78 research outputs found
Pre-equilibrium dileptons look thermal
The dilepton mass distribution from pre-equilibrium matter in
ultrarelativistic nuclear collisions is indistinguishable from a thermally
produced distribution.Comment: CERN-TH.6813/93, 3 pages (latex) plus 1 figure (uuencoded postscript
file
Superluminal Signals: Causal Loop Paradoxes Revisited
Recent results demonstrating superluminal group velocities and tachyonic
dispersion relations reopen the question of superluminal signals and causal
loop paradoxes. The sense in which superluminal signals are permitted is
explained in terms of pulse reshaping, and the self-consistent behavior which
prevents causal loop paradoxes is illustrated by an explicit example.Comment: 6 pages, 3 figure
Evidence for Color Fluctuations in Hadrons from Coherent Nuclear Diffraction}
A QCD-based treatment of projectile size fluctuations is used to compute
inelastic diffractive cross sections for coherent
hadron-nuclear processes. We find that fluctuations near the average size give
the major contribution to the cross section with contribution
from small size configurations.
The computed values of are consistent with the limited
available data. The importance of coherent diffraction studies for a wide range
of projectiles for high energy Fermilab fixed target experiments is emphasized.
The implications of these significant color fluctuations for relativistic heavy
ion collisions are discussed.Comment: Report number DOE/ER 40427-13-N93 11 pages, 3 figures available from
author Mille
A New Phase of Matter: Quark-Gluon Plasma Beyond the Hagedorn Critical Temperature
I retrace the developments from Hagedorn's concept of a limiting temperature
for hadronic matter to the discovery and characterization of the quark-gluon
plasma as a new state of matter. My recollections begin with the transformation
more than 30 years ago of Hagedorn's original concept into its modern
interpretation as the "critical" temperature separating the hadron gas and
quark-gluon plasma phases of strongly interacting matter. This was followed by
the realization that the QCD phase transformation could be studied
experimentally in high-energy nuclear collisions. I describe here my personal
effort to help develop the strangeness experimental signatures of quark and
gluon deconfinement and recall how the experimental program proceeded soon to
investigate this idea, at first at the SPS, then at RHIC, and finally at LHC.
As it is often the case, the experiment finds more than theory predicts, and I
highlight the discovery of the "perfectly" liquid quark-gluon plasma at RHIC. I
conclude with an outline of future opportunities, especially the search for a
critical point in the QCD phase diagram.Comment: To appear in {\em Melting Hadrons, Boiling Quarks} by Rolf Hagedorn
and Johan Rafelski (editor), Springer Publishers, 2015 (open access
Out-of-equilibrium electromagnetic radiation
We derive general formulas for photon and dilepton production rates from an
arbitrary non-equilibrated medium from first principles in quantum field
theory. At lowest order in the electromagnetic coupling constant, these relate
the rates to the unequal-time in-medium photon polarization tensor and
generalize the corresponding expressions for a system in thermodynamic
equilibrium. We formulate the question of electromagnetic radiation in real
time as an initial value problem and consistently describe the virtual
electromagnetic dressing of the initial state. In the limit of slowly evolving
systems, we recover known expressions for the emission rates and work out the
first correction to the static formulas in a systematic gradient expansion.
Finally, we discuss the possible application of recently developed techniques
in non-equilibrium quantum field theory to the problem of electromagnetic
radiation. We argue, in particular, that the two-particle-irreducible (2PI)
effective action formalism provides a powerful resummation scheme for the
description of multiple scattering effects, such as the
Landau-Pomeranchuk-Migdal suppression recently discussed in the context of
equilibrium QCD.Comment: 34 pages, 9 figures, uses JHEP3.cl
Scaling violations: Connections between elastic and inelastic hadron scattering in a geometrical approach
Starting from a short range expansion of the inelastic overlap function,
capable of describing quite well the elastic pp and scattering data,
we obtain extensions to the inelastic channel, through unitarity and an impact
parameter approach. Based on geometrical arguments we infer some
characteristics of the elementary hadronic process and this allows an excellent
description of the inclusive multiplicity distributions in and
collisions. With this approach we quantitatively correlate the violations of
both geometrical and KNO scaling in an analytical way. The physical picture
from both channels is that the geometrical evolution of the hadronic
constituents is principally reponsible for the energy dependence of the
physical quantities rather than the dynamical (elementary) interaction itself.Comment: 16 pages, aps-revtex, 11 figure
On the formation of a Hawking-radiation photosphere around microscopic black holes
We show that once a black hole surpasses some critical temperature
, the emitted Hawking radiation interacts with itself and forms a
nearly thermal photosphere. Using QED, we show that the dominant interactions
are bremsstrahlung and electron-photon pair production, and we estimate
, which when calculated more precisely is
found to be 45 GeV. The formation of the photosphere is
purely a particle physics effect, and not a general relativistic effect, since
the the photosphere forms roughly Schwarzschild radii away from
the black hole. The temperature of the photosphere decreases with distance
from the black hole, and the outer surface is determined by the constraint
(for the QED case), since this is the point at which electrons
and positrons annihilate, and the remaining photons free stream to infinity.
Observational consequences are discussed, and it is found that, although the
QED photosphere will not affect the Page-Hawking limits on primordial black
holes, which is most important for 100MeV black holes, the inclusion of QCD
interactions may significantly effect this limit, since for QCD we estimate
. The photosphere greatly reduces possibility of
observing individual black holes with temperatures greater than ,
since the high energy particles emitted from the black hole are processed
through the photosphere to a lower energy, where the gamma ray background is
much higher. The temperature of the plasma in the photosphere can be extremely
high, and this offers interesting possibilities for processes such as symmetry
restoration.Comment: Latex, 16 pages, 3 postscript figures, submitted to PRD. Also
available at http://fnas08.fnal.gov
Are direct photons a clean signal of a thermalized quark gluon plasma?
Direct photon production from a quark gluon plasma (QGP) in thermal
equilibrium is studied directly in real time. In contrast to the usual S-matrix
calculations, the real time approach is valid for a QGP that formed and reached
LTE a short time after a collision and of finite lifetime ( as expected at RHIC or LHC). We point out that during such
finite QGP lifetime the spectrum of emitted photons carries information on the
initial state. There is an inherent ambiguity in separating the virtual from
the observable photons during the transient evolution of the QGP. We propose a
real time formulation to extract the photon yield which includes the initial
stage of formation of the QGP parametrized by an effective time scale of
formation . This formulation coincides with the S-matrix approach
in the infinite lifetime limit. It allows to separate the virtual cloud as well
as the observable photons emitted during the pre- equilibrium stage from the
yield during the QGP lifetime. We find that the lowest order contribution
which does \emph{not} contribute to the S-matrix
approach, is of the same order of or larger than the S-matrix contribution
during the lifetime of the QGP for a typical formation time . The yield for momenta features a
power law fall-off and is larger than that obtained
with the S-matrix for momenta . We provide a
comprehensive numerical comparison between the real time and S-matrix yields
and study the dynamics of the build-up of the photon cloud and the different
contributions to the radiative energy loss. The reliability of the current
estimates on photon emission is discussed.Comment: 31 pages, 12 eps figures, version to appear in PR
Thermal rates for baryon and anti-baryon production
We use a form of the fluctuation-dissipation theorem to derive formulas
giving the rate of production of spin-1/2 baryons in terms of the fluctuations
of either meson or quark fields. The most general formulas do not assume
thermal or chemical equilibrium. When evaluated in a thermal ensemble we find
equilibration times on the order of 10 fm/c near the critical temperature in
QCD.Comment: 22 pages, 4 tables and 2 figures, REVTe
Correlations and Fluctuations in High-Energy Nuclear Collisions
Nucleon correlations in the target and projectile nuclei are shown to reduce
significantly the fluctuations in multiple nucleon-nucleon collisions, total
multiplicity and transverse energy in relativistic heavy-ion collisions, in
particular for heavy projectile and target. The interplay between cross-section
fluctuations, from color transparency and opacity, and nuclear correlations is
calculated and found to be able to account for large fluctuations in transverse
energy spectra. Numerical implementation of correlations and cross-section
fluctuations in Monte-Carlo codes is discussed.Comment: 30 pages, in Revtex, plus 4 figures. Figures and preprint can be
obtained by mailing address to: [email protected]
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