arXiv subjects

Bedangadas Mohanty

Publications and source records attributed to Bedangadas Mohanty.

At least 73 records · Page 4Linked to original sources

arXiv2013

Elliptic flow of ϕ-meson a sensitive probe for onset of de-confinement transition in high energy heavy-ion collisions

Elliptic flow (v_2) of ϕ-meson is shown to be a sensitive probe of the partonic collectivity using A Multi Phase Transport (AMPT) model. Within the ambit of the AMPT model with partonic interactions (string melting version), the ϕ-meson v_2 at midrapidity is found to have negligible contribution from hadronic interactions for Au+Au collisions at 200 GeV per nucleon. Changing the hadron cascade time in the model calculations does not change the ϕ-meson v_2, while it reduces the v_2 of proton at low transverse momentum (pT). These observations indicate that a substantial reduction in ϕ-mesons v_2 as a function of colliding beam energy would suggest the dominance of hadronic interactions over partonic interactions.

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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

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

Expectation of forward-backward rapidity correlations in $p+p$ collisions at the LHC energies

Forward-backward correlation strength ($b$) as a function of pesudorapidity intervals for experimental data from $p+\bar{p}$ non-singly diffractive collisions are compared to PYTHIA and PHOJET model calculations. The correlations are discussed as a function of rapidity window ($Δη$) symmetric about the central rapidity as well as rapidity window separated by a gap ($η_{gap}$) between forward and backward regions. While the correlations are observed to be independent of $Δη$, it is found to decrease with increase in $η_{gap}$. This reflects the role of short range correlations and justifies the use of $η_{gap}$ to obtain the accurate information about the physics of interest, the long range correlations. The experimental $b$ value shows a linear dependence on $\ln \sqrt{s}$ with the maximum value of unity being reached at $\sqrt{s}$ = 16 TeV, beyond the top LHC energy. However calculations from the PYTHIA and PHOJET models indicate a deviation from linear dependence on $\ln \sqrt{s}$ and saturation in the $b$ values being reached beyond $\sqrt{s}$ = 1.8 TeV. Such a saturation in correlation values could have interesting physical interpretations related to clan structures in particle production. Strong forward-backward correlations are associated with cluster production in the collisions. The average number of charged particles to which the clusters fragments, called the cluster size, are found to also increase linearly with $\ln \sqrt{s}$ for both data and the models studied. The rate of increase in cluster size vs. $\ln \sqrt{s}$ from models studied are larger compared to those from the data and higher for PHOJET compared to PYTHIA. Our study indicates that the forward-backward measurements will provide a clear distinguishing observable for the models studied at LHC energies.

hep-ex↗

arXiv2012

Multiplicity, average transverse momentum and azimuthal anisotropy in U+U collisions at $\sqrt{s_{NN}}$ = 200 GeV using AMPT model

Using a multi-phase transport (AMPT) model that includes the implementation of deformed Uranium nuclei, we have studied the centrality dependence of the charged particle multiplicity, , eccentricity (e2), triangularity (e3), their fluctuations, elliptic flow (v2) and triangular flow (v3) for different configurations of U+U collisions at midrapidity for \sqrt(s_NN)=200 GeV. The results are compared to the corresponding observations from Au+Au collisions. We find that for the U+U collisions the dNch/dηat midrapidity is enhanced by about 15-40% depending on the collision and model configuration chosen, compared to Au+Au collisions. The tip-to-tip collisions leads to the largest values of Nch,transverse energy (ET) and . The and its fluctuation shows a rich centrality dependence, whereas not much variations are observed for and its fluctuations. The U+U side-on-side collision configuration provides maximum values of and minimum values of eccentricity fluctuations, whereas for peripheral collisions and mid-central collisions minimum values of and maximum value of eccentricity fluctuations are observed for body-to-body configuration and the tip-to-tip configuration has minimum value of and maximum value of eccentricity fluctuations for central collisions. The calculated v2 closely correlates with the eccentricity in the model. It is smallest for the body-to-body configuration in peripheral and mid-central collisions while it is minimum for tip-to-tip configuration in central collisions. For peripheral collisions the v2 in U+U can be about 40% larger than in Au+Au whereas for central collisions it can be a factor 2 higher depending on the collision configuration. It is also observed that the v3(pT) is higher for tip-to-tip and body-to-body configurations compared to other systems for the collision centrality studied.

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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.

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arXiv2012

Energy dependence of inclusive photon elliptic flow in heavy-ion collision models

We present a comparison of inclusive photon elliptic flow parameter (v_{2}) measured at RHIC and SPS high energy heavy-ion collision experiments to calculations done using the AMPT and UrQMD models. The new results discussed includes the comparison of the model calculations of photon v_{2} to corresponding measurements at the forward rapidities. We observe that the AMPT model which includes partonic interactions and quark coalescence as a mechanism of hadronization is in good agreement with the measurements even at forward rapidities (2.3 < η< 3.9) at RHIC as was previously observed for measurements at midrapidity. At the top SPS energy the contribution from partonic effects are smaller than that at RHIC energy, based on the comparison of the measured photon v_{2} with those from the AMPT default and UrQMD model calculations. We find that if the measurements in RHIC beam energy scan (BES) and LHC energies would require an energy dependent partonic cross section in the AMPT models, then the observed longitudinal scaling of v_{2} at top RHIC energies (19.6-200 GeV) will be violated. We also discuss the relation between the inclusive photon v_{2} and those of their parent π^{0}'s for the beam energies of 7.7 GeV to 2.76 TeV. The model results show that the transverse momentum (p_{\mathrm T}) integrated v_{2} of π^{0} is larger by about 44% relative to those of the inclusive photons. Finally we present the expectations of inclusive photon v_{2} for the RHIC beam energy scan (BES) program and LHC from the transport models, so that they can be compared to corresponding measurements using the data already collected at RHIC and LHC.

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arXiv2011

Elliptic and Triangular flow in asymmetric heavy-ion collisions

We present a study of the elliptic (v2) and triangular (v3) flow and their corresponding eccentricity fluctuations for asymmetric (Au+Ag, Au+Cu and Au+Si) collisions at \sqrt_NN = 200 GeV. These are compared to the corresponding results from symmetric (Au+Au and Cu+Cu) collisions at the same energy. The study which is carried out using a multi-phase transport (AMPT) model shows that triangularity (ε_3), fluctuations in triangularity and v3 do not show much variation for the different colliding ion sizes studied. However the eccentricity (ε_2), fluctuations in eccentricity and v2 shows a strong dependence on colliding ion size for a given number of participating nucleons. Our study thus indicates that asymmetric heavy-ion collisions could be used to constrain models dealing with flow fluctuations in heavy-ion collisions.

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arXiv2011

Evolution of collectivity as a signal of quark gluon plasma formation in heavy ion collisions

A measurement for studying the mass dependence of the dilepton interferometry in relativistic heavy-ion collision experiments as a tool to characterize the quark-gluon phase is proposed. In calculations involving dileptons, we show that the mass dependence of radii extracted from the virtual photon (dilepton) interferometry provide access to the development of collective flow with time. It is argued that the non-monotonic variation of HBT radii with invariant mass of the lepton pairs signals the formation of quark gluon plasma in heavy ion collisions. Our proposal of experimentally measuring the ratio, $R_{\mathrm out}/R_{\mathrm side}$ for dileptons can be used to estimate the average life times of the partonic as well as the hadronic phases.

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arXiv2011

Statistical Origin of Constituent-Quark Scaling in the QGP hadronization

Nonextensive statistics in a Blast-Wave model (TBW) is implemented to describe the identified hadron production in relativistic p+p and nucleus-nucleus collisions. Incorporating the core and corona components within the TBW formalism allows us to describe simultaneously some of the major observations in hadronic observables at the Relativistic Heavy-Ion Collider (RHIC): the Number of Constituent Quark Scaling (NCQ), the large radial and elliptic flow, the effect of gluon saturation and the suppression of hadron production at high transverse momentum (pT) due to jet quenching. In this formalism, the NCQ scaling at RHIC appears as a consequence of non-equilibrium process. Our study also provides concise reference distributions with a least chi2 fit of the available experimental data for future experiments and models.

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arXiv2011

STAR experiment results from the beam energy scan program at RHIC

We present the first results using the STAR detector from the Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider (RHIC). In this program, Au ion collisions at center of mass energies (\sqrt{s_{NN}}) of 7.7, 11.5 and 39 GeV allowed RHIC to extend the study of the QCD phase diagram from baryonic chemical potential values of 20 MeV to about 400 MeV. For the high net-baryon density matter at midrapidity, formed in these collisions, we report several interesting measurements. These include the observation of difference between anti-particle and particle elliptic flow, disappearance of the difference in dynamical azimuthal correlations with respect to event plane between same and opposite signed charged particles, change in slope of eccentricity at freeze-out and directed flow of protons as a function of \sqrt{s_{NN}} and the deviation of higher order fluctuations from hadron resonance gas and Poissonian expectations. Possible interpretations of these observations are also discussed.

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arXiv2011

Search for the QCD Critical Point: Higher Moments of Net-proton Multiplicity Distributions

Higher moments of event-by-event net-proton multiplicity distributions have been applied to search for the QCD critical point. Model results are used to provide a baseline for this search. The measured moment products, $κσ^2$ and $Sσ$ of net-proton distributions, which are directly connected to the thermodynamical baryon number susceptibility ratio in Lattice QCD and Hadron Resonance Gas (HRG) model, are compared to the transport and thermal model results. We argue that a non-monotonic dependence of $κσ^2$ and $S σ$ as a function of beam energy can be used to search for the QCD critical point.

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arXiv2011

Scale for the Phase Diagram of Quantum Chromodynamics

Matter described by Quantum Chromodynamics (QCD), the theory of strong interactions, may undergo phase transitions when its temperature and the chemical potentials are varied. QCD at finite temperature is studied in the laboratory by colliding heavy-ions at varying beam energies. We present a test of QCD in the non-perturbative domain through a comparison of thermodynamic fluctuations predicted in lattice computations with the experimental data of baryon number distributions in high-energy heavy-ion collisions. This study provides evidence for thermalization in these collisions, and allows us to find the crossover temperature between normal nuclear matter and a deconfined phase called the quark gluon plasma. This value allows us to set a scale for the phase diagram of QCD.

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arXiv2011

Longitudinal scaling of observables in heavy-ion collision models

Longitudinal scaling of pseudorapidity distribution of charged particles ($dN_{\mathrm {ch}}/dη$) is observed when presented as a function of pseudorapidity ($η$) shifted by the beam rapidity ($η$ - $y_{\mathrm {beam}}$) for a wide range of collision systems ($e^{+}+e^{-}$, p+p, $d$+A and A+A) and beam energies. Such a scaling is also observed for the elliptic flow ($v_{2}$) of charged hadrons in A+A collisions. This is a striking observation, as $v_{2}$ is expected to be sensitive to the initial conditions, the expansion dynamics and the degrees of freedom of the system, all of which potentially varies with collision system and colliding energies. We present a study of the longitudinal scalings of $dN_{\mathrm {ch}}/dη$, average transverse momentum ($< p_{\mathrm T}>$) and $v_{2}$ using transport models UrQMD and AMPT for Au+Au collisions at center of mass energies ($\sqrt{s_{\mathrm {NN}}}$) of 19.6, 62.4, 200 GeV and Pb+Pb collisions at 2760 GeV. Only the AMPT models which includes partonic effects and quark coalescence as a mechanism of hadronization, shows longitudinal scaling for $dN_{\mathrm {ch}}/dη$, $< p_{\mathrm T}>$ and $v_{2}$. Whereas the UrQMD and AMPT default versions show longitudinal scaling only for $dN_{\mathrm {ch}}/dη$ and $ $. We also discuss the possibility of longitudinal scaling of $v_{2}$ within two extreme scenarios of models with hydrodynamic and collisionless limits. We find the longitudinal scaling of bulk observables to be an important test for the underlying physics mechanism in models of particle production.

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arXiv2011

Exploring the QCD landscape with high-energy nuclear collisions

Quantum chromodynamics (QCD) phase diagram is usually plotted as temperature (T) versus the chemical potential associated with the conserved baryon number (μ_{B}). Two fundamental properties of QCD, related to confinement and chiral symmetry, allows for two corresponding phase transitions when T and μ_{B} are varied. Theoretically the phase diagram is explored through non-perturbative QCD calculations on lattice. The energy scale for the phase diagram (Λ_{QCD} ~ 200 MeV) is such that it can be explored experimentally by colliding nuclei at varying beam energies in the laboratory. In this paper we review some aspects of the QCD phase structure as explored through the experimental studies using high energy nuclear collisions. Specifically, we discuss three observations related to the formation of a strongly coupled plasma of quarks and gluons in the collisions, experimental search for the QCD critical point on the phase diagram and freeze-out properties of the hadronic phase.

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arXiv2011

Heavy lepton pair production in nucleus-nucleus collisions at LHC energy - a case study

We present a study of $τ^{+}τ^{-}$ lepton pair production in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 5.5 TeV. The larger $τ^{\pm}$ mass ($\sim$ 1.77 GeV) compared to $e^{\pm}$ and $μ^{\pm}$ leads to considerably small hadronic contribution to the $τ^{+}τ^{-}$ pair invariant mass ($M$) distribution relative to the production from thermal partonic sources. The quark-anti-quark annihilation processes via intermediary virtual photon, Z and Higgs bosons have been considered for the production of $ τ^{+}τ^{-}$. We observe that the contribution from Drell-Yan process dominates over thermal yield for $τ^{+}τ^{-}$ pair mass from 4 to 20 GeV at the LHC energy. We also present the ratio of $τ$ lepton pair yields for nucleus-nucleus collisions relative to yields from p+p collisions scaled by number of binary collisions at LHC energies as a function $τ$ pair mass. The ratio is found to be significantly above unity for the mass range 4 to 6 GeV. This indicates the possibility of detecting $τ^{+}τ^{-}$ pair from quark gluon plasma (QGP) in the mass window $4\leq M$(GeV)$\leq 6$.

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arXiv2011

Characterizing quark gluon plasma by dilepton interferometry

The Hanbury-Brown-Twiss (HBT) radii have been calculated from the two particle correlation functions with virtual photons produced in the collisions of two nuclei at ultra-relativistic energies. We show that the variation of the HBT radii with the invariant mass of the virtual photon can be used to characterize and distinguish the hadronic as well as the partonic phase that might have produced initially in the collisions. It has been illustrated that the non-monotonic variation of the HBT radii with invariant mass provides an access to the development of collective flow in the system.

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arXiv2010

Energy dependence of elliptic flow from heavy-ion collision models

We have compared the experimental data on charged particle elliptic flow parameter (v2) in Au+Au collisions at midrapidity for \surd sNN = 9.2, 19.6, 62.4 and 200 GeV with results from various models in heavy-ion collisions like UrQMD, AMPT, and HIJING. We observe that the average from the transport model UrQMD agrees well with the measurements at \surd sNN = 9.2 GeV but increasingly falls short of the experimental values as the beam energy increases. The difference in being of the order of 60% at \surd sNN = 200 GeV. The results from HIJING is consistent with zero, while those from AMPT with default settings, a model based on HIJING with additional initial and final state rescattering effects included, gives a value of about 4% for all the beam energies studied. This is in contrast to increase in with beam energy for the experimental data. A different version of the AMPT model, which includes partonic effects and quark coalescence as a mechanism of hadronization, gives higher values of among the models studied and is in agreement with the measured values at \surd sNN = 200 GeV. These studies show that the experimental < v2 > has substantial contribution from partonic interactions at \surd sNN = 200 GeV whose magnitude reduces with decrease in beam energy. We also compare the available data on the transverse momentum and pseudorapidity dependence of v2 to those from the above models.

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