arXiv subjects

Sourav Sarkar

Publications and source records attributed to Sourav Sarkar.

At least 109 records · Page 6Linked to original sources

arXiv2014

Flat space cosmologies in two dimensions - Phase transitions and asymptotic mass-domination

We study flat space cosmologies in two dimensions by taking the flat space limit of the Achucarro-Ortiz model. We unravel a phase transition between hot flat space and flat space cosmologies, and derive a new dilaton-dependent counterterm required for the consistency of the Euclidean partition function. Our results generalize to asymptotically mass-dominated 2-dimensional dilaton gravity models, whose thermodynamical properties we discuss. The novel case of asymptotic mass-domination is neither covered by the comprehensive discussion of hep-th/0703230 nor by the more recent generalization to dilaton gravity with confining U(1) charges in 1406.7007.

hep-th↗

arXiv2014

Medium effects on the thermal conductivity of a hot pion gas

We investigate the effect of the medium on the thermal conductivity of a pion gas out of chemical equilibrium by solving the relativistic transport equation in the Chapman-Enskog and relaxation time approximations. Using an effective model for the pi-pi cross-section involving rho and sigma meson exchange, medium effects are incorporated through thermal one-loop self-energies. The temperature dependence of the thermal conductivity is observed to be significantly affected.

nucl-th↗

arXiv2014

Shear viscosity of pion gas due to $ρππ$ and $σππ$ interactions

We have evaluated the shear viscosity of pion gas taking into account its scattering with the low mass resonances, $σ$ and $ρ$ during propagation in the medium. The thermal width (or collisional rate) of the pions is calculated from $πσ$ and $πρ$ loop diagrams using effective interactions in the real time formulation of finite temperature field theory. A very small value of shear viscosity by entropy density ratio ($η/s$), close to the KSS bound, is obtained which approximately matches the range of values of $η/s$ used by Niemi et al. \cite{Niemi} in order to fit the RHIC data of elliptic flow.

nucl-th↗

arXiv2014

Positron Annihilation Study of Zn O Nanoparticles Grown under Folic Acid Template

Positron lifetime spectroscopy (PAL), Doppler broadening (DB) as well as coincidence Doppler broadening (CDB) spectroscopy of a new variety of Folic acid (FA) capped zinc oxide nano-particle samples has been performed at room temperature. The results show interesting patterns of observation, hither-to unobserved in ZnO wurtzite crystalline samples, such as predominance of positronium formation, as reflected in the (PAL) analysis, phase transition in the nano crystalline samples at ~0.8-1.0 % FA concentration as depicted from DB results. Also, the chemical environment of the samples has been analysed from the ratio curves of CDB studies. Beside these, other independent results from X-ray diffraction (XRD) data and Debye- Scherrer method, Transmission electron microscopic (TEM) observations and as well as Fourier transformed infrared spectroscopic (FT-IR) analysis are reported for comparison.

cond-mat.mtrl-sci↗

arXiv2013

Effect of in-medium spectral density of $D$ and $D^*$ mesons on the $J/ψ$ dissociation in hadronic matter

The one-loop self-energy of the $D$ and $D^*$ mesons in a hot hadronic medium is evaluated using the real time formalism of thermal field theory. The interaction of the heavy open-charm mesons with the thermalized constituents $(π,K,η)$ of the hadronic matter is treated in the covariant formalism of heavy meson chiral perturbation theory. The imaginary parts are extracted from the discontinuities of the self-energy function across the unitary and the Landau cuts. The non-zero contribution from the latter to the spectral density of $D$ and $D^*$ mesons opens a number of subthreshold decay channels of the $J/ψ$ leading to a significant increase in the dissociation width in hadronic matter.

nucl-th↗

arXiv2013

Viscous coefficients of a hot pion gas

The steps essentially involved in the evaluation of transport coefficients in linear response theory using Kubo formulas are to relate the defining {\em retarded} correlation function to the corresponding {\em time-ordered} one and to evaluate the latter in the conventional perturbation expansion. Here we evaluate the viscosities of a pion gas carrying out both the steps in the {\em real time} formulation. We also obtain the viscous coefficients by solving the relativistic transport equation in the Chapman-Enskog approximation to leading order. An in-medium $ππ$ cross-section in used in which spectral modifications are introduced in the propagator of the exchanged $ρ$.

nucl-th↗

arXiv2013

Analysis of $ω$ self-energy at finite temperature and density in the real-time formalism

Using the real time formalism of field theory at finite temperature and density we have evaluated the in-medium $ω$ self-energy from baryon and meson loops. We have analyzed in detail the discontinuities across the branch cuts of the self-energy function and obtained the imaginary part from the non-vanishing contributions in the cut regions. An extensive set of resonances have been considered in the baryon loops. Adding the meson loop contribution we obtain the full modified spectral function of $ω$ in a thermal gas of mesons, baryons and anti-baryons in equilibrium for several values of temperature and baryon chemical potential.

nucl-th↗

arXiv2013

Heavy flavor suppression: role of hadronic matter

The role of hadronic matter in the suppression of open heavy flavored mesons has been studied. The heavy-quarks (HQs) suppression factors have been calculated and contrasted with the experimental data obtained from nuclear collisions at Relativistic Heavy Ion Collider (RHIC) and Large Hadronic Collider (LHC) experiments. It is found that the suppression in the hadronic phase at RHIC energy is around $20%-25%$ whereas at the LHC it is around $10%-12%$ for the D meson. In case of B meson the hadronic suppression is around $10%-12%$ and $5%-6%$ at RHIC and LHC energies respectively. Present study suggests that the suppression of heavy flavor in the hadronic phase is significant at RHIC. However, the effect of hadronic suppression at LHC is marginal, this makes the characterization of QGP at LHC less complicated.

nucl-th↗

arXiv2013

Medium effects on the viscosities of a pion gas

The bulk and shear viscosities of a pion gas is obtained by solving the relativistic transport equation in the Chapman-Enskog approximation. In-medium effects are introduced in the $ππ$ cross-section through one-loop self-energies in the propagator of the exchanged $ρ$ and $σ$ mesons. The effect of early chemical freeze-out in heavy ion collisions is implemented through a temperature dependent pion chemical potential. These are found to affect the temperature dependence of the bulk and shear viscosities significantly.

hep-ph↗

arXiv2013

Characterizing quark gluon plasma by thermal photons and lepton pairs

The photon spectra measured by the ALICE collaboration in Pb+Pb collisions at Large Hadron Collider (LHC) energies has been analyzed with a view of extracting the properties of thermal system formed in these collisions. The results of the analysis are compared with the previously studied spectra measured at Super Proton Synchrotron (SPS) and Relativistic Heavy Ion Collider (RHIC) energies. The thermal dilepton spectra from the Pb+Pb collision at LHC energy has been predicted for the initial conditions constrained by the thermal photon spectra at the same collision conditions. The slope of the photon has been used to estimate the rise in the effective degeneracy with the charged particle multiplicity of the system. The slopes of the lepton pair spectra for different invariant mass windows have been used to conjecture the average radial flow velocity of the high temperature phase.

nucl-th↗

arXiv2012

Evaluating transport coefficients in real time thermal field theory

Transport coefficients in a hadronic gas have been calculated earlier in the imaginary time formulation of thermal field theory. The steps involved are to relate the defining retarded correlation function to the corresponding time-ordered one and to evaluate the latter in the conventional perturbation expansion. Here we carry out both the steps in the real time formulation.

nucl-th↗

arXiv2012

Effect of spectral modification of $ρ$ on shear viscosity of a pion gas

We evaluate the shear viscosity of a pion gas in the relativistic kinetic theory approach. The in-medium propagator of the $ρ$ meson at finite temperature is used to evaluate the $π-π$ scattering amplitude in the medium. The real and imaginary parts of the self-energy calculated from one-loop diagrams are seen to have noticeable effects on the scattering cross-section. The consequences on temperature dependence of the shear viscosity evaluated in the Chapman-Enskog and relaxation time approximations are studied.

nucl-th↗

arXiv2012

Effect of running coupling on photon emission from quark gluon plasma

We discuss the role of running coupling on the thermal photon yield from quark gluon plasma. It is shown that the photon production rate from the partonic phase is considerably enhanced when running coupling is considered with respect to a fixed value. However, we show by explicit evaluation that although this difference survives the space-time evolution the experimental data cannot distinguish between the two once the hard contribution, which is an essential component of photon production mechanism, is added.

hep-ph↗

arXiv2012

Drag and diffusion coefficients of $B$ mesons in hot hadronic matter

The drag and diffusion coefficients of a hot hadronic medium consisting of pions, kaons and eta using open beauty mesons as a probe have been evaluated. The interaction of the probe with the hadronic matter has been treated in the framework of chiral perturbation theory. It is observed that the magnitude of both the transport coefficients are significant, indicating substantial amount of interaction of the heavy mesons with the thermal bath. The results may have significant impact on the experimental observables like the suppression of single electron spectra originating from the decays of heavy mesons produced in nuclear collisions at RHIC and LHC energies

hep-ph↗

arXiv2012

In-medium vector mesons and low mass lepton pairs from heavy ion collisions

The rho and omega meson self-energy at finite temperature and baryon density have been analysed for an exhaustive set of mesonic and baryonic loops in the real time formulation of thermal field theory. The large enhancement of spectral strength below the nominal rho mass is seen to cause a substantial enhancement in dilepton pair yield in this mass region. The integrated yield after space-time evolution using relativistic hydrodynamics with quark gluon plasma in the initial state leads to a very good agreement with the experimental data from In-In collisions obtained by the NA60 collaboration.

nucl-th↗

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↗

arXiv2011

Observing many body effects on lepton pair production from low mass enhancement and flow at RHIC and LHC energies

The $ρ$ spectral function at finite temperature calculated using the real-time formalism of thermal field theory is used to evaluate the low mass dilepton spectra. The analytic structure of the $ρ$ propagator is studied and contributions to the dilepton yield in the region below the bare $ρ$ peak from the different cuts in the spectral function are discussed. The space-time integrated yield shows significant enhancement in the region below the bare $ρ$ peak in the invariant mass spectra. It is argued that the variation of the inverse slope of the transverse mass ($M_T$) distribution can be used as an efficient tool to predict the presence of two different phases of the matter during the evolution of the system. Sensitivity of the effective temperature obtained from the slopes of the $M_T$ spectra to the medium effects are studied.

nucl-th↗

arXiv2011

$ρ$ self energy at finite temperature and density in the real-time formalism

The $ρ$ meson self-energy in nuclear matter from baryonic loops is analysed in the real time formulation of field theory at finite temperature and density. The discontinuities across the branch cuts of the self-energy function are evaluated for an exhaustive set of resonances in the loops considering the fully relativistic thermal baryon propagator including anti-baryons. Numerical calculations show a significant broadening of the $ρ$ spectral function coming from the Landau cut. Adding the contribution from mesonic loops, the full spectral function of the $ρ$ in a thermal gas of mesons, baryons and antibaryons in equilibrium is evaluated at various values of temperature and baryonic chemical potential.

nucl-th↗