arXiv subjects

Nasser Demir

Publications and source records attributed to Nasser Demir.

5 recordsLinked to original sources

arXiv2026

One-Loop Renormalization of the Improved Energy-Momentum Tensor in Lattice QCD

We present the one-loop renormalization of the improved gluonic energy-momentum tensor (EMT) by employing a three-loop-improved clover discretization of the field-strength tensor in pure SU(3) lattice gauge theory. The renormalization factor is extracted by matching the amputated two-gluon matrix element of the lattice energy-momentum tensor to the continuum MSbar scheme. The one-loop contribution is separated into sail, operator-tadpole, and external-leg corrections, each expressed in terms of a minimal set of scalar Brillouin-zone integrals to obtain the explicit expression for the finite lattice coefficient B_lat(u_0) and the multiplicative renormalization factor Z_T(u_0) associated with the traceless spin-2 component of the energy-momentum tensor. A key result, obtained in this framework, is the clear distinction between two sectors: the spin-2 sector, governed by Z_T, and the scalar trace sector, which encompasses the Yang-Mills trace anomaly. The trace is determined by the scalar operator F_{\rho\sigma}F_{\rho\sigma} and the Yang-Mills beta function, rather than by the spin-2 renormalization factor. The renormalized EMT modifies the normalization and short-distance behaviour of energy-density correlators through both traceless and scalar-channel contributions. Comparison with existing lattice thermodynamic data demonstrates that the improved operator accurately reproduces the expected temperature dependence of the trace anomaly and offers a systematically improvable framework for investigating the equation of state, gluon condensate, and transport coefficients in lattice QCD.

hep-lat↗

arXiv2023

Entropy Density and Speed of Sound from Improved Energy-Momentum Tensor in Lattice QCD

We present a lattice calculation of the entropy density $s/T^{3}$ and speed of sound $c_{s}^{2}$ of gluedynamics near the critical temperature, $T_{c}$, in the deconfined phase. By exploring the temperature dependence of entropy density in this region, we aim to analyse the significant discrepancies between the previous computations. The calculation of entropy density is carried out by numerical simulations of $O(a^{4})$ mean-field improved energy-momentum tensor (EMT) of SU(3) gauge theory on the lattice. We expand on reaching $O(a^{4})$ improvement using tadpole-improved Symanzik action. The entropy density is calculated directly from the expectation value of the space-time component of the improved EMT in the presence of shifted boundary conditions at several lattice spacings ($a \approx 0.043 - 0.012$ fm). The absence of ultraviolet divergences and the minimal finite-size effects allow for the precision determination of the entropy density and its extrapolation to the continuum limit. As expected, the resulting entropy density displays the expected behaviour of rapid increase near the critical temperature in the deconfined phase followed by a slow increase in $2T_{c}\leq T\leq 3T_{c}$ region, suggesting a logarithmic dependence on the temperature. A quantitative comparison of $s/T^{3}$ shows good agreement with Pade approximation and lattice results of previous high-precision data obtained using the gradient flow method. We observe that at temperatures of about $3T_{c}$, deviations of entropy density from the Stefan-Boltzmann value for a free theory are about 10$\%$. It is shown that the speed of sound in SU(3) gluedynamics is found to be $c_{s}^{2}\leq 0.333$ in the temperature region $1.06T_{c}\leq T\leq 3.05T_{c}$ explored in this study. The results are found to agree with the corresponding analytic and numerical estimates.

hep-lat↗

arXiv2022

Nonextensive hydrodynamics of boost-invariant plasmas

We use quasiparticle anisotropic hydrodynamics to study the non-conformal and non-extensive dynamics of a system undergoing boost-invariant Bjorken expansion. To introduce nonextensivity, we use an underlying Tsallis distribution with a time-dependent nonextensivity parameter $q$. By taking moments of the quasiparticle Boltzmann equation in the relaxation-time approximation, we obtain dynamical equations which allow us to determine the time evolution of all microscopic parameters including $q$. We compare numerical solutions for bulk observables obtained using the nonextensive evolution with results obtained using quasiparticle anisotropic hydrodynamics with a Boltzmann distribution function ($q \rightarrow 1$). We show that the evolution of the temperature, pressure ratio, and scaled energy density, are quite insensitive to which distribution function is assumed. However, we find significant differences in the early-time evolution of the bulk pressure which are observed for even small deviations from the Boltzmann distribution function. Finally, we discuss the existence of non-conformal hydrodynamic attractors for the longitudinal and transverse pressures, the bulk and shear viscous corrections, and the nonextensivity parameter $q$.

nucl-th↗

arXiv2018

Effect of the GUP on the Entropy, Speed of Sound, and Bulk to Shear Viscosity Ratio of an ideal QGP

In this work we compute the entropy density, speed of sound, and the resulting impact on the bulk viscosity to shear viscosity ratio of an ideal Quark Gluon Plasma when the effects of a generalized uncertainty principle are taken into consideration. When the parameter of the generalized uncertainty principle tends to zero, i.e., $α\rightarrow 0$, we obtain the value of the speed of sound for the ideal gas of massless particles, i.e., $c^{2}_{s}\rightarrow 1/3$, and we recover the expected result that the bulk viscosity $ζ\rightarrow 0$ when $α\rightarrow 0$. In addition, in the high temperature limit, i.e., $T\rightarrow \infty$, the speed of sound satisfies the equation $c^{2}_{s}\rightarrow 1/4$. The consequence this has on the bulk viscosity is that in the high temperature limit, the ratio of the bulk to shear viscosity $ζ/η\rightarrow 5/48$. Our results suggest that the GUP introduces a scale into the system breaking the a priori conformal invariance of a system of massless noninteracting particles.

hep-th↗

arXiv2009

Shear-Viscosity to Entropy Density Ratio of a Relativistic Hadron Gas

Ultrarelativistic heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) are thought to have produced a state of matter called the Quark-Gluon-Plasma, characterized by a very small shear viscosity to entropy density ratio $η/s$, near the lower bound predicted for that quantity by Anti-deSitter space/Conformal Field Theory (AdS/CFT) methods. As the produced matter expands and cools, it evolves through a phase described by a hadron gas with rapidly increasing $η/s$. We calculate $η/s$ as a function of temperature in this phase and find that its value poses a challenge for viscous relativistic hydrodynamics, which requires small values of $η/s$ throughout the entire evolution of the reaction in order to successfully describe the collective flow observables at RHIC. We show that the inclusion of non-unit fugacities will reduce $η/s$ in the hadronic phase, yet not sufficiently to be compatible with viscous hydrodynamics. We therefore conclude that the origin of the low viscosity matter at RHIC must be in the partonic phase of the reaction.

nucl-th↗