arXiv subjects

A. K. Chaudhuri

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

At least 19 recordsLinked to original sources

arXiv2015

Fluctuations in slope parameter in event-by-event hydrodynamics and momentum anisotropy in heavy ion collisions

In event by event hydrodynamic model, we have simulated 30-40\% Au+Au collisions at RHIC and computed the slope parameter from the invariant pion distribution. In each event, the slope parameter fluctuates azimuthally. Fourier expansion coefficients $T_n$ for the slope parameter and the Fourier expansion coefficients $v_n$ for the azimuthal distribution $\frac{dN}{dϕ}$ are found to be strongly correlated. Strong correlation between the two expansion coefficients suggests that in addition to azimuthal distribution, fluctuations in the slope parameter of the invariant distribution can as well be used to study the final state momentum anisotropy in relativistic energy heavy ion collisions. If measured experimentally, they can serve as additional constraint for hydrodynamical modeling.

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arXiv2014

Effect of high frequency modes of medium on an open quantum system

We present a method to calculate the real time effective propagator of a generic open quantum system, immersed in a medium using a wave function based framework. The medium is characterised by a set of harmonic oscillators having a continuous span of frequencies. This technique has been applied to the Caldeira-Leggett model showing that high frequency modes of the medium do not contribute towards decay of the population of states of the open system. In fact, they cause a Rabi type oscillation. Moreover, our wave function based approach provides an excellent alternative to conventional formalisms involving the density matrix.

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arXiv2014

Parton shadowing and $J/ψ$-to-Drell-Yan ratio in nuclear collisions at SPS and FAIR

We have analyzed the data on $J/ψ$-to-Drell-Yan production cross section ratio in proton-nucleus ($p+A$) and nucleus-nucleus ($A+A$) collisions, measured by the NA50 collaboration in the SPS energy domain. Two component QVZ model has been employed to calculate $J/ψ$ production cross sections. For both $J/ψ$ and Drell-Yan production, nuclear modifications to the free nucleon structure functions are taken into account. In heavy-ion collisions, such modifications are assumed to be proportional to the local nuclear density resulting in centrality dependent initial state effects. Differences in quark and gluon shadowing leads to a new source of impact parameter dependence of the $J/ψ$ to Drell-Yan production ratio. For $J/ψ$, final state interaction of the produced $c\bar{c}$ pairs with the nuclear medium is also taken into account. A satisfactory description of the data in both $p+A$ and $A+A$ collisions is obtained. Role of the shadowing corrections are investigated in detail. Model calculations are extrapolated to predict the centrality dependence of $J/ψ$-to-Drell-Yan ratio in the FAIR energy regime.

hep-ph↗

arXiv2013

Charmonium suppression in a baryon rich quark-gluon plasma

We have investigated the charmonium survival probability, in a high baryon density parton plasma, expected to be produced in nuclear collisions at FAIR. Charmonia are assumed to undergo complete dissociation by color screening, if the in-medium Debye radius becomes comparable to the spatial size of the corresponding bound state. Results indicate a non-trivial dependence of the suppression pattern on the plasma evolution dynamics. A much larger magnitude of suppression is foreseen induced by cold nuclear matter compared to that due to plasma screening.

hep-ph↗

arXiv2013

Direct photon production and interferometry in $\sqrt{s}_{NN}$=200 GeV Au+Au and in $\sqrt{s}_{NN}$=2.76 TeV Pb+Pb collisions

Direct photon production, in $\sqrt{s_{NN}}$=200 GeV Au+Au and in $\sqrt{s_{NN}}$=2.76 TeV Pb+Pb collisions, are studied in a hydrodynamic model. Ideal hydrodynamic model, initialised to reproduce experimentally charged particle spectra in RHIC and LHC energy, also reproduces the PHENIX and ALICE measurements for the direct photon spectra in Au+Au collisions at RHIC and Pb+Pb collisions at LHC. The model however produces less elliptic flow than in experiment. Discrepancy between experiment and hydrodynamic simulation is comparatively less in Pb+Pb collisions at LHC than in Au+Au collisions at RHIC. We also studied direct photon correlation and determined the HBT radii. In 0-10%-50-60% collisions, HBT radii in Au+Au or in Pb+Pb collisions do not show large centrality dependence. Energy dependence of the HBT radii for direct photons is also not large.

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arXiv2013

Elliptic and Hexadecapole flow of charged hadron in viscous hydrodynamics with Glauber and Color Glass Condensate initial conditions for Pb-Pb collision at $\sqrt{s_{NN}}$=2.76 TeV

The experimentally measured elliptic ($v_{2}$) and hexadecapole ($v_{4}$) flow of charged particles as a function of transverse momentum ($p_{T}$) at midrapidity in Pb-Pb collisions at $\sqrt{s_{\mathrm NN}}$ = 2.76 TeV are compared with the relativistic viscous hydrodynamic model simulations. The simulations are carried out for two different initial energy density profiles obtained from (i) Glauber model, and (ii) Color Glass Condensate (CGC) model. Comparison to experimental data for 10-20% to 40-50% centrality, shows that a centrality dependent shear viscosity to entropy density ($η/s$) ratio with values ranging between 0.0 to 0.12 are needed to explain the $v_{2}$ data for simulations with the Glauber based initial condition. Whereas for the CGC based initial conditions a slightly higher value of $η/s$ is preferred, around 0.08 to 0.16. From the comparison of the $v_{4}$ simulated results to the corresponding experimental measurements we observe that for the centralities 20-30% to 40-50% the $η/s$ values lies between 0.0 to 0.12 for both the initial conditions studied. The $η/s$ values obtained from our studies for Pb-Pb collisions at $\sqrt{s_{\mathrm NN}}$ = 2.76 TeV are compared to other studies which uses both transport and hydrodynamic approaches.

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arXiv2013

Effect of charged particle's multiplicity fluctuations on flow harmonics in even-by-event hydrodynamics

In nucleon-nucleon collisions, charged particle's multiplicity fluctuates. We have studied the effect of multiplicity fluctuation on flow harmonics in nucleus-nucleus collision in event-by-event hydrodynamics. Assuming that the charged particle's multiplicity fluctuations are governed by the negative binomial distribution, the Monte-Carlo Glauber model of initial condition is generalised to include the fluctuations. Explicit simulations with the generalised Monte-Carlo Glauber model initial conditions indicate that the multiplicity fluctuations do not have large effect on the flow harmonics.

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arXiv2012

Even-by-event hydrodynamical simulations for $\sqrt{s}_{NN}$=200 GeV Au+Au collisions and the correlation between flow coefficients and initial asymmetry measures

Centrality dependence of charged particles multiplicity, transverse momentum spectra, integrated and differential elliptic flow, in $\sqrt{s}_{NN}$=200 GeV Au+Au collisions are analyzed using event by event ideal hydrodynamics. Monte-Carlo Glauber model of initial condition, constrained to reproduce experimental charged particle's multiplicity in 0-10% Au+Au collisions, reasonably well reproduces all the experimental observables, e.g. centrality dependence of charged particles multiplicity, integrated and differential elliptic flow. Model predictions for higher flow harmonics, $v_3$, $v_4$ however overestimate the experimental data, more in the peripheral collisions than in the central collisions. Correlation between initial (spatial) asymmetry measures and flow coefficients are also studied. With exception of the elliptic flow, for all the higher flow coefficients ($v_n$,n=3-5), correlation is reduced with collision centrality. In peripheral collisions, higher flow coefficients are only weakly correlated to the asymmetry measures. Elliptic flow however, remains strongly correlated with initial eccentricity in all the collision centralities.

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arXiv2012

Temperature dependence of QGP viscosity over entropy ratio from hydrodynamical analysis of ALICE data in $\sqrt{s_{NN}}$=2.76 TeV Pb+Pb collisions

Within Israel-Stewart's theory of dissipative hydrodynamics, we have analyzed ALICE data for the centrality dependence of charged particles multiplicity, elliptic flow and $p_T$ spectra in $\sqrt{s}_{NN}$=2.76 TeV Pb+Pb collisions and obtained the temperature dependence of the QGP viscosity over the entropy ratio ($\etas$). If temperature dependence of $\etas$ is parameterized as $\etas=α\frac{T-T_c}{T_c}+\frac{1}{4π}$, experimental data favor $α$ in the range 0-0.2. $α\geq 0.4$ is not favored by the data. Experimental data in $\sqrt{s}_{NN}$=200 GeV Au+Au collisions however, prefer $α$=0.4.

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arXiv2012

A short course on Relativistic Heavy Ion Collisions

Some ideas/concepts in relativistic heavy ion collisions are discussed. To a large extent, the discussions are non-comprehensive and non-rigorous. It is intended for fresh graduate students of Homi Bhabha National Institute, Kolkata Centre, who are intending to pursue career in theoretical /experimental high energy nuclear physics. Comments and criticisms will be appreciated.

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arXiv2012

Comparison of results from a 2+1D relativistic viscous hydrodynamic model to elliptic and hexadecapole flow of charged hadrons measured in Au-Au collisions at $\sqrt{s_{\rm {NN}}}$ = 200 GeV

Simulated results from a 2+1D relativistic viscous hydrodynamic model have been compared to the experimental data on the centrality dependence of invariant yield, elliptic flow ($v_{2}$), and hexadecapole flow ($v_{4}$) as a function of transverse momentum ($p_{T}$) of charged hadrons in Au-Au collisions at $\sqrt{s_{\rm {NN}}}$ = 200 GeV. Results from two types of initial transverse energy density profile, one based on the Glauber model and other based on Color-Glass-Condensate (CGC) are presented. We observe no difference in the simulated results on the invariant yield of charged hadrons for the calculations with different initial conditions. The comparison to the experimental data on invariant yield of charged hadrons supports a shear viscosity to entropy density ratio ($η/s$) between 0 to 0.12 for the 0-10% to 40-50% collision centralities. The simulated $v_{2}(p_{T})$ is found to be higher for a fluid with CGC based initial condition compared to Glauber based initial condition for a given collision centrality. Consequently the Glauber based calculations when compared to the experimental data requires a lower value of $η/s$ relative to CGC based calculations. In addition, a centrality dependence of the estimated $η/s$ is observed from the $v_{2}(p_{T})$ study. The $v_{4}(p_{T})$ for the collision centralities 0-10% to 40-50% supports a $η/s$ value between 0 - 0.08 for a CGC based initial condition. While simulated results using the Glauber based initial condition for the ideal fluid evolution under estimates the $v_{4}(p_{T})$ for collision centralities 0-10% to 30-40%.

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arXiv2012

J/$ψ$ suppression in a dense baryonic medium

We have examined the available latest SPS data on $J/ψ$ suppression in Pb+Pb and In+In collisions at 158 A GeV. Our employed model, with parameters fixed by $p$-$p$ and $p$-nucleus collisions, gives excellent description of NA50 and NA60 data on centrality dependence of $J/ψ$ suppression. The model is then applied to predict the centrality dependence of $J/ψ$ production in Au+Au collisions at FAIR energy domain. A much larger suppression of $J/ψ$ is predicted. In addition the possible effects of a baryon rich medium on $J/ψ$ production is also investigated.

hep-ph↗

arXiv2012

Fluctuating initial condition and smoothening effect on elliptic and triangular flow

In heavy ion collisions, event-by-event fluctuations in participating nucleon positions can lead to triangular flow. Generally, one uses Monte-Carlo Glauber model to obtain the participating nucleon positions. To use in a hydrodynamic model, the positions needs to be smoothened. We study the effect of smoothening of Glauber Monte-Carlo initial conditions on elliptic and triangular flow. It is shown that integrated as well as differential elliptic and triangular flow remain largely unaltered, irrespective of functional form of the smoothening function, or the smoothening parameter

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arXiv2012

$J/ψ$ suppression and QCD phase diagram

QCD phase diagram is obtained by analysing centrality dependence of $J/ψ$ suppression in $\snn$=17.3 Pb+Pb and $\snn$=200 GeV Au+Au collisions. $J/ψ$'s produced in initial interactions are assumed to dissolve if local temperature exceeds a threshold temperature. The threshold temperature depends on the (local) fluid temperature and baryonic chemical potential, which are obtained, under certain assumptions, from experimentally determined quantities e.g. rapidity density, net baryon density. QCD critical line with curvature parameter $κ\approx 0.018-0.23$ is consistent with experimental $J/ψ$ data. $κ$ is factor of $\sim$3 larger than in lattice QCD calculations.

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arXiv2012

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.

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arXiv2012

2+1 dimensional hydrodynamics including bulk viscosity: a systematics study

We have studied the effect of nonzero bulk viscosity with peak near the lattice QCD predicted crossover temperature $T_{co}\sim 175 MeV$ on charged particle transverse momentum spectra and elliptic flow. The Israel-Stewart theory of 2nd order causal dissipative relativistic fluid dynamics is used to simulate the space time evolution of the matter formed in Au-Au collisions at $\sqrt{s_{NN}}$=200 GeV assuming longitudinal boost invariance. A systematic comparison of temperature, transverse velocity, spatial and momentum anisotropy evolution of the ideal, bulk and shear viscous fluid has been carried out. Two different temperature dependent forms of $ζ/s$ and a constant $η/s$ was used. Both the bulk and shear viscous correction to the ideal freezeout distribution function are included. The dissipative correction to the freezeout distribution for bulk viscosity was calculated using Grad's fourteen moment method. From our simulation we show that the method is applicable only for $ζ/s < 0.005\timesη/s|_{KSS}$ for freezeout temperatures 130 and 160 MeV.

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arXiv2012

Bulk viscosity in heavy ion collision

The effect of a temperature dependent bulk viscosity to entropy density ratio~($ζ/s$) along with a constant shear viscosity to entropy density ratio~($η/s$) on the space time evolution of the fluid produced in high energy heavy ion collisions have been studied in a relativistic viscous hydrodynamics model. The boost invariant Israel-Stewart theory of causal relativistic viscous hydrodynamics is used to simulate the evolution of the fluid in 2 spatial and 1 temporal dimension. The dissipative correction to the freezeout distribution for bulk viscosity is calculated using Grad's fourteen moment method. From our simulation we show that the method is applicable only for $ζ/s<0.004$.

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arXiv2012

Dilepton to photon ratio, a viscometer of QGP

In the Israel-Stewart's 2nd order hydrodynamics, a viscous effect on dilepton emission from a QGP medium is investigated. Dileptons are strongly affected by QGP viscosity. Large invariant mass dileptons, due to their lower velocity, are less affected by viscosity than the low invariant mass dileptons. We also show that the ratio of photon to dilepton is sensitive to the viscosity and can serve as a viscometer for QGP.

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