23,668 research outputs found
Strongly Coupled Plasmas in High-Energy Physics
One of the main activities in high-energy and nuclear physics is the search
for the so-called quark-gluon plasma, a new state of matter which should have
existed a few microseconds after the Big Bang. A quark-gluon plasma consists of
free color charges, i.e. quarks and gluons, interacting by the strong (instead
of electromagnetic) force. Theoretical considerations predict that the critical
temperature for the phase transition from nuclear matter to a quark-gluon
plasma is about 150 - 200 MeV. In the laboratory such a temperature can be
reached in a so-called relativistic heavy-ion collision in accelerator
experiments. Using the color charge instead of the electric charge, the Coulomb
coupling parameter of such a system is of the order 10 - 30. Hence the
quark-gluon plasma is a strongly coupled, relativistic plasma, in which also
quantum effects are important. In the present work the experimental and
theoretical status of the quark-gluon plasma physics will be reviewed,
emphasizing the similarities and differences with usual plasma physics.
Furthermore, the mixed phase consisting of free quarks and gluons together with
hadrons (e.g. pions) will be discussed, which can be regarded as a complex
plasma due to the finite extent of the hadrons.Comment: 5 pages, 5 figures, to be published in the Proceedings of the 10th
Workshop on the Physics of Dusty Plasmas (St. Thomas, US Virgin Islands
Van Hove Singularities in the Quark-Gluon Plasma
General arguments as well as different approximations for the in-medium quark
propagator in a quark-gluon plasma lead to quark dispersion relations that
exhibit a minimum in one branch (plasmino). This minimum causes Van Hove
singularities in the dilepton production rate and mesonic correlators, which
might have observable consequences.Comment: 9 pages, LaTex, 5 PostScript figures and style file included, to be
published in the proceedings of the conference "New Frontiers in Soft Physics
and Correlations on the Threshold of the Third Millenium" (12-17 June 2000,
Torino, Italy
Leontovich Relations in Thermal Field Theory
The application of generalized Kramers-Kronig relations, the so-called
Leontovich relations, to thermal field theory is discussed. Medium effects
contained in the full, thermal propagators can easily be taken into account by
this method. As examples the collisional energy loss of a charged particle in a
relativistic plasma and the radiation of energetic photons from a quark-gluon
plasma are considered. Within the leading logarithmic approximation the results
based on the hard thermal loop resummation technique are reproduced easily.
However, the method presented here is more general and provides exact
expressions, which allow in principle non-perturbative calculations.Comment: 14 pages, 4 figure
Absence of Thermophoretic Flow in Relativistic Heavy-Ion Collisions as an Indicator for the Absence of a Mixed Phase
If a quark-gluon plasma is formed in relativistic heavy-ion collisions, there
may or may not be a mixed phase of quarks, gluons and hadronic clusters when
the critical temperature is reached in the expansion of the fireball. If there
is a temperature gradient in the fireball, the hadronic clusters, embedded in
the heat bath of quarks and gluons, are subjected to a thermophoretic force. It
is shown that even for small temperature gradients and short lifetimes of the
mixed phase, thermophoresis would lead to a flow essentially stronger than the
observed one. The absence of this strong flow provides support for a rapid or
sudden hadronization mechanism without a mixed phase.Comment: 3 pages, 1 figure, revised version to be published in Phys. Rev. Let
Direct Photons from Relativistic Heavy-Ion Collisions
Direct photons have been proposed as a promising signature for the
quark-gluon plasma (QGP) formation in relativistic heavy-ion collisions.
Recently WA98 presented the first data on direct photons in Pb+Pb-collisions at
SPS. At the same time RHIC started with its experimental program. The discovery
of the QGP in these experiments relies on a comparison of data with theoretical
predictions for QGP signals. In the case of direct photons new results for the
production rates of thermal photons from the QGP and a hot hadron gas as well
as for prompt photons from initial hard parton scatterings have been proposed
recently. Based on these rates a variety of different hydrodynamic models,
describing the space-time evolution of the fireball, have been adopted for
calculating the direct photon spectra. The results have been compared to the
WA98 data and predictions for RHIC and LHC have been made. So far the
conclusions of the various models are controversial.
The aim of the present review is to provide a comprehensive and up-to-date
survey and status report on the experimental and theoretical aspects of direct
photons in relativistic heavy-ion collisions.Comment: 91 pages, 44 figures, revised version to be published in Phys. Re
Renormalization of Entanglement Entropy and the Gravitational Effective Action
The entanglement entropy associated with a spatial boundary in quantum field
theory is UV divergent, with the leading term proportional to the area of the
boundary. For a class of quantum states defined by a path integral, the
Callan-Wilczek formula gives a geometrical definition of the entanglement
entropy. We show that, for this class of quantum states, the entanglement
entropy is rendered UV-finite by precisely the counterterms required to cancel
the UV divergences in the gravitational effective action. In particular, the
leading contribution to the entanglement entropy is given by the renormalized
Bekenstein-Hawking formula, in accordance with a proposal of Susskind and
Uglum. We show that the subleading UV-divergent terms in the entanglement
entropy depend nontrivially on the quantum state. We compute new subleading
terms in the entanglement entropy and find agreement with the Wald entropy
formula for black hole spacetimes with bifurcate Killing horizons. We speculate
that the entanglement entropy of an arbitrary spatial boundary may be a
well-defined observable in quantum gravity.Comment: 26 pages, 2 figures. v2: minor corrections and clarification
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