171 research outputs found
Nuclear absorption of Charmoniums in pA and AA collisions
We have analysed the latest NA50 data on production in pA and AA
collisions. The production is assumed to be a two step process, (i)
formation of pairs, perturbatively calculable, and (ii) formation of
from the pair, a non-perturbative process, which is conviniently
parametrized. In a nuclear medium, as the pair passes through the
nuclear medium, it gain relative square momentum and some of the pairs can gain
enough square momentum to cross the threshold for open charm meson, leading to
suppression in nuclear medium. Few parameters of the model were fixed from the
latest high statistics NA50 pA and NA38 SU total cross sectional data.
The model then reproduces the centrality dependence of over Drell-Yan
ration in 200 GeV/c S+U and 158 GeV/c Pb+Pb collisions. We also discuss the
centrality dependence of suppression at RHIC energy.Comment: 4 pages including 3 figures, Revised version, to be published in
Phys.Rev.
Transverse energy dependence of J/Psi suppression in Au+Au collisions at RHIC energy
Prediction for transverse energy dependence of to Drell-Yan ratio in
Au+Au collisions at RHIC energy was obtained in a model which assume 100%
absorption of above a threshold density. The threshold density was
obtained by fitting the NA50 data on suppression in Pb+Pb collisions
at SPS energy. At RHIC energy, hard processes may be important. Prediction of
suppression with and without hard processes were obtained. With hard
processes included, 's are strongly suppressed.Comment: 4 pages, 2 figures (revised version
Single photons from Pb + Pb collisions at CERN SPS, QGP versus hadronic gas
In a hydrodynamic model, we have analysed the direct photon data obtained by the WA98 collaboration in 158 A GeV Pb+Pb collisions at CERN SPS. The transverse expansion of the system was taken into account. Two scenarios (i) formation of quark-gluon plasma and (ii) formation of hot hadronic gas were considered. Equally well description to the data were obtained in both the scenarios. However, hadronic gas scenario require very high initial temperature ( 300 MeV) and it is difficult to conceive existence of hadron gas at that high temperature. If the hadronic fluid has small radial velocity (0.2c-0.3c) initially, the data are well explained in the hadronic gas scenario with reasonable initial temperatures.In a hydrodynamic model, we have analysed the direct photon data obtained by the WA98 collaboration in 158 A GeV Pb+Pb collisions at CERN SPS. The transverse expansion of the system was taken into account. Two scenarios (i) formation of quark-gluon plasma and (ii) formation of hot hadronic gas were considered. Equally well description to the data were obtained in both the scenarios. However, hadronic gas scenario require very high initial temperature ( 300 MeV) and it is difficult to conceive existence of hadron gas at that high temperature. If the hadronic fluid has small radial velocity (0.2c-0.3c) initially, the data are well explained in the hadronic gas scenario with reasonable initial temperatures
suppression in Pb+Pb collisions and broadening
We have analysed the NA50 data, on the centrality dependence of
broadening of 's, in Pb+Pb collisions, at the CERN-SPS. The data were
analysed in a QCD based model, where 's are suppressed in 'nuclear'
medium. Without any free parameter, the model could explain the NA50
broadening data. The data were also analysed in a QGP based threshold model,
where suppression is 100% above a critical density. The QGP based
model could not explain the NA50 broadening data. We have also predicted
the centrality dependence of suppression and broadening at RHIC
energy. Both the models, the QGP based threshold model and the QCD based
nuclear absorption model, predict broadening very close to each other.Comment: The paper was completely revised. The conclusion is also changed. 5
pages, 4 figure
Charged particle's elliptic flow in 2+1D viscous hydrodynamics at LHC (= 2.76 TeV) energy in Pb+Pb collision
In the Israel-Stewart's theory of second order hydrodynamics, we have
simulated =2.76 TeV Pb+Pb collisions. ALICE data for the centrality
dependence of charged particles multiplicity, spectra in 0-5% collisions,
centrality dependence of integrated and differential elliptic flow are
analysed. Analysis indicate that while ALICE data on charged particles
multiplicity or spectra do not demand any viscosity, viscosity is
demanded by the elliptic flow data. From a simulataneous fit to all the data
sets, viscosity to entropy ratio in =2.76 TeV Pb+Pb collisions
is extracted as, .Comment: Significantly revised version. New figures are added. Accepted in
Phys. Lett. B. 6 pages, 6 figure
Influence of shear viscosity on the correlation between the triangular flow and initial spatial triangularity
In a hydrodynamic model, with fluctuating initial conditions, the correlation
between triangular flow and initial spatial triangularity is studied. The
triangular flow, even in ideal fluid, is only weakly correlated with the
initial triangularity. The correlation is largely reduced in viscous fluid.
Elliptic flow on the other hand appears to be strongly correlated with initial
eccentricity. Weak correlation between triangular flow and initial
triangularity indicate that a part of triangular flow is unrelated to initial
triangularity. Triangularity acquired during the fluid evolution also
contributes to the triangular flow.Comment: 4 pages, 2 figures. arXiv admin note: text overlap with
arXiv:1108.555
Nuclear absorption and anomalous J/psi suppression in Pb+Pb collisions
We have studied the J/psi suppression in 158 GeV/c Pb+Pb collisions at CERN
SPS. J/psi production is assumed to be a two step process, (i) formation of c
bar{c} pair, which is accurately calculable in QCD and (ii) formation of J/psi
meson from the c bar{c} pair, which can be conveniently parameterized. In a
pA/AA collision, as the c bar{c} pair pass through the nuclear medium, it gains
relative square momentum. As a result, some of the c bar{c} pairs can gain
enough momentum to cross the threshold to become open charm meson, leading to
suppression in pA/AA collisions. The model without any free parameter could
describe the of NA50 data on centrality dependence of the ratio's; J/psi over
Drell-Yan, J/psi over minimum bias and also the Drell-Yan over minimum bias.
The model was used to predict J/psi suppression at RHIC energy. At RHIC energy,
hard processes may be important. With hard processes included, J/psi's are
strongly suppressed, in agreement with other model calculations. We also show
that centrality dependence of J/psi over minimum bias ratio can be used to
determine the fraction of hard processes in the collision.Comment: 10 pages, 8 figure
Nearly perfect fluid in Au+Au collisions at RHIC
In the Israel-Stewart's theory of dissipative hydrodynamics, we have analysed
the STAR data on meson production in Au+Au collisions at =200
GeV. From a simultaneous fit to mesons multiplicity, mean and
integrated , we obtain a phenomenological estimate of QGP viscosity,
, the first error is due to the experimental
uncertainty in STAR measurements, the second reflects the uncertainties in
initial and final conditions of the fluid.Comment: 5 pages, 6 figures. Version accepted for publication in Phys. Lett.
Multiplicity, mean , -spectra and elliptic flow of identified particles in Pb+Pb collisions at LHC
Israel-Stewart's causal theory of dissipative hydrodynamics, with the ADS/CFT
lower limit of shear viscosity to entropy ratio (=0.08), give
consistent description of a number of experimental observables in Au+Au
collisions at RHIC (c.m. energy =200 GeV) \cite{Chaudhuri:2008sj}.
Assuming that in Pb+Pb collisions at LHC (c.m. energy =5.5 TeV),
except for the initial temperature, other parameters of the fluid remain
unchanged, we have predicted for the centrality dependence of multiplicity,
mean , -spectra, elliptic flow. The central temperature of the fluid
is adjusted to =421 MeV such that in a Pb+Pb collision, with participant
number =350, average charge particle multiplicity is 900 and
is consistent with the experimental trend observed at lower energies. Compare
to Au+Au collisions at RHIC, in Pb+Pb collisions at LHC, on the average,
particle multiplicity increases by a factor of 1.6, the mean is
increased by 10% only. The elliptic flow on the other hand decreases by
15%.Comment: 6 pages, 8 figures. Revised version. To be published in Physics
Letters
Fluctuating initial conditions and fluctuations in elliptic and triangular flow
In heavy ion collisions, event-by-event fluctuations in participating nucleon
positions can lead to triangular flow. With fluctuating initial conditions,
flow coefficients will also fluctuate. In a hydrodynamic model, we study the
fluctuations in elliptic and triangular flow, due to fluctuating initial
conditions. Both elliptic and triangular flow fluctuates strongly, triangular
flow more strongly than the elliptic flow. Strong fluctuations greatly reduce
the sensitivity of elliptic and triangular flow to viscosity.Comment: Final version to be published in Phys. Lett.B. 5 pages, 6 figure
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