5,518 research outputs found
Flash of photons from the early stage of heavy-ion collisions
The dynamics of partonic cascades may be an important aspect for particle
production in relativistic collisions of nuclei at CERN SPS and BNL RHIC
energies. Within the Parton-Cascade Model, we estimate the production of single
photons from such cascades due to scattering of quarks and gluons q g -> q
gamma, quark-antiquark annihilation q qbar -> g gamma, or gamma gamma, and from
electromagnetic brems-strahlung of quarks q -> q gamma. We find that the latter
QED branching process plays the dominant role for photon production, similarly
as the QCD branchings q -> q g and g -> g g play a crucial role for parton
multiplication. We conclude therefore that photons accompanying the parton
cascade evolution during the early stage of heavy-ion collisions shed light on
the formation of a partonic plasma.Comment: 4 pages including 3 postscript figure
A QCD space-time analysis of quarkonium formation and evolution in hadronic collisions
The production of heavy quarkonium as QQbar bound-states in hadron-hadron
collisions is considered within the framework of a space-time description,
combining parton-cascade evolution with a coalescence model for bound-state
formation. The `hard' production of the initial QQbar, directly or via gluon
fragmentation and including both color-singlet and color-octet contributions,
is calculated from the PQCD cross-sections. The subsequent development of the
QQbar system is described within a space-time generalization of the DGLAP
parton-evolution formalism in position- and momentum-space. The actual
formation of the bound-states is accomplished through overlap of the QQbar pair
and a spectrum of quarkonium wave-functions. This coalescence can only occur
after sufficent gluon radiation reduces the QQbar relative velocity to a value
commensurate with the non-relativistic kinematics of these bound systems. The
presence of gluon participants in the cascade then is both necessary and leads
to the natural inclusion of both color-singlet and color-octet mechanisms. The
application of this approach to pp (ppbar) collisions from sqrt(s)= 30 GeV - 14
TeV reveals very decent agreement with available data from ISR and Tevatron -
without the necessity of introducing fit parameters. Moreover, production
probabilities are calculated for a complete spectrum of charmonium and
bottonium states, with the relative significance compared to open charm
(bottom) production. An analysis of the space-time development is carried
through which sheds light on the relevance of gluon radiation and
color-structure, suggesting a correponding experimental investigation.Comment: 37 pages including 16 postscript figure
Excitation Function of Energy Density and Partonic Degrees of Freedom in Relativistic Heavy Ion Collisions
We estimate the energy density pile-up at mid-rapidity in central Pb+Pb
collisions from 2 - 200 GeV/nucleon. The energy density is decomposed into
hadronic and partonic contributions. A detailed analysis of the collision
dynamics in the framework of a microscopic transport model shows the importance
of partonic degrees of freedom and rescattering of leading (di)quarks in the
early phase of the reaction for lab-energies > 30 GeV/nucleon. In Pb+Pb
collisions at 160 GeV/nucleon the energy density reaches up to 4 GeV/fm^3, 95%
of which are contained in partonic degrees of freedom.Comment: 10 pages, 4 figure
Critique of a Pion Exchange Model for Interquark Forces
I describe four serious defects of a widely discussed pion exchange model for
interquark forces: it doesn't solve the "spin-orbit problem" as advertised, it
fails to describe the internal structure of baryon resonances, it leads to
disastrous conclusions when extended to mesons, and it is not reasonably
connected to the physics of heavy-light systems.Comment: 20 pages, 6 figures; some clarifications and references adde
Validation of gyrokinetic modelling of light impurity transport including rotation in ASDEX Upgrade
Upgraded spectroscopic hardware and an improved impurity concentration
calculation allow accurate determination of boron density in the ASDEX Upgrade
tokamak. A database of boron measurements is compared to quasilinear and
nonlinear gyrokinetic simulations including Coriolis and centrifugal rotational
effects over a range of H-mode plasma regimes. The peaking of the measured
boron profiles shows a strong anti-correlation with the plasma rotation
gradient, via a relationship explained and reproduced by the theory. It is
demonstrated that the rotodiffusive impurity flux driven by the rotation
gradient is required for the modelling to reproduce the hollow boron profiles
at higher rotation gradients. The nonlinear simulations validate the
quasilinear approach, and, with the addition of perpendicular flow shear,
demonstrate that each symmetry breaking mechanism that causes momentum
transport also couples to rotodiffusion. At lower rotation gradients, the
parallel compressive convection is required to match the most peaked boron
profiles. The sensitivities of both datasets to possible errors is
investigated, and quantitative agreement is found within the estimated
uncertainties. The approach used can be considered a template for mitigating
uncertainty in quantitative comparisons between simulation and experiment.Comment: 19 pages, 11 figures, accepted in Nuclear Fusio
Reaction dynamics in Pb+Pb at the CERN/SPS: from partonic degrees of freedom to freeze-out
We analyze the reaction dynamics of central Pb+Pb collisions at 160
GeV/nucleon. First we estimate the energy density pile-up at mid-rapidity and
calculate its excitation function: The energy density is decomposed into
hadronic and partonic contributions. A detailed analysis of the collision
dynamics in the framework of a microscopic transport model shows the importance
of partonic degrees of freedom and rescattering of leading (di)quarks in the
early phase of the reaction for E > 30 GeV/nucleon. The energy density reaches
up to 4 GeV/fm^3, 95% of which are contained in partonic degrees of freedom. It
is shown that cells of hadronic matter, after the early reaction phase, can be
viewed as nearly chemically equilibrated. This matter never exceeds energy
densities of 0.4 GeV/fm^3, i.e. a density above which the notion of separated
hadrons loses its meaning. The final reaction stage is analyzed in terms of
hadron ratios, freeze-out distributions and a source analysis for final state
pions.Comment: 10 pages, 7 figures, Proceedings of the Erice School on Nuclear
Physics in Erice, Sicily, Italy, September 17 -25 1998; to be published in
Progress in Particle and Nuclear Physics Vol. 4
Soft Electromagnetic Radiations From Equilibrating Quark-Gluon Plasma
We evaluate the bremsstrahlung production of low mass dileptons and soft
photons from equilibrating and transversely expanding quark gluon plasma which
may be created in the wake of relativistic heavy ion collisions. We use initial
conditions obtained from the self screened parton cascade model. We consider a
boost invariant longitudinal and cylindrically symmetric transverse expansion
of the parton plasma and find that for low mass dileptons ( GeV)
and soft photons ( GeV), the bremsstrahlung contribution is
rather large compared to annihilation process at both RHIC and LHC energies. We
also find an increase by a factor of 15-20 in the low mass dileptons and soft
photons yield as one goes from RHIC to LHC energies.Comment: 8 pages, including 7 figures To appear in Phys. Rev.
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