38 research outputs found
Excitation of Color Degrees of Freedom of Nuclear Matter and Suppression
In high energy nuclear collisions, the conventional Glauber model is commonly
used to evaluate the contribution to suppression originating from the
inelastic interaction with colorless bound nucleons. This requires an effective
value for the -nucleon absorption cross section which is larger than
theoretically expected. On the other hand, multiple nucleon-nucleon collisions
mediated by color exchange interactions, excite their color degrees of freedom.
We investigate the importance of this effect and find that these excited states
provide a larger cross section for absorption. We conclude that the
related corrections are important to explain the effective value extrapolated
from experiment.Comment: 21 pages Latex, 8 postscript figure
Charmonium Suppression in Heavy Ion Collisions by Prompt Gluons
In relativistic heavy ion collisions, also the bremsstrahlung of gluons in
the fragmentation regions of the nuclei suppresses the produced charmonium
states. In the energy range of the SPS, the radiation of semi-hard gluons
occurs in the Bethe-Heitler regime and the density of gluons and therefore the
suppression goes like , where and are the nucleon
numbers of the projectile and target nuclei. In contrast, the suppression via
collisions with nucleons is proportional to .
Parameter free perturbative QCD calculations are in a good agreement with the
data on and suppression in heavy ion collisions at SPS CERN.
At higher energies (RHIC, LHC) the number of gluons which are able to break-up
the charmonium substantially decreases and the additional suppression is
expected to vanish.Comment: One figure is adde
Transverse Momentum Dependence of Anomalous J/\psi Suppression in Pb-Pb Collisions
The recently published data for for production in Pb-Pb
collisions at 158 A GeV are analyzed. For low values of transverse energy
, where normal suppression dominates, scales with the path
length of the gluons which fuse to make the . In the domain of
anomalous suppression is found to rise linearly with the
relative amount of anomalous suppression. This empirical law is reproduced
within an analytically solvable transport model which allows high
's to escape anomalous suppression. Interpreted in this way, the data
for lead to an estimate of fm/ for the duration
of anomalous suppression.Comment: 10 pages and 3 figure
Multiple Scattering Theory for the Photoproduction of Vector Mesons off Nuclei
The integrated cross section for the incoherent photoproduction of vector
mesons on nuclei , , is calculated within Glauber
theory and as a function of the photon energy. The inverse of the longitudinal
momentum transfer is called coherence length and depends on the
virtuality and the energy of the photon. Nuclear transmission factors strongly
depend on ( is the nuclear radius) and this effect may interfere
with the search for color transparency effects.Comment: 9 pages, 1 Postscript figur
Coherence Phenomena in Charmonium Production off Nuclei at the Energies of RHIC and LHC
In the energy range of RHIC and LHC the mechanisms of nuclear suppression of
charmonia are expected to be strikingly different from what is known for the
energy of the SPS. One cannot think any more of charmonium produced on a bound
nucleon which then attenuates as it passes through the rest of the nucleus. The
coherence length of charmonium production substantially exceeds the nuclear
radius in the new energy range. Therefore the production amplitudes on
different nucleons, rather than the cross sections, add up and interfere, i.e.
shadowing is at work. So far no theoretical tool has been available to
calculate nuclear effects for charmonium production in this energy regime. We
develop a light-cone Green function formalism which incorporates the effects of
the coherence of the production amplitudes and of charmonium wave function
formation, and is the central result of this paper. We found a substantial
deviation from QCD factorization, namely, gluon shadowing is much stronger for
charmonium production than it is in DIS. We predict for nuclear effects
scaling which is violated at lower energies by initial state energy loss which
must be also included in order to compare with available data. In this paper
only the indirect J/Psi originating from decay of P-wave charmonia are
considered. The calculated x_F-dependence of J/Psi nuclear suppression is in a
good accord with data. We predict a dramatic variation of nuclear suppression
with x_F in pA and a peculiar peak at x_F=0 in AA collisions at RHIC.Comment: 51 pages including 12 figures. Two references and comments are added
at the en
Time Structure of Anomalous J/psi and psi' suppression in nuclear collisions
The data for the mean squared transverse momentum P_t as function of
transverse energy E_t of J/Psi and Psi' produced in Pb-Pb collisions at the
CERN-SPS are analyzed and it is claimed that they contain information about the
time structure of anomalous suppression. A transport equation which describes
transverse motion of J/Psi and Psi' in the absorptive medium is proposed and
solved for a QGP and a comover scenario of suppression. While the comover
approach accounts for the data fairly well without adjusting any parameter, the
fit to the data within the QGP scenario requires to assume anomalous
suppression to become effective rather late, 3-4 fm/c after the nuclear
overlap.Comment: 11 pages, 4 figure
Critical Scattering at the Chiral Phase Transition and low-p_t enhancement of mesons in ultra-relativistic heavy-ion collisions
The enhancement of pions and kaons observed at small transverse momenta in
ultra-relativistic heavy-ion collisions may at least partly reflect critical
scattering expected to occur in the neighborhood of a second order phase
transition. Kinetic equations in the relaxation time approximation are proposed
for the time evolution of the quark distribution function into that of the
pions. Relaxation times for thermalization and hadronization processes are
functions of momenta and approach zero in the limit p->0, a consequence of
criticality at the phase transition. Data can be reproduced for suitably chosen
parameters.Comment: 9 pages, two postscript figure
Scanning the Quark-Gluon Plasma with Charmonium
We suggest the variation of charmonium suppression with Feynman x_F in heavy
ion collisions as a novel and sensitive probe for the properties of the matter
created in such reactions. In contrast to the proton-nucleus case where nuclear
suppression is weakest at small x_F, final state interactions with the comoving
matter create a minimum at x_F=0, which is especially deep and narrow if a
quark-gluon plasma is formed. While a particularly strong effect is predicted
at SPS, at the higher RHIC energy it overlaps with the expected sharp variation
with x_F of nuclear effects and needs comparison with proton-nucleus data. If
thermal enhancement of J/\Psi production takes over at the energies of RHIC and
LHC, it will form an easily identified peak, rather than dip in x_F dependence.
We predict a steep dependence on centrality and suggest that this new probe is
complementary to the dependence on transverse energy, and is more sensitive to
a scenario of final state interactions.Comment: 5 pages including 3 figures. Stylistic and clarifying corrections are
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Inelastic Photoproduction off Nuclei: Gluon Enhancement or Double Color Exchange?
The nuclear enhancement observed in inelastic photoproduction of
should not be interpreted as evidence for an increased gluon density in nuclei.
The nuclear suppression of the production rate due to initial and final state
interactions is calculated and a novel two-step color exchange process is
proposed, which is able to explain the data.Comment: Latex file, 23 pages including 5 Postscript figure