arXiv subjects

C. P. Singh

Publications and source records attributed to C. P. Singh.

At least 37 records · Page 2Linked to original sources

arXiv2013

The effect of flow on Hadronic Spectra in an Excluded-Volume Model

Recently, we proposed a thermodynamically consistent excluded-volume model for the HG fireball and we noticed that our model gives a suitable description for various properties of multiparticle production and their ratios in the entire range of temperatures and baryon densities. The aim in this paper is to obtain the variations of freeze-out volume in a slice of unit rapidity i.e. $dV/dy$ as well as total volume of the fireball with respect to center-of-mass energy $(\sqrt{s_{NN}})$ and confront our model calculations with the corresponding thermal freeze-out volume obtained from the Hanbury-Brown-Twiss (HBT) pion interferometry method. We also test the validity of our model in extracting the total multiplicities as well as the central rapidity densities of various hadrons and comparing them with the recent results. We further calculate the rapidity as well as transverse momentum spectra of various particles produced in different heavy-ion collider experiments in order to examine the role of flow by matching our predictions suitably with the available experimental results. Finally, we extend our analysis for the production of light nuclei, hypernuclei and their antinuclei over a broad energy range from Alternating Gradient Synchrotron (AGS) to Large Hadron Collider (LHC) energies.

hep-ph↗

arXiv2012

Some Exact Solutions of Magnetized viscous model in String Cosmology

In this paper, we study anisotropic Bianchi-V universe with magnetic field and bulk viscous fluid in string cosmology. Exact solutions of the field equations are obtained by using the equation of state for a cloud of strings, and a relationship between bulk viscous coefficient and scalar expansion. The bulk viscous coefficient is assumed to be inversely proportional to the expansion scalar. It is interesting to examine the effects of magnetized bulk viscous string model in early and late stages of the evolution of the universe. This paper presents the different string models like geometrical (Nambu string), Takabayashi (p-string) and Reddy string models by taking certain physical conditions. We discuss the nature of the classical potential for viscous fluid with and without magnetic field. We find that the introduction of bulk viscosity with and without magnetic field results in rapid change in density parameters and in the classical potential. The presence of bulk viscosity prevents the universe to be empty in its future evolution. The other physical and geometrical aspects of each string model are discussed in detail.

gr-qc↗

arXiv2012

A Hybrid Model for QCD Deconfining Phase Boundary

Intensive search for a proper and realistic equations of state (EOS) is still continued for studying the phase diagram existing between quark gluon plasma (QGP) and hadron gas (HG) phases. Lattice calculations provide such EOS for the strongly interacting matter at finite temperature ($T$) and vanishing baryon chemical potential ($μ_{B}$). These calculations are of limited use at finite $μ_{B}$ due to the appearance of notorious sign problem. In the recent past, we had constructed a hybrid model description for the QGP as well as HG phases where we make use of a new excluded-volume model for HG and a thermodynamically-consistent quasiparticle model for the QGP phase and used them further to get QCD phase boundary and a critical point. Since then many lattice calculations have appeared showing various thermal and transport properties of QCD matter at finite $T$ and $μ_{B}=0$. We test our hybrid model by reproducing the entire data for strongly interacting matter and predict our results at finite $μ_{B}$ so that they can be tested in future. Finally we demonstrate the utility of the model in fixing the precise location, the order of the phase transition and the nature of CP existing on the QCD phase diagram. We thus emphasize the suitability of the hybrid model as formulated here in providing a realistic EOS for the strongly interacting matter.

hep-ph↗

arXiv2012

Viscous FRW model with particle creation in the early universe

We discuss the dynamical effects of bulk viscosity and particle creation on the early evolution of the Friedmann -Robertson -Walker model in the framework of open thermodynamical systems. We consider bulk viscosity and Particle creation as separate irreversible processes. Exact solutions of the Einstein field equations are obtained by using the "gamma-law" equation of state $p=(γ-1)ρ$, where the adiabatic parameter $γ$ varies with scale factor of the metric. We consider the cosmological model to study the early phases of the evolution of the universe as it goes from an inflationary phase to a radiation -dominated era in the presence of bulk viscosity and particle creation. Analytical solutions are obtained for particle number density and entropy for all models. It is seen that, by choosing appropriate functions for particle creation rate and bulk viscous coefficient, the models exhibit singular and non-singular beginnings.

gr-qc↗

arXiv2012

Prediction of Charged Hadron Multiplicity at LHC and CBM Experiments

A systematic study of charged hadron multiplicities ($n_{ch}$) at various collision energies is very much important in understanding the basic production mechanism of the hadrons in nucleus-nucleus collision experiments. Furthermore, the variations of $n_{ch}$ in nucleus-nucleus collisions with respect to the colliding energy and mass number can provide a potential probe for the formation of quark gluon plasma (QGP) in the laboratory. In this paper, we propose a phenomenological model based on the constituent quark-quark interactions to calculate the average multiplicity ($n_{ch}$) and pseudorapidity density at mid-rapidity ($(dn_{ch}/dη)_{η=0}$) of charged hadrons at various center-of-mass energies $(\sqrt{s_{NN}})$ for nucleus-nucleus ($A-A$) collisions. We first propose a new parametrization for $n_{ch}^{pp}$ and $(dn_{ch}/dη)^{pp}_{η=0}$ in $p-p$ interactions based on some initial inputs which fit the experimental data very well. We further extend this parametrization by using simple phenomenological assumptions regarding mean number of participating quarks and mean number of collisions to obtain the $n_{ch}$ and $(dn_{ch}/dη)_{η=0}$ for $A-A$ collisions and show their dependencies on the mass number of colliding nuclei as well as on $\sqrt{s_{NN}}$. We also compare the results obtained from our model with the results obtained from the modified Glauber model in order to demonstrate the difference between the two formalisms. Finally, we compare the charged hadron multiplicity and pseudorapidity density at mid-rapidity for $A-A$ collisions obtained from our model with the available experimetal data from various heavy-ion collision experiments and give our predictions for $A-A$ collisions at the Large Hadron Collider (LHC) and at Compressed Baryonic Matter (CBM) experiments.

hep-ph↗

arXiv2012

Critical Point on the QCD Deconfining Phase Boundary

Ambiguities regarding the physics and the existence of the critical point (CP) on the QCD phase boundary still exist and the mist regarding the conjectured QCD phase boundary has not yet cleared. In this paper we extend our earlier study where we constructed a deconfining phase boundary using Gibbs' equilibrium conditions after using a quasiparticle equation of state (EOS) for quark gluon plasma (QGP) and an excluded volume EOS for the hadron gas (HG) and find the presence of a critical point on this phase boundary where the first order phase transition terminates. In this paper, we plot the difference in the normalized entropy density ($s/T^{3}$) between HG and QGP phases along the deconfining phase boundary and find that it vanishes at CP. Further we have shown the variation of the square of speed of sound ($c_{s}^{2}$) for the HG and QGP separately and find that the difference ($Δc_{s}^{2}$) between them along the deconfining phase boundary again vanishes at the CP of the boundary. We also plot the variation of shear viscosity to entropy density ratio ($η/s$) in HG as well as in QGP phases separately with respect to temperature at different baryonic chemical potential ($μ_{B}$). The presence of a cusp like structure in $η/s$ again confirms the existence of CP on the deconfining phase boundary as predicted by some authors. These studies thus firmly indicate the precise location of CP as a second order phase transition point.

hep-ph↗

arXiv2011

Description of Hot and Dense Hadron Gas Properties in a New Excluded-Volume model

A new equation of state for a hot and dense hadron gas (HG) is obtained where the finite hard-core size of baryons has been incorporated in a thermodynamically consistent formulation of excluded volume correction. Our model differs from other existing approaches on the following points. We assign a hard-core volume only to each baryon and mesons though possess a small volume but they can fuse and interpenetrate into one another. Use of the full quantum statistics is made in obtaining the grand canonical partition function where excluded-volume correction has been incorporated by explicitly integrating over volume. We thus find that the new model works even for the cases of extreme temperatures and/or densities where most of other approaches fail. The model does not violate causality even at extreme densities. The temperature and density dependence of various thermodynamical quantities, e.g. pressure, baryon density, entropy and energy density compare well with the results of other microscopic HG models. After suitable parametrization of the centre-of-mass energy in terms of temperature and baryon chemical potential, we explore some new freeze-out criteria which exhibit full independence of the collision energy and of the structures of the colliding nuclei. We further demonstrate the suitability of our model in explaining various experimental results of the multiplicity-ratios of various particles and their antiparticles. Finally, we use our excluded-volume model to obtain the transport behaviour of the hot and/or dense HG such as shear viscosity to entropy ratio, speed of sound etc. and compare the results with earlier calculations.

hep-ph↗

arXiv2011

Baryon-Size Dependent Location of QCD Critical Point

The physics regarding the existence of the critical end point (CEP) on the QCD phase boundary still remains unclear and its precise location is quite uncertain. In this paper we propose that the hard-core size of the baryons used in the description of the hot and dense hadron gas (HG) plays a decisive role in the existence of CEP. Here we construct a deconfining phase transition using Gibbs' equilibrium conditions after using a quasiparticle equation of state (EOS) for QCD plasma and excluded-volume EOS for the HG. We find that the first order transition results only when we assign a hard-core size to each baryon in the description of HG and the phase boundary thus obtained terminates at CEP beyond which a cross-over region occurs. The mean field approach for the HG lends support to this finding where unless we include an excluded-volume effect in the approach, CEP does not materialize on the QCD boundary. This investigation provides an intuitive reasoning regarding the origin of CEP and the cross-over transition on the QCD phase boundary.

hep-ph↗

arXiv2011

On Locating the Critical End Point in QCD Phase Diagram

We use the available two different self-consistent formulations of quasiparticle models and extend their applications for the description of quark gluon plasma (QGP) at non-vanishing baryon chemical potentials. The thermodynamical quantities calculated from these models are compared with the values obtained from lattice simulations and a good agreement between theoretical calculations and lattice QCD data suggests that the values of the parameters used in the paper are consistent. A new equation of state (EOS) for a gas of extended baryons and pointlike mesons is presented here which incorporates the repulsive hard-core interactions arising due to geometrical size of baryons. A first order deconfining phase transition is constructed using Gibb's equilibrium criteria between the hadron gas EOS and quasiparticle model EOS for the weakly interacting quark matter. This leads to an interesting finding that the phase transition line ends at a critical end point beyond which a crossover region exists in the phase diagram.

hep-ph↗

arXiv2010

QCD Critical Point in a Quasiparticle Model

Recent theoretical investigations have unveiled a rich structure in the quantum chromodynamics (QCD) phase diagram which consists of quark gluon plasma (QGP) and the hadronic phases but also supports the existence of a cross-over transition ending at a critical end point (CEP). We find a too large variation in determination of the coordinates of the CEP in the temperature (T), baryon chemical potential ($μ_{B}$) plane and, therefore, its identification in the current heavy-ion experiments becomes debatable. Here we use an equation of state (EOS) for a deconfined QGP using a thermodynamically consistent quasiparticle model involving quarks and gluons having thermal masses. We further use a thermodynamically consistent excluded volume model for the hadron gas (HG) which was recently proposed by us. Using these equations of state, a first order deconfining phase transition is constructed using Gibbs' criteria. This leads to an interesting finding that the phase transition line ends at a critical point (CEP) beyond which a cross-over region exists. Using our thermal HG model, we obtain a chemical freeze out curve and we find that the CEP lies in close proximity to this curve as proposed by some authors. The coordinates of CEP are found to lie within the reach of RHIC experiment.

hep-ph↗

arXiv2009

QCD Phase Boundary and Critical Point in a Bag Model Calculation

Location of critical point and mapping the QCD phase boundary still exists as one of the most interesting and studied problems of heavy-ion physics. A new equation of state (EOS) for a gas of extended baryons and pointlike mesons is presented here which accounts for the repulsive hard-core interactions arising due to the geometrical size of the baryons. A first order deconfining phase transition is obtained using Gibbs' equilibrium criteria and a bag model EOS for the weakly interacting quark matter. It is interesting to find that the phase transition line ends at a critical point beyond which a cross-over region exists between hot-dense meson gas and quark-antiquark gluon matter. Our curve resembles in shape closely with the predictions of the available lattice gauge calculations and also reproduces the conjectured phase boundary.

hep-ph↗

arXiv2008

J/ψsuppression at forward rapidity as a potential probe for QGP formation in colour screening scenario

In order to study the properties of $J/ψ$ (1S) in the deconfining medium, we extend our previous formalism [Phys. Lett. B {\bf 656}, 45 (2007)] on $J/ψ$ suppression at mid-rapidity using the colour screening framework. Our formalism is more general as the complete rapidity, transverse momentum and centrality dependence including $J/ψ$ suppression at forward as well as mid-rapidity can be computed directly from it. Careful attention is paid to the role of the medium's proper time in determining the locus of the screening region where $J/ψ$ gets suppressed. Other important ingredients in the calculation are bag model equation of state for QGP, the longitudinal expansion of the QGP fluid obeying Bjorken's boost invariant scaling law and non-sequential/sequential melting of $χ_c$ (1P) as well as $ψ^{'}$ (2S) higher resonances. Upon comparison with the recent data of PHENIX collaboration on $J/ψ$ suppression at forward and mid-rapidity regions, we find that our model shows a reasonable agreement with the data without incorporating any sequential decay mechanism of higher charmonia states. Furthermore, we observe a larger suppression at forward rapidity in our model which is again well supported by the PHENIX data and also gives a hint that a scenario based on directly produced $J/ψ$'s is preferable.

hep-ph↗

arXiv2008

Particle multiplicities and particle ratios in excluded volume model

One of the most surprising results is to find that a consistent description of all the experimental results on particle multiplicities and particle ratios obtained from the lowest AGS to the highest RHIC energies is possible within the framework of a thermal statistical model. We propose here a thermodynamically consistent excluded-volume model involving an interacting multi-component hadron gas. We find that the energy dependence of the total multiplicities of strange and non-strange hadrons obtained in this model agrees closely with the experimental results. It indicates that the freeze out volume of the fireball is uniformly the same for all the particles. We have also compared the variation of the particle ratios such as $ /<π^{+}>, < K^{-}>/<π^{-}>, K^{-}/K^{+}, \bar{p}/p, \barΛ/Λ, \barΞ/Ξ, \barΩ/Ω, < Λ>/<π>, <Ξ^{-}>/<π>, <Ω+\barΩ>/<π>$ and $<Φ>/<π>$ with respect to the center-of-mass energy as predicted by our model with the recent experimental data.

hep-ph↗

arXiv2007

On the chemical freeze-out criteria in a hot and dense fireball

Intensive investigations of freeze-out criteria in a hot and dense fireball provide important information regarding particle emission from the fireball. A systematic comparison of these proposals is presented here in the framework of a thermodynamically consistent excluded volume model which has been found to describe the properties of hadron gas (HG) quite well. We find that the impact of excluded volume correction is considerably large and the average energy per hadron is 0.9 GeV, $n_b+n_{\bar b}$ stays nearly constant at 0.12/fm$^3$ and the normalized entropy density $s/T^3\approx 6$ in this model. Moreover, these values are independent of the beam or center-of-mass energy and also of the target and beam nuclei. In ideal HG model these quantities show substantial energy dependence. Further we have compared the predictions of various excluded volume models in the precise determination of these criteria and we find that the thermodynamically consistent excluded volume models give the best results. In addition, we find another important criterion that entropy per hadron has a constant value at 6 in our model. We hope that these findings will throw considerable light on the expansion dynamics and the bulk thermodynamic properties of the fireball before chemical freeze-out.

hep-ph↗

arXiv2007

Freeze-out volume of hot dense fireball

A thermodynamically consistent excluded volume model is proposed to account for the particle multiplicities obtained from lowest SIS energies to the highest RHIC energies. The chemical freeze-out volumes lying in a slice of one unit of rapidity for pions and kaons are separately inferred from this analysis and the results are compared with the corresponding thermal freeze-out volumes obtained from the Hanbury-Brown Twiss (HBT) pion interferometry. Furthermore, we extract the variations of freeze-out number densities for pions and nucleons with the center-of-mass energy in our model and compare them with the HBT data.

hep-ph↗

arXiv2007

$J/ψ$ suppression in Au+Au collisions at RHIC : colour screening scenario in the bag model at variable participant numbers

We have modified the colour screening theory of Chu and Matsui by properly incorporating bag model equation of state for quark gluon plasma (QGP). We have also chosen the pressure parametrization rather than parametrizing energy density in the transverse plane. We assume that the QGP dense medium is expanding in the longitudinal direction obeying Bjorken boost invariant scaling law. Sequential melting of $χ_c$, $ψ^{'}$ and $J/ψ$ is also considered in this scenario. We have applied above formulation to the recent PHENIX experimental data of $J/ψ$ suppression in Au + Au collisions at RHIC. We find that the model gives a good description of data at mid-rapidity in terms of survival probability versus number of participants without any necessity of implementing (3+1)-dimensional expansion of the deconfined medium.

hep-ph↗

arXiv2007

Effect of geometrical size of the particles in a hot and dense hadron gas

Incorporation of the finite size of baryons into the equation of state (EOS) of a hot and dense hadron gas (HG) in a thermodynamically consistent manner has been a much studied problem. We first review its current status. Various models have been proposed in order to account for the repulsive force generated by the hard-core geometrical size of the baryons resulting in an excluded volume effect in the EOS. We examine the criterion of the thermodynamical consistency of these models and summarize their shortcomings. In order to remove the shortcomings, we propose a new model which incorporates the excluded volume effect in a thermodynamically consistent manner. We find that the new model works even for the cases of extremely large temperatures and densities where most of other approaches fail. Furthermore, the new expressions for thermodynamical variables resemble in form with those obtained from thermodynamically inconsistent models and thus a useful correction factor has been suggested here which converts inconsistent expressions into thermodynamically consistent ones. Finally we compare the predictions of new model with those obtained from various old models.

hep-ph↗

arXiv1999

Baryon Number Penetrability as a Measure of Isothermal Baryon Number Fluctuations in the Early Universe

We have examined the efficiency of baryon-number transport mechanism across the phase boundary in a cosmological quark-hadron phase transition through the proper estimate of baryon-number penetrability $<Σ_h>$. For this purpose we have derived first the double-pair creation probability $P_b$ in terms of single-pair creation probabilty per unit time and unit volume $κ_m$ and then obtained an analytical expression for $<Σ_h >$. Our calculation is free from the uncertainty of the value of double-pair creation probability per unit time and unit volume $κ_b$ which was used as a free parameter in earlier calculations. Finally the variations of double-pair creation probability $P_b$ as well as $<Σ_h>$ with temperature are shown and compared with other known results.

hep-ph↗