arXiv subjects

Sourendu Gupta

Publications and source records attributed to Sourendu Gupta.

At least 73 records · Page 4Linked to original sources

arXiv2008

Eigenvalues and Eigenvectors of the Staggered Dirac Operator at Finite Temperature

We examine the eigenvalues and eigenvectors of the staggered Dirac operator on thermal ensembles created in QCD with two flavours of staggered quarks. We see that across the phase transition a gap opens in the spectrum. For finite volume lattices in the low-temperature phase the eigenvectors are extended, but generic field configurations in the high temperature phase give rise to localized eigenstates. We examine measures of the stability of such localization and find that at finite volumes localization occurs through Mott's mechanism of the formation of mobility edges. However, the band gap between the localized and extended states seem to scale to zero in the limit of large volume.

hep-lat↗

arXiv2007

Aspects of causal viscous hydrodynamics

We investigate the phenomenology of freely expanding fluids, with different material properties, evolving through the Israel-Stewart (IS) causal viscous hydrodynamics, and compare our results with those obtained in the relativistic Eckart-Landau-Navier-Stokes (ELNS) acausal viscous hydrodynamics. Through the analysis of scaling invariants we give a definition of thermalization time which can be self-consistently determined in viscous hydrodynamics. Next we construct the solutions for one-dimensional boost-invariant flows. Expansion of viscous fluids is slower than that of one-dimensional ideal fluids, resulting in entropy production. At late times, these flows are reasonably well approximated by solutions obtained in ELNS hydrodynamics. Estimates of initial energy densities from observed final values are strongly dependent on the dynamics one chooses. For the same material, and the same final state, IS hydrodynamics gives the smallest initial energy density. We also study fluctuations about these one-dimensional boost-invariant backgrounds; they are damped in ELNS hydrodynamics but can become sound waves in IS hydrodynamics. The difference is obvious in power spectra due to clear signals of wave-interference in IS hydrodynamics, which is completely absent in ELNS dynamics.

nucl-th↗

arXiv2007

The phase diagram of QCD at small chemical potentials

We investigate the phase diagram of QCD at small chemical potentials, ie, when chiral and flavour symmetry breaking involves the pairing of a quark and an antiquark. The phase diagram of two-flavour QCD at small chemical potentials involves chiral symmetry restoration and charged pion condensation. We extend previous studies of the topology of the phase diagram, in sections with high degree of symmetry, to the physical case of fully broken flavour symmetry, using generic thermodynamic arguments. We argue that the extension is unique and present the result. In three flavour QCD the phase diagram for chiral symmetry restoration is less well constrained. However, we argue that present lattice data allows just two different phase diagrams, which we discuss.

hep-ph↗

arXiv2007

Renormalized Polyakov loops in many representations

We present a renormalization procedure for Polyakov loops which explicitly implements the fact that the renormalization constant depends only on the ultraviolet cutoff. Using this we study the renormalized Polyakov loops in all representations upto the {\bf 27} of the gauge group SU(3). We find good evidence for Casimir scaling of the Polyakov loops and for approximate large-N factorization. By studying many loops together, we are able to show that there is a matrix model with a single coupling which can describe the high temperature phase of QCD, although it is hard to construct explicitly. We present the first results for the non-vanishing renormalized octet loop in the thermodynamic limit below the SU(3) phase transition, and estimate the associated string breaking distance and the gluelump binding energy. By studying the connection of the direct renormalization procedure with a generalization of an earlier suggestion which goes by the name of the $Q\bar Q$ renormalization procedure, we find that they are functionally equivalent.

hep-lat↗

arXiv2007

Renormalization of Polyakov loops in fundamental and higher representations

We compare two renormalization procedures, one based on the short distance behavior of heavy quark-antiquark free energies and the other by using bare Polyakov loops at different temporal extent of the lattice and find that both prescriptions are equivalent, resulting in renormalization constants that depend on the bare coupling. Furthermore these renormalization constants show Casimir scaling for higher representations of the Polyakov loops. The analysis of Polyakov loops in different representations of the color SU(3) group indicates that a simple perturbative inspired relation in terms of the quadratic Casimir operator is realized to a good approximation at temperatures $T \gsim T_c$ for renormalized as well as bare loops. In contrast to a vanishing Polyakov loop in representations with non-zero triality in the confined phase, the adjoint loops are small but non-zero even for temperatures below the critical one. The adjoint quark-antiquark pairs exhibit screening. This behavior can be related to the binding energy of gluelump states.

hep-lat↗

arXiv2007

A systematic expansion for relativistic causal hydrodynamics

A systematic expansion of the Boltzmann equation for the diffusion of dilute tracers, in powers of the Knudsen number, carried out to next-to-leading order (NLO), gives the relativistic causal diffusion equation (Kelly's equation). Using dimensionless combinations of dynamical quantities, we show when the small NLO term plays a crucially important role. We proceed to show that a derivation of Kelly's equation from a microscopic theory of the correlation function of the number density of diffusers is possible. The correlator fulfils the Green-Kubo relation for the diffusion constant, as well as an f-sum which goes beyond a purely phenomenological causal theory. We argue that the construction generalizes to the full hydrodynamic equations.

nucl-th↗

arXiv2006

Towards continuum limit of screening lengths with chiral Fermions

We investigate mesonic screening correlators at T=2T_c using the overlap Fermions in the quenched approximation, where T_c is the QCD phase transition temperature. Using lattices with temporal extent up to 8, we found that both pseudoscalar and vector correlators exhibit a nice $cosh$ behaviour, leading to a plateau behaviour in the local screening masses as a function of distance. The rho and pi masses so determined show very little variation with the lattice spacing a. This augurs well for the use of chiral Fermions, and further suggests the small deviations of these masses from the ideal gas values are genuine effects of interactions.

hep-lat↗

arXiv2006

The critical end point in QCD

In this talk I present the logic behind, and examine the reliability of, estimates of the critical end point (CEP) of QCD using the Taylor expansion method.

hep-lat↗

arXiv2006

Lattice QCD Results on Strangeness and Quasi-Quarks in Heavy-Ion Collisions

Fluctuations of conserved quantities in heavy-ion collisions have been argued to be diagnostic tools for the nature of the produced phase. These can be related to the predictions of quark number susceptibilities (QNS) from lattice QCD. Using the diagonal QNS, we extracted the Wroblewski parameter in a dynamical QCD computation. Our results on the cross correlations χ_{BQ}, χ_{BY}, χ_{BS} and χ_{QY} allow us to explore the charge and baryon number of objects that carry flavour. We present evidence that in the high temperature phase of QCD the different flavour quantum numbers are excited in linkages which are exactly the same as one expects from quarks.

hep-ph↗

arXiv2005

The critical end point of QCD

We investigate the critical end point of QCD with two flavours of light dynamical quarks at finite lattice cutoff a=1/4T using a Taylor expansion of the baryon number susceptibility. We find a strong volume dependence of the position of the critical end point. In the large volume limit we obtain T^E/T_c \~ 0.95 and mu_B^E/T^E ~ 1.1, where T_c is the cross over temperature at zero chemical potential, and T^E and mu_B^E are the temperature and the baryon chemical potential at the critical end point. The small value of mu_B^E places it in the range of observability in energy scans at the RHIC.

hep-lat↗

arXiv2005

Fluctuations, strangeness and quasi-quarks in heavy-ion collisions from lattice QCD

We report measurements of diagonal susceptibilities for the baryon number, chi_B, electrical charge, chi_Q, third component of isospin, chi_I, strangeness, chi_S, and hypercharge, chi_Y, as well as the off-diagonal chi_BQ, chi_BY, chi_BS, etc. We show that the ratios of susceptibilities in the high temperature phase are robust variables, independent of lattice spacing, and therefore give predictions for experiments. We also investigate strangeness production and flavour symmetry breaking matrix elements at finite temperature. Finally, we present evidence that in the high temperature phase of QCD the different flavour quantum numbers are excited in linkages which are exactly the same as one expects from quarks. We present some investigations of these quark-like quasi particles.

hep-lat↗

arXiv2005

The critical end point of quantum chromodynamics

We investigate the critical end point (CEP) of QCD with two flavours of light dynamical quarks at finite lattice cutoff a=1/4T using a Taylor expansion of the baryon number susceptibility. We find a strong volume dependence of the position of the critical end point. In the large volume limit we obtain\ T^E/T_c approximately 0.95 and mu_B^E/T^E approximately 1.1, where T_c is the cross over temperature at zero chemical potential, and T^E and mu_B^E are the temperature and the baryon chemical potential at the critical end point. The small value of mu_B^E places it in the range of observability in energy scans at the RHIC.

hep-lat↗

arXiv2005

A new method for computation of QCD thermodynamics: EOS, specific heat and speed of sound

We propose a new variant of the operator method for the computation of the equation of state of QCD, which yields positive pressure for all temperatures and all values of temporal lattice spacings. Using this new method, we calculate the continuum limit of pressure, energy density, entropy density, specific heat, and the speed of sound, in quenched QCD, for 0.9 \le T/T_c \le 3.

hep-lat↗

arXiv2005

Simple patterns for non-linear susceptibilities near T_c

Non-linear susceptibilities upto the eighth order have been constructed in QCD with 2 flavours of dynamical quarks. Beyond leading order, they exhibit peaks at the cross over temperature, T_c. By analyzing their behaviour in detail, we find that the dominant contributions near T_c come from a set of operators with a remarkably simple topology. Any effective theory of QCD near T_c must be able to explain these regularities.

hep-lat↗

arXiv2005

Dissipation, hydrodynamics and the fireball

We investigate the hydrodynamics of the QCD plasma using dimensionless numbers built from the thermodynamics and transport theory of the plasma and characteristic dimensions of the fireball produced in heavy-ion collisions. We find that by the usual measures, dissipation is strong, and the fireball is on the borderline of equilibrium. As a result, the system is richer in phenomena than ideal hydrodynamics would predict. One general implication is that it may be possible to get a direct view of the QCD plasma phase rather than having to infer its existence indirectly from signals that come from the freezeout isotherm after the fireball has cooled into the hadronic phase.

hep-ph↗

arXiv2005

A transport coefficient: the electrical conductivity

I describe the lattice determination of the electrical conductivity of the quark gluon plasma. Since this is the first extraction of a transport coefficient with a degree of control over errors, I next use this to make estimates of other transport related quantities using simple kinetic theory formulae. The resulting estimates are applied to fluctuations, ultra-soft photon spectra and the viscosity. Dimming of ultra-soft photons is exponential in the mean free path, and hence is a very sensitive probe of transport.

hep-ph↗

arXiv2005

Quark Number Susceptibilities & The Wroblewski Parameter

The Wroblewski parameter is a convenient indicator of strangeness production and can be employed to monitor a signal of quark-gluon plasma production : enhancement of strangeness production. It has been shown to be about a factor two higher in heavy ion collisions than in hadronic collisions. Using a method proposed by us earlier, we obtained lattice QCD results for the Wroblewski parameter from our simulations of QCD with two light quarks both below and above the chiral transition. Our first principles based and parameter free result compare well with the A-A data from SPS and RHIC.

hep-ph↗

arXiv2005

The speed of sound and specific heat in the QCD plasma: hydrodynamics, fluctuations and conformal symmetry

We report the continuum limits of the speed of sound, c_s, and the specific heat at constant volume, c_v, in quenched QCD at temperatures of 2Tc and 3Tc. At these temperatures, c_s is within 2-sigma of the ideal gas value, whereas c_v differs significantly from the ideal gas. However, both are compatible with results expected in a theory with conformal symmetry. We investigate effective measures of conformal symmetry in the high temperature phase of QCD.

hep-lat↗