arXiv subjects

Sourendu Gupta

Publications and source records attributed to Sourendu Gupta.

At least 91 records · Page 5Linked to original sources

arXiv2004

Chiral dynamics in QCD at finite chemical potential

We present the renormalized Taylor expansion of the chiral condensate with chemical potential. The relation with the Taylor coefficients of meson susceptibilities and the quark mass dependence of the quark number susceptibilities are obtained through chiral Ward identities and Maxwell relations. The continuum limit is obtained in quenched and two flavour QCD. In Nf=2 QCD, the quadratic response coefficients (QRCs) of the chiral condensate to chemical potential indicate that pion fluctuations decrease at finite chemical potential toward the critical end point. We discuss how these observables can be used to test other features of the QCD phase diagram.

hep-lat↗

arXiv2004

Here's looking at you, fireball

Plasmas screen electromagnetic waves of frequency less than the plasma frequency, omega_p, with a skin depth, delta, specified by the electrical conductivity, sigma. Current estimates of the transport properties of the QCD plasma indicates that photons with energy less than 250--500 MeV would be screened with a skin depth of 2--4 fm. In a hadron gas, a little below T_c, screening occurs for much softer photons, of energy less than approximately 50 MeV, albeit with similar values of delta. If the QCD plasma is indeed strongly interacting, then it can be proven by merely looking at the brightness of a fireball from different angles.

hep-ph↗

arXiv2004

The electrical conductivity and soft photon emissivity of the QCD plasma

The electrical conductivity in the hot phase of the QCD plasma is extracted from a quenched lattice measurement of the Euclidean time vector correlator for 1.5 < T/T_c < 3. The spectral density in the vicinity of the origin is examined using a method specially adapted to this region, and a peak at small energies is seen. The continuum limit of the electrical conductivity, and the closely related soft photon emissivity of the QCD plasma, are then extracted from a fit to the Fourier transform of the temporal vector correlator.

hep-lat↗

arXiv2004

The Wroblewski Parameter from Lattice QCD

Enhancement of strangeness production has since long been proposed as a promising signal of quark-gluon plasma production. A convenient indicator for it is the Wroblewski parameter which has been shown to be about a factor two higher in heavy ion 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.

hep-ph↗

arXiv2003

Taylor expansions in chemical potential

Properties of QCD at finite chemical potential are extracted using Taylor series expansions. The continuum limits of lattice results are presented. The result of expanding the free energy density, ie, the pressure, to 6th order in the expansion is shown. The Taylor coefficients of the chiral condensate are also shown. Relations between various Taylor coefficients are demonstrated. All this information is utilised to remove various lattice artifacts from the determination of the Wroblewski parameter in strangeness production.

nucl-th↗

arXiv2003

Pressure and non-linear quark number susceptibilities in QCD

We report the first measurements of sixth order non-linear quark number susceptibilities in continuum QCD. This extends our earlier computation of the continuum limit of non-linear quark number susceptibilities and the pressure in QCD for m/Tc=0.1. For T/Tc > 1.5, no phase transitions are observable for mu<30T. Systematics of off-diagonal susceptibilities are reported. The equation of state in the high temperature phase of the QCD plasma is constructed.

hep-lat↗

arXiv2003

Analyticity and the phase diagram of QCD

Some consequences of the analyticity of the free energy (pressure) of QCD at finite chemical potential are deduced. These include a method for numerical exploration of the full phase diagram by a novel use of simulations at imaginary chemical potential to extract Yang-Lee zeroes of the grand-canonical partition function. We make use of, and comment on, CPT symmetries, positivity of non-linear susceptibilities and the finiteness of screening lengths. We also comment on the structure of zeroes expected for the usual picture of the phases of QCD, following a discussion of the physics of imaginary chemical potential.

hep-lat↗

arXiv2003

Pressure and non-linear susceptibilities in QCD at finite chemical potentials

When the free energy density of QCD is expanded in a series in the chemical potential, mu, the Taylor coefficients are the non-linear quark number susceptibilities. We show that these depend on the prescription for putting chemical potential on the lattice, making all extrapolations in chemical potential prescription dependent at finite lattice spacing. To put bounds on the prescription dependence, we investigate the magnitude of the non-linear susceptibilities over a range of temperature, T, in QCD with two degenerate flavours of light dynamical quarks at lattice spacing 1/4T. The prescription dependence is removed in quenched QCD through a continuum extrapolation, and the dependence of the pressure, P, on mu is obtained.

hep-lat↗

arXiv2003

Speed and Adaptability of Overlap Fermion Algorithms

We compare the efficiency of four different algorithms to compute the overlap Dirac operator, both for the speed, i.e., time required to reach a desired numerical accuracy, and for the adaptability, i.e., the scaling of speed with the condition number of the (square of the) Wilson Dirac operator. Although orthogonal polynomial expansions give good speeds at moderate condition number, they are highly non-adaptable. One of the rational function expansions, the Zolotarev approximation, is the fastest and is adaptable. The conjugate gradient approximation is adaptable, self-tuning, and nearly as fast as the ZA.

hep-lat↗

arXiv2003

The Quark Gluon Plasma: lattice computations put to experimental test

I describe how lattice computations are being used to extract experimentally relevant features of the quark gluon plasma. I deal specifically with relaxation times, photon emissivity, strangeness yields, event by event fluctuations of conserved quantities and hydrodynamic flow. Finally I give evidence that the plasma is liquid-like in some ways.

hep-ph↗

arXiv2002

Lattice QCD for RHIC

I briefly introduce the methods by which lattice QCD predictions for RHIC are obtained. Next I deal with lattice determinations of strangeness production and event-to-event fluctuations of conserved quantities. I also present a new diagrammatic method for computing derivatives with respect to chemical potentials, and conclude with discussions of some tests of thermal perturbation theory which follow.

hep-ph↗

arXiv2002

Valence quarks in the QCD plasma: quark number susceptibilities and screening

We have investigated the quark sector of quenched QCD for 1.5\le T/Tc\le3 in the continuum limit, using two different lattice discretisations of quarks and extrapolating from lattice spacings between 1/4T and 1/14T. At these temperatures, the flavour off-diagonal susceptibility, chi_{ud}/T^2, is compatible with zero at each lattice spacing, and hence also in the continuum limit. In the continuum limit, the light quark susceptibilities are about 10% less than the ideal gas results even at the highest T, in agreement with hard thermal loop predictions but marginally below a resummed perturbative computation. For the mass range appropriate to the strange quark, the flavour diagonal susceptibility is significantly smaller. Our estimate of the Wroblewski parameter is compatible with observations at RHIC and SPS. The continuum limit of screening masses in all (local) quark-bilinears is very close to the ideal gas results.

hep-lat↗

arXiv2002

The phase transition in QCD with broken SU(2) flavour symmetry

We report the first investigation of the QCD transition temperature, T_c, for two flavours of staggered quarks with unequal masses at lattice spacings of 1/4T. On changing the u/d quark mass ratio in such a way that m(pi_0)^2/m(pi_+)^2 changes from 1 to 0.78, thus bracketing the physical value of this ratio, we find that T_c remains unchanged in units of both m_rho and Lambda_MSbar.

hep-lat↗

arXiv2002

Does the QCD plasma contain propagating gluons?

Comparison of two appropriately chosen screening masses of colour singlet operators in the pure glue QCD plasma indicates that at sufficiently high temperature it contains a weakly-interacting massive quasi-particle with the quantum numbers of the electric gluon. Still in the deconfined phase, but closer to T_c, the same mass ratio is similar to that at zero temperature, indicating that the propagating modes are more glueball-like, albeit with a lower scale for the masses. We observe a continuity between these two regimes.

hep-lat↗

arXiv2002

Critical behaviour in QCD at finite isovector chemical potential

We report an investigation of criticality in QCD at finite isovector chemical potential, mu_3, and at zero temperature. At the critical point, mu_3^c = m_pi, we find that an uncharged scalar and pseudoscalar and a charged pseudoscalar meson become massless within the resolution of our measurement. The effective long distance theory therefore breaks O(4) symmetry by charged pion condensation. This results in a rising quark number susceptibility. The baryon remains massive, as indicated by a vanishing baryon number susceptibility.

hep-lat↗

arXiv2002

The continuum limit of quark number susceptibilities

We report the continuum limit of quark number susceptibilities in quenched QCD. Deviations from ideal gas behaviour at temperature T increase as the lattice spacing is decreased from T/4 to T/6, but a further decrease seems to have very little effect. The measured susceptibilities are 20% lower than the ideal gas values, and also 10% below the hard thermal loop (HTL) results. The off-diagonal susceptibility is several orders of magnitude smaller than the HTL results. We verify a strong correlation between the lowest screening mass and the susceptibility. We also show that the quark number susceptibilities give a reasonable account of the Wroblewski parameter, which measures the strangeness yield in a heavy-ion collision.

hep-lat↗

arXiv2001

Susceptibilities and screening masses in two flavor QCD

We studied QCD with two flavors of dynamical staggered quarks at finite temperature, with a bare sea quark mass of about 17 MeV. We report investigations of baryon, isospin, charge and strangeness susceptibilities, as well as screening masses obtained from correlators of local and one-link separated meson operators. These were studied as functions of valence quark mass at several temperatures. Our results for susceptibilities deviate significantly from ideal gas values, and even more from the weak coupling series. We also report the first measurement of off-diagonal quark number susceptibilities below the transition temperature, Tc, where they are the main contribution to charge fluctuations. We present evidence for a close connection between the susceptibilities and the screening masses.

hep-lat↗

arXiv2001

Quenched QCD at finite temperature with overlap Fermions

We study quenched QCD just above the phase transition temperature using overlap Fermions. Exact zero modes of the overlap operator are localized. Chiral symmetry is restored, as indicated by the behavior of the chiral condensate after subtracting the effects of zero modes. The vector and pseudo-scalar screening masses are close to ideal gas values.

hep-lat↗