arXiv subjects

A. K. Chaudhuri

Publications and source records attributed to A. K. Chaudhuri.

At least 73 records · Page 4Linked to original sources

arXiv2007

Dissipative hydrodynamics in 2+1 dimensions

In a first order theory of dissipative hydrodynamics, we have simulated hydrodynamic evolution of QGP fluid with dissipation due to shear viscosity only. Simulation confirms that compared to an ideal fluid, energy density or temperature of a viscous fluid evolve slowly. Transverse expansion is also more in viscous fluid. We also study the effect of viscosity on particle production. Particle production is enhanced, more at large $p_T$. The elliptic flow on the otherhand decreases and shows a tendency to saturate at large $p_T$.

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arXiv2006

$J/ψ$ production in Au+Au collisions at RHIC and the nuclear absorption

It is shown that a QCD based nuclear absorption model, with few parameters fixed to reproduce experimental $J/ψ$ yield in 200 GeV pp/pA and 450 GeV pA collisions can explain the preliminary PHENIX data on the centrality dependence of $J/ψ$ suppression in Cu+Cu collisions at RHIC energy, $\sqrt{s_{NN}}$=200 GeV. However, the model does not give satisfactory description to the preliminary PHENIX data on the centrality dependence of $J/ψ$ suppression in Au+Au collisions. The analysis suggest that in Au+Au collisions, $J/ψ$ are suppressed in a medium unlike the medium produced in SPS energy nuclear collisions or in RHIC energy Cu+Cu collisions.

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arXiv2006

Conical flow due to partonic jets in central Au+Au collisions

In jet quenching, a hard QCD parton, before fragmenting into a jet of hadrons, deposits a fraction of its energy in the medium, leading to suppressed production of high-$p_T$ hadrons. The process can generate shock waves. We study the distortion of Mach shock waves due to jet quenching in central Au+Au collisions and its effect on particle production. Finite fluid velocity and inhomogeneity of the medium can distort the Mach shock front significantly such that the inside shock front disappear and the outside shock front is opened up. We also show that the STAR data on azimuthal distribution of background subtracted secondaries, associated with high $p_T$ trigger, are reasonably well explained by the excess pions produced due to partonic energy loss.

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arXiv2006

$J/ψ$ production in Au+Au/Cu+Cu collisions at $\sqrt{s}_{NN}$=200 GeV and the threshold model

Using the QGP motivated threshold model, where all the $J/ψ$'s are suppressed above a threshold density, we have analyzed the preliminary PHENIX data on the centrality dependence of nuclear modification factor for $J/ψ$'s in Cu+Cu and in Au+Au collisions, at RHIC energy, $\sqrt{s}_{NN}$=200 GeV. Centrality dependence of $J/ψ$ suppression in Au+Au collisions are well explained in the model for threshold densities in ranges of 3.6-3.7 $fm^{-2}$. $J/ψ$ suppression in Cu+Cu collisions on the other hand are not explained in the model.

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arXiv2006

Dissipative hydrodynamics in 2+1 dimension

In 2+1 dimension, we have simulated the hydrodynamic evolution of QGP fluid with dissipation due to shear viscosity. Comparison of evolution of ideal and viscous fluid, both initialised under the same conditions e.g. same equilibration time, energy density and velocity profile, reveal that the dissipative fluid evolves slowly, cooling at a slower rate. Cooling get still slower for higher viscosity. The fluid velocities on the otherhand evolve faster in a dissipative fluid than in an ideal fluid. The transverse expansion is also enhanced in dissipative evolution. For the same decoupling temperature, freeze-out surface for a dissipative fluid is more extended than an ideal fluid. Dissipation produces entropy as a result of which particle production is increased. Particle production is increased due to (i) extension of the freeze-out surface and (ii) change of the equilibrium distribution function to a non-equilibrium one, the last effect being prominent at large transverse momentum. Compared to ideal fluid, transverse momentum distribution of pion production is considerably enhanced. Enhancement is more at high $p_T$ than at low $p_T$. Pion production also increases with viscosity, larger the viscosity, more is the pion production. Dissipation also modifies the elliptic flow. Elliptic flow is reduced in viscous dynamics. Also, contrary to ideal dynamics where elliptic flow continues to increase with transverse momentum, in viscous dynamics, elliptic flow tends to saturate at large transverse momentum. The analysis suggest that initial conditions of the hot, dense matter produced in Au+Au collisions at RHIC, as extracted from ideal fluid analysis can be changed significantly if the QGP fluid is viscous.

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arXiv2006

Effects of jet quenching on the hydrodynamical evolution of quark-gluon plasma

We study the effects of jet quenching on the hydrodynamical evolution of the quark-gluon plasma (QGP) fluid created in a heavy-ion collision. In jet quenching, a hard QCD parton, before fragmenting into a jet of hadrons, deposits a fraction of its energy in the medium, leading to suppressed production of high-pT hadrons. Assuming that the deposited energy quickly thermalizes, we simulate the subsequent hydrodynamic evolution of the QGP fluid. For partons moving at supersonic speed, v_p > c_s, and sufficiently large energy loss, a shock wave forms leading to conical flow [1]. The PHENIX Collaboration recently suggested that observed structures in the azimuthal angle distribution [2] might be caused by conical flow. We show here that conical flow produces different angular structures than predicted in [1] and that, for phenomenologically acceptable values of parton energy loss, conical flow effects are too weak to explain the structures seen by PHENIX [2].

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arXiv2005

Direct photons from Au+Au collisions at RHIC: QGP vs. hot hadronic gas

We have analysed the preliminary PHENIX data on the transverse momentum distribution of direct photons in 0-20% centrality Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV. In ideal hydrodynamics, data are explained if Au+Au collision produces Quark-Gluon-Plasma at the temperature $T_i$=400 MeV, at an initial time $τ_i$=0.6 fm. PHENIX data are not explained in the alternate scenario when Au+Au collisions produces hot hadronic gas with initial temperature within physically acceptable limit.

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arXiv2005

Heavy flavor production at RHIC and LHC energy

In a leading order pQCD model, we have studied the heavy flavor production in p+p collisions at RHIC and LHC energy. Leading order pQCD models require a K-factor. At RHIC energy, $\sqrt{s}$=200 GeV, we fix K such that the model reproduces the integrated charm yield, $dN^{c\bar{c}}/dy$, estimated by the STAR and the PHENIX collaboration in p+p collisions. The model then explains the STAR data on the transverse momentum distribution of open charm mesons $(D^0)$ and decay electrons in p+p collisions. The p+p predictions, scaled by the number of binary collisions, also explain the electron spectra in STAR p+d collisions and PHENIX Au+Au collisions in different centrality bins. Assuming that at LHC energy K-factor is of the order of unity, we have used the model to predict the transverse momentum distribution of $D$ and $B$ mesons and also of electrons from semileptonic decay of $D\to e$ and $B\to e$, in p+p collisions at LHC energy, $\sqrt{s}$=14 TeV.

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arXiv2005

Can $J/ψ$ suppression and $p_T$ broadening signal the deconfinement transition at RHIC?

We have analyzed the latest NA50 data on $J/ψ$ suppression in Pb+Pb collisions at CERN SPS. It is shown that a QCD based nuclear absorption model, where $J/ψ$'s are absorbed in nuclear medium could explain the latest NA50 data on the centrality dependence of the $J/ψ$ over Drell-Yan ratio. The model also explains the NA50 data on $J/ψ$ over minimum bias ratio and the $p_T$ broadening of $J/ψ$'s. A QGP based threshold model where all the $J/ψ$'s are suppressed above a threshold density, also explains the data sets with smeared threshold density. Even at RHIC energy, centrality dependence of $J/ψ$ suppression or $p_T$ broadening could not distinguish between the two models.

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arXiv2005

Initiative in Nuclear Theory at the Variable Energy Cyclotron Centre

We recall the path breaking contributions of the nuclear theory group of the Variable Energy Cyclotron Centre, Kolkata. From a beginning of just one person in 1970s, the group has steadily developed into a leading group in the country today, with seminal contributions to almost the entire range of nuclear physics, viz., low energy nuclear reactions, nuclear structure, deep inelastic collisions, fission, liquid to gas phase transitions, nuclear matter, equation of state, mass formulae, neutron stars, relativistic heavy ion collisions, medium modification of hadron properties, quark gluon plasma, and cosmology of early universe.

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arXiv2005

Jet quenching and neutral pion production in Au+Au collisions at RHIC

In the jet quenching model, we have analysed the PHENIX data on nuclear modification factor of $π^0$, in Au+Au collisions at $\sqrt{s}$=200 GeV, and extracted the initial gluon density of the medium produced. In jet quenching, partons lose energy in the medium, depending on the medium density as well as on the in-medium path length. Systematic analysis indicate that in most central (0-10% centrality) collisions, medium density is very large $dN_g/dy \sim$ 2150. Medium density decreases exponentially as the collision centrality decreases and in very peripheral (70-92% centrality) collisions, $dN_g/dy \sim$ 3. Initial energy density of the medium also decreases smoothly from $ε_0 \sim$ 20 $GeV/fm^3$ in most central collisions to $ε_0 \sim$ 3 $GeV/fm^3$ in most peripheral collisions. Very large $dN_g/dy$ or $ε_0$ indicate very dense matter formation in central Au+Au collisions.

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arXiv2005

Hydrodynamical evolution of dissipative QGP fluid

In the framework of the Mueller-Israel-Stewart theory of dissipative fluid dynamics, we have studied the space-time evolution of a QGP fluid. For simplicity, we have considered shear viscosity only and neglected bulk viscosity and heat conductivity. Shear viscosity opposes the expansion and cooling of the fluid. As a result, the lifetime of the fluid is extended. We also find that the parton $p_T$ distribution is considerably flattened.

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arXiv2004

Centrality dependence of high $p_T$ suppression in Au+Au collisions suggest quark matter formation

In a pQCD-based model, we have analyzed the STAR data on the high $p_T$ suppression of charged hadrons, in Au+Au collisions at $\sqrt{s}$=200 GeV. In the jet quenching or the energy loss picture, $p_T$ spectra of charged hadrons as well as the $p_T$ dependence of nuclear modification factor, in all the centrality ranges, are well explained, with nearly a constant relative energy loss, $ΔE/E=0.56\pm 0.03$. Centrality independence of relative energy loss indicate that the matter produced in central and in peripheral collisions are different, otherwise relative energy loss would have shown strong centrality dependence. Qualitatively, centrality independence of relative energy loss can be understood, if in central Au+Au collisions deconfined matter is produced and the matter remain confined in peripheral collisions.

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arXiv2003

Centrality dependence of $ψ\prime$ over $J/ψ$ : a signal of QGP

If nuclear collisions lead to QGP formation then the ratio of $ψ\prime$ over $J/ψ$ will remain constant with $E_T$ as both $J/ψ$ and $ψ\prime$ are melted in the QGP. On the other hand, if hot hadronic matter is produced, the ratio will continually fall with $E_T$, as $ψ\prime$'s are more suppressed in hadronic matter than the $J/ψ$'s. From the existing NA50 Pb+Pb data, we have constructed the ratio $σ(ψ\prime)/σ(J/ψ)$, as a function of $E_T$. The ratio give indication of QGP formation, but definite conclusion can not be reached. We have also given the prediction for the ratio at RHIC energy. If QGP is formed at RHIC, it could be detected from the ratio.

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arXiv2003

Nuclear absorption and anomalous $J/ψ$ suppression in Pb+Pb collisions

We discuss $J/ψ$ suppression in a QCD based nuclear absorption model. Centrality dependence of $J/ψ$ suppression in S+U and in Pb+Pb collisions are explained in the model. However, the model fails to explain the centrality dependence of $ψ\prime$ suppression. $ψ\prime$ suppression in S+U or in Pb+Pb collisions require additional suppression. Additional suppression of $ψ\prime$, due to hadronic comovers or due to QGP formation could not be distinguished in Pb+Pb collisions. We then show that the centrality dependence of the ratio, $ψ\prime$ over $J/ψ$, could possibly distinguish two scenario (e.g. QGP or hadronic comover) at RHIC energy.

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arXiv2003

Threshold model and the latest NA50 data on $J/ψ$ suppression in Pb+Pb collisions

Using the QGP motivated threshold model, where all the $J/ψ$'s are suppressed above a threshold density, we have analyzed the latest version of the NA50 data on the centrality dependence of the $J/ψ$ over Drell-Yan ratio. The data are not well explain in the model, unless the threshold density is largely smeared. Large smeared threshold density effectively excludes creation of any deconfined medium in the collision.

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arXiv2003

Nuclear absorption of Charmoniums in pA and AA collisions

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

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arXiv2003

$J/ψ$ suppression in Pb+Pb collisions and $p_T$ broadening

We have analysed the NA50 data, on the centrality dependence of $p_T$ broadening of $J/ψ$'s, in Pb+Pb collisions, at the CERN-SPS. The data were analysed in a QCD based model, where $J/ψ$'s are suppressed in 'nuclear' medium. Without any free parameter, the model could explain the NA50 $p_T$ broadening data. The data were also analysed in a QGP based threshold model, where $J/ψ$ suppression is 100% above a critical density. The QGP based model could not explain the NA50 $p_T$ broadening data. We have also predicted the centrality dependence of $J/ψ$ suppression and $p_T$ broadening at RHIC energy. Both the models, the QGP based threshold model and the QCD based nuclear absorption model, predict $p_T$ broadening very close to each other.

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