arXiv subjects

Victor Roy

Publications and source records attributed to Victor Roy.

50 records · Page 3Linked to original sources

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

nucl-ex↗

arXiv2012

Elliptic flow of thermal dileptons as a probe of QCD matter

We study the variation of elliptic flow of thermal dileptons with transverse momentum and invariant mass of the pairs for Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. The dilepton productions from quark gluon plasma (QGP) and hot hadrons have been considered including the spectral change of light vector mesons in the thermal bath. The space time evolution has been carried out within the frame work of 2+1 dimensional ideal hydrodynamics with lattice+hadron resonance gas equation of state. We find that a judicious selection of invariant mass(M) and transverse momentum (p_T) windows can be used to extract the collective properties of quark matter, hadronic matter and also get a distinct signature of medium effects on vector mesons. Our results indicate a reduction of elliptic flow (v_2) for M beyond phi mass, which if observed experimentally would give the measure of v_2 of the partonic phase.

nucl-th↗

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.

nucl-th↗

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 $ζ/s<0.004$.

nucl-th↗

arXiv2011

Hydrodynamical analysis of centrality dependence of charged particle's multiplicity in $\sqrt{s}_{NN}$=2.76 TeV Pb+Pb collisions

In Israel-Stewart's theory of dissipative hydrodynamics, we have analyzed the recent ALICE data for the centrality dependence of charged particle multiplicity, centrality dependence of integrated elliptic flow and 0-5% charged particles $p_T$ spectra in $\sqrt{s}_{NN}$=2.76 TeV Pb+Pb collisions and determined the initial or the thermalisation time. Analysis indicate that the ALICE data disfavor very early thermalsiation $τ_i$=0.2 fm, or very late thermalsation $τ_i$= 4fm. Data are best explained for thermalisation time $τ_i=0.88^{-0.14}_{+0.68}$ fm.

nucl-th↗

arXiv2011

Charged particle's elliptic flow in 2+1D viscous hydrodynamics at LHC ($\sqrt{s}$= 2.76 TeV) energy in Pb+Pb collision

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

nucl-th↗

arXiv2011

The phase diagram of QCD in a BAG+HRG based equation of state: appearance of a pseudo-critical point

Mapping the QCD phase boundary and locating critical end point still remains as an open problem in strong interaction physics. Predictions about the co-ordinates of the critical point in the $(T, μ_B)$ plane, from different QCD motivated models show a wide variation. Lattice QCD calculations are also available, that give an estimation of the critical point for chiral phase transition, where the transition changes its nature from rapid cross over to first order transition. Recently co-ordinates of the critical point for deconfinement phase transition are claimed to be found as an endpoint of the first order phase transition line, in Bag model scenario. In the present paper we have shown that Bag model gives a complete first order phase transition line in the $(T, μ_B)$ plane, and one can not have any point where the transition changes its nature.

hep-ph↗

arXiv2011

Charged particle's $p_T$ spectra and elliptic flow in $\sqrt{s_{NN}}$=200 GeV Au+Au collisions: QGP vs. hadronic resonance gas

We show that if the hadronic resonance gas (HRG), with viscosity to entropy ratio $η/s\approx$0.24, is physical at temperature $T\approx$220 MeV, charged particles $p_T$ spectra and elliptic flow in Au+Au collisions at RHIC, over a wide range of collision centrality do not distinguish between initial QGP fluid and initial hadronic resonance gas. Unambiguous identification of bulk of the matter produced in Au+Au collisions require clear demonstration that HRG is unphysical at temperature $T<$200 MeV. It calls for precise lattice simulations with realistic boundary conditions.

nucl-th↗

arXiv2010

Elliptic flow ($v_2$) in pp collisions at energies available at the CERN Large Hadron Collider: A hydrodynamical approach

At Large Hadron Collider energy, the expected large multiplicities suggests the presence of collective behavior even in pp collisions. A hydrodynamical approach has been applied to estimate the expected elliptic flow measured by the azimuthal asymmetry parameter $v_2$, in pp collisions at $\surd$s = 14 TeV. $v_2$ of $π^-$ is found to be strongly dependent on the parton density profile inside a proton [e.g., surface diffuseness parameter ($ξ$)]. For $ξ$ = 0.105, $v_2$ is found to be positive while at $ξ$ = 0.25, $v_2$ is close to zero and approaches negative values at large $p_t$. The impact parameter dependence of $v_2$ has also been studied.

nucl-th↗

arXiv2010

Hadronic resonance gas and charged particle's $p_T$ spectra and elliptic flow in $\sqrt{s}$=200 GeV Au+Au collisions

Charged particles $p_T$ spectra and elliptic flow in 0-60% Au+Au collisions at RHIC are analyzed in a hydrodynamic model with hot, hadronic resonance gas in the initial state. Physically conceivable hadronic resonance gas, thermalized in the time scale $τ_i$=1 fm, at a (central) temperature $T_i$=220 MeV, with viscosity to entropy ratio $η/s$=0.24, reasonably well explains $p_T$ spectra in all the collision centralities. Centrality dependence of elliptic flow however demands continual increase of viscosity to entropy ratio with centrality.

nucl-th↗

arXiv2010

Transverse momentum spectra and elliptic flow in ideal hydrodynamics and geometric scaling

In an ideal hydrodynamic model, with an equation of state where the confinement-deconfinement transition is a cross-over at $T_{co}=196 MeV$, we have simulated $\sqrt{s}$=200 GeV Au+Au collisions. Simultaneous description of the experimental charged particle's $p_T$ spectra and elliptic flow require that in central (0-10%) Au+Au collisions, initial energy density scales with the binary collision number density. In less central collisions, experimental data demand scaling with the participant density. Simulation studies also indicate that in central collisions viscous effects are minimal.

nucl-th↗

arXiv2010

Lattice based equation of state and transverse momentum spectra of identified particles in ideal and viscous hydrodynamics

Assuming that in Au+Au collisions, a baryon free fluid is produced, transverse momentum spectra of identified particles ($π$, $K$, $p$ and $ϕ$), in evolution of ideal and viscous fluid is studied. Hydrodynamic evolution is governed by a lattice based equation of state (EOS), where the confinement-deconfinement transition is a cross-over at $T_{co}$=196 MeV. Ideal or viscous fluid was initialised to reproduce $ϕ$ meson multiplicity in 0-5% Au+Au collisions. Ideal or minimally viscous ($η/s$=0.08) fluid evolution reasonably well explain the transverse momentum spectra of pion's, kaon's, and $ϕ$ meson's in central and mid-central Au+Au collisions. Description to the data is much poorer in viscous fluid evolution with $η/s \geq$0.12. The model however under estimate proton production by a factor $\sim$ 2.

nucl-th↗

arXiv2010

A lattice based equation of state and $ϕ$ meson production in $\sqrt{s}$=6-200 GeV Pb+Pb and Au+Au collisions

In a boost-invariant hydrodynamic model, we have analyzed $ϕ$ meson production in Pb+Pb and Au+Au collisions, in the centre of mass energy range $\sqrt{s}$= 6-200 GeV. Hydrodynamic evolution is governed by a lattice based equation of state with a confinement-deconfinement {\em cross over} at $T_{co}$=196 MeV. We also look for the threshold energy above which the confined matter undergoes a deconfinement transition. Analysis indicate that above a threshold energy, ${\sqrt{s}}_{th}$=$13.65 \pm 3.06$ GeV, the fluid medium crosses over from a confined phase to a deconfined phase.

nucl-th↗

arXiv2010

Equation of state dependence of Mach cone like structures in 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. As the parton moves nearly with speed of light, much greater that the speed of sound of the medium, quenching jet can generate Mach shock wave. We have examined the possibility of Mach shock wave formation due to jet quenching. Assuming that the deposited energy quickly thermalize, we simulate the hydrodynamic evolution of the QGP fluid with a quenching jet and subsequent particle production. Angular distribution of pions, averaged over all the jet trajectories, resembles 'conical flow' due to Mach shock wave formation. However, speed of sound dependence of the simulated Mach angles are at variance with that due to shock wave formation in a static medium or in a medium with finite velocity.

nucl-th↗