864 research outputs found
Parton Percolation in Nuclear Collisions
An essential prerequisite for quark-gluon plasma production in nuclear
collisions is cross-talk between the partons from different nucleons in the
colliding nuclei. The initial density of partons is determined by the parton
distribution functions obtained from deep inelastic lepton-hadron scattering
and by the nuclear geometry; it increases with increasing and/or .
In the transverse collision plane, this results in clusters of overlapping
partons, and at some critical density, the cluster size suddenly reaches the
size of the system. The onset of large-scale cross-talk through color
connection thus occurs as geometric critical behavior. Percolation theory
specifies the details of this transition, which leads to the formation of a
condensate of deconfined partons. Given sufficient time, this condensate could
eventually thermalize. However, already the onset of parton condensation in the
initial state, without subsequent thermalization, leads to a number of
interesting observable consequences.Comment: 15 pages, 18 figures; Lectures at the International School of Physics
"Enrico Fermi", Varenna/Italy, 6.-16. 8. 200
Phase Transitions in QCD
At high temperatures or densities, hadronic matter shows different forms of
critical behaviour: colour deconfinement, chiral symmetry restoration, and
diquark condensation. I first discuss the conceptual basis of these phenomena
and then consider the description of colour deconfinement in terms of symmetry
breaking, through colour screening and as percolation transition.Comment: 19 pages, 14 figure
Causality Constraints on Hadron Production In High Energy Collisions
For hadron production in high energy collisions, causality requirements lead
to the counterpart of the cosmological horizon problem: the production occurs
in a number of causally disconnected regions of finite space-time size. As a
result, globally conserved quantum numbers (charge, strangeness, baryon number)
must be conserved locally in spatially restricted correlation clusters. This
provides a theoretical basis for the observed suppression of strangeness
production in elementary interactions (pp, e^+e^-). In contrast, the space-time
superposition of many collisions in heavy ion interactions largely removes
these causality constraints, resulting in an ideal hadronic resonance gas in
full equilibrium.Comment: 16 pages,8 figure
Hawking-Unruh Hadronization and Strangeness Production in High Energy Collisions
The thermal multihadron production observed in different high energy
collisions poses many basic problems: why do even elementary, and
hadron-hadron, collisions show thermal behaviour? Why is there in such
interactions a suppression of strange particle production? Why does the
strangeness suppression almost disappear in relativistic heavy ion collisions?
Why in these collisions is the thermalization time less than fm/c?
We show that the recently proposed mechanism of thermal hadron production
through Hawking-Unruh radiation can naturally answer the previous questions.
Indeed, the interpretation of quark- antiquark pairs production, by the
sequential string breaking, as tunneling through the event horizon of colour
confinement leads to thermal behavior with a universal temperature, Mev,related to the quark acceleration, a, by . The resulting
temperature depends on the quark mass and then on the content of the produced
hadrons, causing a deviation from full equilibrium and hence a suppression of
strange particle production in elementary collisions. In nucleus-nucleus
collisions, where the quark density is much bigger, one has to introduce an
average temperature (acceleration) which dilutes the quark mass effect and the
strangeness suppression almost disappears.Comment: Contribution to special issue of Adv. High Energy Phys. entitled
"Experimental Tests of Quantum Gravity and Exotic Quantum Field Theory
Effects
Quarkonium Feed-Down and Sequential Suppression
About 40-50 % of the quarkonium ground states J/psi(1S) and Upsilon(1S)
produced in hadronic collisions originate from the decay of higher excitations.
In a hot medium, these higher states are dissociated at lower temperatures than
the more tightly bound ground states, leading to a sequential suppression
pattern. Using new finite temperature lattice results, we specify the in-medium
potential between heavy quarks and determine the dissociation points of
different quarkonium states. On the basis of recent CDF data on bottomonium
production, we then obtain first predictions for sequential Upsilon suppression
in nuclear collisions.Comment: 19 pages, LaTeX, 11 figure
Sequential charmonium dissociation
Finite temperature lattice QCD indicates that the charmonium ground state
J/psi can survive in a quark-gluon plasma up to 1.5 T_c or more, while the
excited states chi_c and psi-prime are dissociated just above T_c. We assume
that the chi_c suffers the same form of suppression as that observed for the
psi-prime in SPS experiments, and that the directly produced J/psi is
unaffected at presently available energy densities. This provides a
parameter-free description of J/psi and psi-prime suppression which agrees
quite well with that observed in SPS and RHIC data.Comment: 10 pages, 8 figure
Sequential Quarkonium Suppression
We use recent lattice data on the heavy quark potential in order to determine
the dissociation temperatures of different quarkonium states in hot strongly
interacting matter. Our analysis shows in particular that certain quarkonium
states dissociate below the deconfinement point.Comment: Talk presented on the International Workshop on the Physics of the
Quark - Gluon Plasma, September 4-7, 2001, Palaisea
Thermal Hadronization and Hawking-Unruh Radiation in QCD
We conjecture that because of color confinement, the physical vacuum forms an
event horizon for quarks and gluons which can be crossed only by quantum
tunneling, i.e., through the QCD counterpart of Hawking radiation by black
holes. Since such radiation cannot transmit information to the outside, it must
be thermal, of a temperature determined by the chromodynamic force at the
confinement surface, and it must maintain color neutrality. We explore the
possibility that the resulting process provides a common mechanism for thermal
hadron production in high energy interactions, from annihilation to
heavy ion collisions.Comment: 29 pages, 14 figure
Deconfinement through Chiral Symmetry Restoration in Two-Flavour QCD
We propose that in QCD with dynamical quarks, colour deconfinement occurs
when an external field induced by the chiral condensate strongly aligns the
Polyakov loop. This effect sets in at the chiral symmetry restoration
temperature T-chi and thus makes deconfinement and chiral symmetry restoration
coincide. The predicted singular behaviour of Polyakov loop susceptibilities at
T-chi is shown to be supported by finite temperature lattice calculations.Comment: 7 pages, 6 figure
- âŠ