376 research outputs found
Mott Effect and J/Psi Dissociation at the Quark-Hadron Phase Transition
We investigate the in-medium modification of pseudoscalar and vector mesons
in a QCD motivated chiral quark model by solving the Dyson-Schwinger equations
for quarks and mesons at finite temperature for a wide mass range of meson
masses, from light (pi, rho) to open-charm (D, D*) states. At the chiral /
deconfinement phase transition, the quark-antiquark bound states enter the
continuum of unbound states and become broad resonances (the hadronic Mott
effect). We calculate the in-medium cross sections for charmonium dissociation
due to collisions with light hadrons in a chiral Lagrangian approach, and show
that the D and D* meson spectral broadening lowers the threshold for charmonium
dissociation by pi and rho meson impact. This leads to a step-like enhancement
in the reaction rate. We suggest that this mechanism for enhanced charmonium
dissociation may be the physical mechanism underlying the anomalous J/Psi
suppression observed by NA50.Comment: 3 pages, 3 figures, uses EPJ style (included), contribution to the
Proceedings of the International Conference on Quark Nuclear Physics, June 9
- 14, 2002, Juelich, Germany; 1 figure added, text adde
Color mixing in high-energy hadron collisions
The color mixing of mesons propagating in a nucleus is studied with the help
of a color-octet Pomeron partner present in the two-gluon model of the Pomeron.
For a simple model with four meson-nucleon channels, color mixings are found to
be absent for pointlike mesons and very small for small mesons. These results
seem to validate the absorption model with two independent color components
used in recent analyses of the nuclear absorption of mesons produced
in nuclear reactions.Comment: 3 journal-style page
Quark-Gluon Plasma: Status of Heavy Ion Physics
Lattice quantum chromodynamics (QCD), defined on a discrete space time
lattice, leads to a spectacular non-perturbative prediction of a new state of
matter, called quark-gluon plasma (QGP), at sufficiently high temperatures or
equivalently large energy densities. The experimental programs of CERN, Geneva
and BNL, New York of relativistic heavy ion collisions are expected to produce
such energy densities, thereby providing us a chance to test the above
prediction. After a brief introduction of the necessary theoretical concepts, I
will present a critical review of the experimental results already obtained by
the various experiments in order to examine whether QGP has already been
observed by them.Comment: 11 Pages, LaTeX, Plenary talk given at 6th Workshop in High Energy
Physics Phenomenology (WHEPP 6), Chennai (Madras), India, 3-15 Jan 2000 To
appear in Pramana, Journal of Physic
J/Psi suppression in colliding nuclei: statistical model analysis
We consider the suppression at a high energy heavy ion collision. An
ideal gas of massive hadrons in thermal and chemical equilibrium is formed in
the central region. The finite-size gas expands longitudinally in accordance
with Bjorken law. The transverse expansion in a form of the rarefaction wave is
taken into account. We show that suppression in such an environment,
when combined with the disintegration in nuclear matter, gives correct
evaluation of NA38 and NA50 data in a broad range of initial energy densities.Comment: 14 pages, 13 figures. Accepted for publication in Phys. Rev.
J/\Psi production, polarization and Color Fluctuations
The hard contributions to the heavy quarkonium-nucleon cross sections are
calculated based on the QCD factorization theorem and the nonrelativistic
quarkonium model. We evaluate the nonperturbative part of these cross sections
which dominates at GeV at the Cern Super Proton
Synchrotron (SPS) and becomes a correction at TeV at
the CERN Large Hadron Collider (LHC). \J production at the CERN SPS is well
described by hard QCD, when the larger absorption cross sections of the
states predicted by QCD are taken into account. We predict an -dependent
polarization of the states. The expansion of small wave packets is
discussed.Comment: 13 pages REVTEX, 1 table, 2 PostScript, corrected some typo
Quark exchange model for charmonium dissociation in hot hadronic matter
A diagrammatic approach to quark exchange processes in meson-meson scattering
is applied to the case of inelastic reactions of the type
(Q\barQ)+(q\barq)\rightarrow (Q\barq) + (q\barQ), where and refer to
heavy and light quarks, respectively. This string-flip process is discussed as
a microscopic mechanism for charmonium dissociation (absorption) in hadronic
matter. The cross section for the reaction is
calculated using a potential model, which is fitted to the meson mass spectrum.
The temperature dependence of the relaxation time for the \J/Psi distribution
in a homogeneous thermal pion gas is obtained. The use of charmonium for the
diagnostics of the state of hot hadronic matter produced in ultrarelativistic
nucleus-nucleus collisions is discussed.Comment: 24 pages, 3 tables, 7 figure
Charmonium suppression from purely geometrical effects
The extend to which geometrical effects contribute to the production and
suppression of the and minijet pairs in general is
investigated for high energy heavy ion collisions at SPS, RHIC and LHC
energies. For the energy range under investigation, the geometrical effects
referred to are shadowing and anti-shadowing, respectively. Due to those
effects, the parton distributions in nuclei deviate from the naive
extrapolation from the free nucleon result; . The strength
of the shadowing/anti-shadowing effect increases with the mass number. The
consequences of gluonic shadowing effects for the distribution of
's at GeV, GeV and TeV are
calculated for some relevant combinations of nuclei, as well as the
distribution of minijets at midrapidity for in the final state.Comment: corrected some typos, improved shadowing ratio
Phenomenology of Jet Quenching in Heavy Ion Collisions
We derive an analytical expression for the quenching factor in the strong
quenching limit where the spectrum of hard partons is dominated by
surface emission. We explore the phenomenological consequences of different
scaling laws for the energy loss and calculate the additional suppression of
the away-side jet.Comment: Substantially modified manuscrip
Evidence for Statistical Production of J/psi Mesons in Nuclear Collisions at 158--200 A GeV
The hypothesis of statistical production of J/psi mesons at hadronization is
formulated and checked against experimental data. It explains in the natural
way the observed scaling behavior of the J/psi to pion ratio at the CERN SPS
energies. Using the multiplicities of J/psi and eta mesons the hadronization
temperature T_H = 175 MeV is found, which agrees with the previous estimates of
the temperature parameter based on the analysis of the hadron yield
systematics.Comment: revised version, to appear in Phys. Rev. Let
Color-Octet Fraction in J/Psi Production and Absorption
The cross section between a pair and a nucleon is small and
sensitive to the separation if the pair is in a color-singlet
state, but very large and insensitive to the separation if it is in a
color-octet state. We use this property in an absorption model involving both
color components to deduce the color structure of pairs produced in
reactions. Our analysis shows that the NA3, NA38 and E772
data are not inconsistent with the theoretical picture that color-octet and
color-singlet precursors are produced in roughly equal proportions if the
produced color-singlet precursors are pointlike and transparent. However, if
the color-singlet precursors are not transparent but have a cross section of a
few mb, these data do show a definite preference for a larger fraction of
color-singlet precursors. In either case, the color-octet fraction increases
with , approaching unity as becomes large.Comment: 9 pages, updated to include new result
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