characterized by a supercritical flow on one side of the obstacle and a train of waves on the other. However, in the case of a single submerged object, the generalized hydraulic falls are unphysical because the waves do not satisfy the radiation condition. In this paper new solutions for the flow past two obstacles of arbitrary shape are computed. These solutions are characterized by a train of waves ‘trapped’ between the obstacles. The generalized hydraulic falls are shown to describe locally the flow over one of the two obstacles when the distance between the two obstacles is large
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