arXiv subjects

Subrata Pal

Publications and source records attributed to Subrata Pal.

At least 55 records · Page 3Linked to original sources

arXiv2012

Boltzmann equation with a nonlocal collision term and the resultant dissipative fluid dynamics

Starting with the relativistic Boltzmann equation where the collision term was generalized to include gradients of the phase-space distribution function, we recently presented a new derivation of the equations for the relativistic dissipative fluid dynamics. We compared them with the corresponding equations obtained in the standard Israel-Stewart and related approaches. Our method generates all the second-order terms that are allowed by symmetry, some of which have been missed by the traditional approaches, and the coefficients of other terms are altered. The first-order or Navier-Stokes equation too receives a small correction. Here we outline this work for the general audience.

nucl-th↗

arXiv2012

Medium information from anisotropic flow and jet quenching in relativistic heavy ion collisions

Within a multiphase transport (AMPT) model, where the initial conditions are obtained from the recently updated HIJING 2.0 model, the recent anisotropic flow and suppression data for charged hadrons in Pb+Pb collisions at the LHC center of mass energy of 2.76 TeV are explored to constrain the properties of the partonic medium formed. In contrast to RHIC, the measured centrality dependence of charged hadron multiplicity dN_ch/deta at LHC provides severe constraint to the largely uncertain gluon shadowing parameter s_g. We find final-state parton scatterings reduce considerably hadron yield at midrapidity and enforces a smaller s_g to be consistent with dN_ch/deta data at LHC. With the parton shadowing so constrained, hadron production and flow over a wide transverse momenta range are investigated in AMPT. The model calculations for the elliptic and triangular flow are found to be in excellent agreement with the RHIC data, and predictions for the flow coefficients v_n(p_T, cent) at LHC are given. The magnitude and pattern of suppression of the hadrons in AMPT are found consistent with the measurements at RHIC. However, the suppression is distinctly overpredicted in Pb+Pb collisions at the LHC energy. Reduction of the QCD coupling constant alpha_s by ~30% in the higher temperature plasma formed at LHC reproduces the measured hadron suppression.

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arXiv2012

Suppression of high transverse momenta hadrons in Pb+Pb collisions at LHC

Hadron production and their suppression in Pb+Pb collisions at LHC at a center-of-mass energy of s_NN = 2.76 TeV are studied within a multiphase transport (AMPT) model whose initial conditions are obtained from the recently updated HIJING 2.0 model. The centrality dependence of charged hadron multiplicity dN_ch/deta at midrapidity was found quite sensitive to the largely uncertain gluon shadowing parameter s_g that determines the nuclear modification of the gluon distribution. We find final-state parton scatterings reduce considerably hadron yield at midrapidity and enforces a smaller gluon shadowing to be consistent with dN_ch/deta data at LHC. With such a constrained parton shadowing, charged hadron and neutral pion production over a wide transverse momenta range are investigated in AMPT. Relative to nucleon-nucleon collisions, the particle yield in central heavy ion collisions is suppressed due to parton energy loss. While the calculated magnitude and pattern of suppression is found consistent with that measured in Au+Au collisions at s_NN = 0.2 TeV at RHIC, at the LHC energy the suppression is overpredicted which may imply the medium formed at LHC is less opaque than expected from simple RHIC extrapolations. Reduction of the QCD coupling constant alpha_s by ~30% in the higher temperature plasma formed at LHC as compared to that at RHIC was found to reproduce the measured suppression at LHC.

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arXiv2010

Role of isospin physics in supernova matter and neutron stars

We investigate the liquid-gas phase transition of hot protoneutron stars shortly after their birth following supernova explosion and the composition and structure of hyperon-rich (proto)neutron stars within a relativistic mean-field model where the nuclear symmetry energy has been constrained from the measured neutron skin thickness of finite nuclei. Light clusters are abundantly formed with increasing temperature well inside the neutrino-sphere for an uniform supernova matter. Liquid-gas phase transition is found to suppress the cluster yield within the coexistence phase as well as decrease considerably the neutron-proton asymmetry over a wide density range. We find symmetry energy has a modest effect on the boundaries and the critical temperature for the liquid-gas phase transition, and the composition depends more sensitively on the number of trapped neutrinos and temperature of the protoneutron star. The influence of hyperons in the dense interior of stars makes the overall equation of state soft. However, neutrino trapping distinctly delays the appearance of hyperons due to abundance of electrons. We also find that a softer symmetry energy further makes the onset of hyperon less favorable. The resulting structures of the (proto)neutron stars with hyperons and with liquid-gas phase transition are discussed.

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arXiv2010

Shear viscosity to entropy density ratio in nuclear multifragmentation

Nuclear multifragmentation in intermediate energy heavy ion collisions has long been associated with liquid-gas phase transition. We calculate the shear viscosity to entropy density ratio eta/s for an equilibrated system of nucleons and fragments produced in multifragmentation within an extended statistical multifragmentation model. The temperature dependence of eta/s exhibits surprisingly similar behavior as that for water. In the coexistence phase of fragments and light particles, the ratio eta/s reaches a minimum of comparable depth as that for water in the vicinity of the critical temperature for liquid-gas phase transition. The effects of freeze-out volume and surface symmetry energy on eta/s in multifragmentation are studied.

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arXiv2010

Nuclear symmetry energy effects on liquid-gas phase transition in hot asymmetric nuclear matter

The liquid-gas phase transition in hot asymmetric nuclear matter is investigated within relativistic mean-field model using the density dependence of nuclear symmetry energy constrained from the measured neutron skin thickness of finite nuclei. We find symmetry energy has a significant influence on several features of liquid-gas phase transition. The boundary and area of the liquid-gas coexistence region, the maximal isospin asymmetry and the critical values of pressure and isospin asymmetry all of which systematically increase with increasing softness in the density dependence of symmetry energy. The critical temperature below which the liquid-gas mixed phase exists is found higher for a softer symmetry energy.

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arXiv2010

Shear viscosity to entropy density ratio of a relativistic Hagedorn resonance gas

The new state of matter produced at Relativistic Heavy Ion Collider reveals a strongly coupled quark-gluon plasma with an extremely small shear viscosity to entropy density ratio eta/s. We calculate the eta/s of an equilibrated hadron matter characterized by a relativistic hadron resonance gas with a Hagedorn mass spectrum that grows exponentially with the hadron mass. We find with increase in temperature of the system the eta/s value decreases due to rapid increase in the multiplicity of massive resonances. In the vicinity of the critical temperature for deconfinement transition, the minimum value of eta/s in the Hagedorn resonance gas is found to be consistent with the current estimates for a strongly coupled quark-gluon plasma.

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arXiv2008

Dynamical transition in translational and rotational dynamics of water in the grooves of DNA duplex at low temperature

We have simulated structure and dynamics of water in the grooves of a DNA duplex using moleculear dynamics simulations. We find signatures of a dynamical transition in both translational and orientational dynamics of water molecules in both the major and the minor grooves of a DNA duplex. The transition occurs at a slightly higher temperature ($T_{GL} \approx $ 255 K) than the temperature ($T_L\approx$ 247 K) where the bulk water is conjectured to undergo a dynamical transition. Groove water, however, exhibits markedly different temperature dependence of its properties from the bulk. Entropy calculations reveal that the minor groove water is ordered even at room temperature and the transition at $T \approx$ 255 K can be characterized as a {\em strong-to-strong} dynamical transition. The low temperature water is characterized by pronounced tetrahedral order, as manifested in the sharp rise near $109^{\circ}$ in the O-O-O angle distribution. We find that Adams-Gibbs relation between configurational entropy and translational diffusion holds quite well when the two quantities are plotted together in a master plot for different region of aqueous DNA duplex (bulk, major and minor grooves) at different temperatures. The activation energy for the transfer of water molecules between different regions of DNA is found to be independent of temperature.

cond-mat.soft↗

arXiv2005

A Multi-Phase Transport Model for Relativistic Heavy Ion Collisions

We describe in detail how the different components of a multi-phase transport (AMPT) model, that uses the Heavy Ion Jet Interaction Generator (HIJING) for generating the initial conditions, Zhang's Parton Cascade (ZPC) for modeling partonic scatterings, the Lund string fragmentation model or a quark coalescence model for hadronization, and A Relativistic Transport (ART) model for treating hadronic scatterings, are improved and combined to give a coherent description of the dynamics of relativistic heavy ion collisions. We also explain the way parameters in the model are determined, and discuss the sensitivity of predicted results to physical input in the model. Comparisons of these results to experimental data, mainly from heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC), are then made in order to extract information on the properties of the hot dense matter formed in these collisions.

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arXiv2005

Hadron production from resonance decay in relativistic collisions

A statistical model for decay and formation of heavy hadronic resonances is formulated. The resonance properties become increasingly uncertain with increasing resonance mass. Drawing on analogy with the situation in low-energy nuclear physics, we employ the Weisskopf approach to the resonance processes. In the large-mass limit, the density of resonance states in mass is governed by a universal Hagedorn-like temperature T_H. As resonances decay, progressively more and more numerous lighter states get populated. For T_H \simeq 170 MeV, the model describes data for the hadron yield ratios at the RHIC and SPS energies under the extreme assumption of a single heavy resonance giving rise to measured yields.

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arXiv2004

Quark recombination and elliptic flow

Elliptic flow systematics for different hadron species have been explained by quark coalescence models. It has been argued that the elliptic asymmetry v_2 should scale with the number of quarks that comprise the hadron. We show how these arguments are sensitive to the relative role of asymmetries in the phase space density vs. asymmetries in the effective volume of emission. We also discuss the degree to which coalescence arguments differ from thermal models. Illustrative calculations based on solving the Boltzmann equation are presented along with the results of blast-wave models. Although the issue is complicated, ambiguities might be clarified by measurements of source-size parameters for nucleons at higher transverse momenta.

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arXiv2004

Xi production at AGS energies

A relativistic transport model is used to study Xi- production in 2-11A GeV Au+Au collisions. Introducing the strangeness-exchange reactions between antikaons and hyperons as the sources for Xi-, we find that the cascade yield in these collisions is in reasonable agreement with the data. Although the Xi- abundance does not reach chemical equilibrium unless the cross section for strangeness-exchange reactions is enhanced by six times, it exhibits the strongest enhancement with increasing centrality of collision and with increasing beam energy.

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arXiv2003

Entropy Production at RHIC

For central heavy ion collisions at the RHIC energy, the entropy per unit rapidity dS/dy at freeze-out is extracted with minimal model dependence from available experimental measurements of particle yields, spectra, and source sizes estimated from two-particle interferometry. The extracted entropy rapidity density is consistent with lattice gauge theory results for a thermalized quark-gluon plasma with an energy density estimated from transverse energy production at RHIC.

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arXiv2003

Finding the remnants of lost jets at RHIC

A fast jet propagating through partonic matter loses energy by medium-induced gluon radiation. We propose a method to detect the energy loss of jets and the final-state interaction of the radiated gluons and the jets with the partonic medium. By embedding jets and radiated energy into a parton cascade, we find that the fate of lost jets can be studied by analyzing the momentum distribution of soft hadrons relative to the jet axis.

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arXiv2003

Dynamics of Bound and Free Water in an Aqueous Micellar Solution : Analysis of the Lifetime and Vibrational Frequencies of Hydrogen Bonds at a Complex Interface

In order to understand the nature and dynamics of interfacial water molecules on the surface of complex systems, large scale molecular dynamics simulations of an aqueous micelle of cesium perfluorooctanoate surfactant molecules have been carried out. The lifetime and the intermolecular vibrational frequencies of the hydrogen bonds that the water molecules form with the polar headgroups of the surfactants, are calculated. Our earlier classification of the interfacial water molecules, based on structural and energetic considerations, into bound and free type is further validated by their dynamics. Lifetime correlation functions of the water-surfactant hydrogen bonds show the long lived nature of the bound water species. The water molecules that are singly hydrogen bonded to the surfactants have longer lifetime than those that form two such hydrogen bonds. The free water molecules that do not form any such hydrogen bonds behave similar to bulk water in their reorientational dynamics. A few water molecules that form two such hydrogen bonds are orientationally locked in for durations of the order of few hundreds of picoseconds. The intermolecular vibrational frequencies of these interfacial water molecules shows a significant blue shift in the librational band apart from a similar shift in the near neighbor bending modes, relative to water molecules in bulk. These blue shifts suggest an increase in rigidity in the structure around interfacial water molecules. This is in good agreement with recent incoherent, inelastic neutron scattering data on macromolecular solutions. The results of the present simulations should be relevant to the understanding of dynamics of water near any hydrophilic surface.

cond-mat.soft↗

arXiv2002

Evidence for Bound and Free Water Species in the Hydration Shell of an Aqueous Micelle

Our atomistic molecular dynamics simulations reveal the existence of bound and free water molecules in the hydration layer of an aqueous micelle. The bound water molecules can be either singly or doubly hydrogen bonded to the polar head group on the surface of the micelle. The ratio of bound to free water is found to be approximately equal to 9:1 at 300 K.

physics.bio-ph↗

arXiv2002

Partonic Effects on Pion Interferometry at the Relativistic Heavy Ion Collider

Using a multiphase transport (AMPT) model that includes both initial partonic and final hadronic interactions, we study the pion interferometry at the Relativistic Heavy Ion Collider. We find that the two-pion correlation function is sensitive to the magnitude of the parton scattering cross section, which controls the parton density at which the transition from the partonic to hadronic matter occurs. Also, the emission source of pions is non-Gaussian, leading to source radii that can be more than twice larger than the radius parameters extracted from a Gaussian fit to the correlation function.

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arXiv2002

Hydrogen Bond Dynamics Near A Micellar Surface: Origin of the Universal Slow Relaxation at Complex Aqueous Interfaces

The dynamics of hydrogen bonds among water molecules themselves and with the polar head groups (PHG) at a micellar surface have been investigated by long molecular dynamics simulations. The lifetime of the hydrogen bond between a PHG and a water molecule is found to be much longer than that between any two water molecules, and is likely to be a general feature of hydrophilic surfaces of organized assemblies. Analyses of individual water trajectories suggest that water molecules can remain bound to the micellar surface for more than a hundred picosecond. The activation energy for such a transition from the bound to a free state for the water molecules is estimated to be about 3.5kcal/mole.

cond-mat.soft↗