171 research outputs found

    Nuclear absorption of Charmoniums in pA and AA collisions

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    We have analysed the latest NA50 data on J/ψJ/\psi production in pA and AA collisions. The J/ψJ/\psi production is assumed to be a two step process, (i) formation of ccˉc\bar{c} pairs, perturbatively calculable, and (ii) formation of J/ψJ/\psi from the pair, a non-perturbative process, which is conviniently parametrized. In a nuclear medium, as the ccˉc\bar{c} 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 J/ψJ/\psi cross sectional data. The model then reproduces the centrality dependence of J/ψJ/\psi over Drell-Yan ration in 200 GeV/c S+U and 158 GeV/c Pb+Pb collisions. We also discuss the centrality dependence of J/ψJ/\psi 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

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    Prediction for transverse energy dependence of J/ψJ/\psi to Drell-Yan ratio in Au+Au collisions at RHIC energy was obtained in a model which assume 100% absorption of J/ψJ/\psi above a threshold density. The threshold density was obtained by fitting the NA50 data on J/ψJ/\psi suppression in Pb+Pb collisions at SPS energy. At RHIC energy, hard processes may be important. Prediction of J/ψJ/\psi suppression with and without hard processes were obtained. With hard processes included, J/ψJ/\psi's are strongly suppressed.Comment: 4 pages, 2 figures (revised version

    Single photons from Pb + Pb collisions at CERN SPS, QGP versus hadronic gas

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    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 (\sim 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 (\sim 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

    J/ψJ/\psi suppression in Pb+Pb collisions and pTp_T broadening

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    We have analysed the NA50 data, on the centrality dependence of pTp_T broadening of J/ψJ/\psi's, in Pb+Pb collisions, at the CERN-SPS. The data were analysed in a QCD based model, where J/ψJ/\psi's are suppressed in 'nuclear' medium. Without any free parameter, the model could explain the NA50 pTp_T broadening data. The data were also analysed in a QGP based threshold model, where J/ψJ/\psi suppression is 100% above a critical density. The QGP based model could not explain the NA50 pTp_T broadening data. We have also predicted the centrality dependence of J/ψJ/\psi suppression and pTp_T broadening at RHIC energy. Both the models, the QGP based threshold model and the QCD based nuclear absorption model, predict pTp_T 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 (s\sqrt{s}= 2.76 TeV) energy in Pb+Pb collision

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    In the Israel-Stewart's theory of second order hydrodynamics, we have simulated s\sqrt{s}=2.76 TeV Pb+Pb collisions. ALICE data for the centrality dependence of charged particles multiplicity, pTp_T 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 pTp_T 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 sNN\sqrt{s}_{NN}=2.76 TeV Pb+Pb collisions is extracted as, η/s=0.06±0.02\eta/s=0.06\pm 0.02.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

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    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

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    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

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    In the Israel-Stewart's theory of dissipative hydrodynamics, we have analysed the STAR data on ϕ\phi meson production in Au+Au collisions at s\sqrt{s}=200 GeV. From a simultaneous fit to ϕ\phi mesons multiplicity, mean pTp_T and integrated v2v_2, we obtain a phenomenological estimate of QGP viscosity, η/s=0.07±0.03±0.14\eta/s =0.07 \pm 0.03 \pm 0.14, 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 pTp_T, pTp_T-spectra and elliptic flow of identified particles in Pb+Pb collisions at LHC

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    Israel-Stewart's causal theory of dissipative hydrodynamics, with the ADS/CFT lower limit of shear viscosity to entropy ratio (η/s\eta/s=0.08), give consistent description of a number of experimental observables in Au+Au collisions at RHIC (c.m. energy s\sqrt{s}=200 GeV) \cite{Chaudhuri:2008sj}. Assuming that in Pb+Pb collisions at LHC (c.m. energy s\sqrt{s}=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 pTp_T, pTp_T-spectra, elliptic flow. The central temperature of the fluid is adjusted to TiT_i=421 MeV such that in a Pb+Pb collision, with participant number NpartN_{part}=350, average charge particle multiplicity is \sim 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 \sim1.6, the mean pTp_T is increased by \sim10% only. The elliptic flow on the other hand decreases by \sim15%.Comment: 6 pages, 8 figures. Revised version. To be published in Physics Letters

    Fluctuating initial conditions and fluctuations in elliptic and triangular flow

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    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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