arXiv subjects

F. Taghinavaz

Publications and source records attributed to F. Taghinavaz.

11 recordsLinked to original sources

arXiv2025

(2+2)D Collective Model based on a relativistic Boltzmann equation in the Isotropization Time Approximation: CoMBolt-ITA

A new model based on the relativistic Boltzmann equation in the isotropization time approximation is developed to investigate the collective behavior of the quark-gluon plasma produced in high-energy heavy-ion collisions. The equation is solved in (2+2)D (two spatial and two momentum-space dimensions). This framework couples pre-equilibrium dynamics with hydrodynamic evolution to simulate the dynamics of quasiparticle evolution. A numerical scheme based on the method of characteristics enables the evolution to begin from a specified initial Boltzmann distribution. In this work, the spatial structure of the initial distribution is modeled using the TrENTo framework. Our results show that a medium initialized at $\tau_0$ on the order of 1 [fm/$c$] with a small shear viscosity to entropy density ratio ($\eta/s = 0.008$) evolves consistently with hydrodynamic simulations, such as those performed using the VISH2+1 code, while discrepancies arise for a medium with $\eta/s = 0.8$. Furthermore, when initialized with a highly anisotropic momentum distribution in the longitudinal direction at early times, the system exhibits spatially non-uniform thermalization in the transverse plane, leading to the emergence of a nontrivial hypersurface that marks the onset of hydrodynamic applicability. Finally, we compute the $p_T$-spectra for a non-fluctuating initial condition using the hybrid version of CoMBolt-ITA. In this hybrid setup, the description is switched from quasiparticles to hadrons, and UrQMD is used to model the hadron gas dynamics. We compare these results with those obtained from the hybrid VISH2+1 initialized within the same setup. For a small shear viscosity, $\eta/s = 0.08$, the two results show a good level of consistency, whereas for a larger value, $\eta/s = 0.8$, a noticeable discrepancy emerges.

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arXiv2023

Relativistic hydrodynamics with phase transition

Assessing the applicability of hydrodynamic expansions close to phase transition points is crucial from either theoretical or phenomenological points of view. We explore this within the gauge/gravity duality, using the Einstein-Klein-Gordon model, a bottom-up string theory construction. This model incorporates a parameter, $B_4$, that simulates different types of phase transitions in the strongly coupled field theory existing at the boundary. We thoroughly examine the thermodynamics and dynamics of time-dependent, linearized perturbations in the spin-2, spin-1, and spin-0 sectors. Our findings suggest that "hydrodynamic series breakdown near transition points" is valid exclusively for second-order phase transitions, not for crossovers or first-order phase transitions. Additionally, we observe that the high-temperature and low-temperature limits of the radius of convergence for the hydrodynamic series ($q^2_c$) are equal. We also discover that the relationship $(\text{Max}\vert q^2_c \vert)_{\text{spin-2}} < (\text{Max}\vert q^2_c\vert)_{\text{spin-0}} < (\text{Max}\vert q^2_c \vert)_{\text{spin-1}}$ is consistent for different spin sectors, regardless of the phase transition type. At the chaos point, we observe the emergence of pole-skipping behavior for both gravity and scalar perturbations at $\omega_n = - 2\pi T n i$. Lastly, comparing the chaos momentum with $q^2_c$, we find that $q^2_{ps} < q^2_c$, except for extremely high temperatures.

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arXiv2023

Chaos Near to the Critical Point: Butterfly Effect and Pole-Skipping

We study the butterfly effect and pole-skipping phenomenon for the 1RCBH model which enjoys a critical point in its phase diagram. Using the holographic idea, we compute the butterfly velocity and interestingly find that this velocity can probe the critical behavior of this model. We calculate the dynamical exponent of this quantity near the critical point and find a perfect agreement with the value of the other quantity's dynamical exponent near this critical point. We also find that at chaos point, the phenomenon of pole-skipping appears which is a sign of a multivalued retarded correlation function. We briefly address the butterfly velocity and pole-skipping for the AdS-RN black hole solution which on its boundary a strongly coupled charged field theory lives. For both of these models, we find $v_B^2\geq c_s^2$ at each point of parameter space where $c_s$ is the speed of sound wave propagation.

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arXiv2022

Chiral phase transition of a dense, magnetized and rotating quark matter

We investigate the chiral symmetry restoration/breaking of a dense, magnetized and rotating quark matter within the Nambu Jona-Lasinio model including $N_f=2$ and $N_c=3$ numbers of flavors and colors, respectively. Imposing the spectral boundary conditions, as well as the positiveness of energy levels, lead to a correlation between the magnetic and rotation fields such that strongly magnetized plasma can not rotate anymore. We solve the gap equation at zero and finite temperature. At finite temperature and baryon chemical potential $\mu_B$, we sketch the phase diagrams $T_c(\mu_B)$ and $T_c(R\Omega)$ in different cases. As a result, we always observe inverse-rotational catalysis mean to decrease $T_c$ by increasing $R\Omega$. But the magnetic field has a more complex structure in the phase diagram. For slowly rotating plasma, we find that $T_c$ decreases by increasing $eB$, while in the fast rotating plasma we see that $T_c$ increases by increasing $eB$. Also, we locate exactly the position of Critical End Point by solving the equations of first and second derivatives of effective action with respect to the order parameters, simultaneously.

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arXiv2021

Inverse magneto-rotational catalysis and the phase diagram of a rotating hot and magnetized quark matter

We study the properties of a hot and magnetized quark matter in a rotating cylinder in the presence of a constant magnetic field. To do this, we solve the corresponding Dirac equation using the Ritus eigenfunction method. This leads to the energy dispersion relation, Ritus eigenfunctions, and the quantization relation for magnetized fermions. To avoid causality-violating effects, we impose a certain global boundary condition, and study its effect, in particular, on the energy eigenmodes and the quantization relations of fermions. Using the fermion propagator arising from this method, we then solve the gap equation at zero and nonzero temperatures. At zero temperature, the dynamical mass $\bar{m}$ does not depend on the angular frequency, as expected. We thus study its dependence on the distance $r$ relative to the axis of rotation and the magnetic field $B$, and explore the corresponding finite size effect for various couplings $G$. We then consider the finite temperature case. The dependence of $\bar{m}$ on the temperature $T$, magnetic field $B$, angular frequency $\Omega$, and distance $r$ for various $G$ is studied. We show that $\bar{m}$ decreases, in general, with $B$ and $\Omega$. This is the ''inverse magneto-rotational catalysis (IMRC)'' or the ''rotational magnetic inhibition'', previously discussed in the literature. To explore the evidence of this effect in the phase diagrams of our model, we examine the phase portraits of the critical temperature $T_c$ as well as the critical angular frequency $\Omega_c$ with respect to $G, B,\Omega$, and $r$ as well as $G, B, T$, and $r$, respectively. We show that $T_{c}$ and $\Omega_c$ decrease, in particular, with $B$. This is interpreted as clear evidence for IMRC.

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arXiv2020

Conformal Bjorken flow in the general frame and its attractor: Similarities and discrepancies with the M\"uller-Israel-Stewart formalism

We investigate the implications of the general frame approach for conformal Bjorken flow beyond the earlier studies. We show that the power series solution at late times is not unique and is accompanied by an exact solution of the form $1/\tau$, which becomes unphysical if taken on shell. In contrast to the M\"uller-Israel-Stewart formalism, a matching between $\NFSYM$ results and the hydro expansion is only possible up to the first order, which gives rise to $\eta/s=1/4\pi$. Matching the results to the next order gives rise to causality/stability-violating values. Furthermore, we show that the pressure anisotropy in the general frame cannot capture the hydrodynamization, and we introduce an alternative measure to find the attractor. Using slow-roll expansion, we find an analytical approximation form for the attractor. We also show that the early-time behavior of attractors is related to stability and causality conditions. The attractor solutions outside the stable and causal regime give rise to reheating and negative longitudinal pressures in early times, in contrast to the stable and causal ones. We also comment on the violation of the second law of thermodynamics by the off-shell parameters. We show that for the stable and causal choice of parameters, the off-shell canonical entropy of the attractors, which is not a physical quantity, has a negative divergence in early times before tending to its on-shell limit. On the other hand, the unstable and acausal attractors have non-negative entropy divergence. We speculate that the violation of the second law by stable and causal off-shell parameters is required for stability of the first-order hydrodynamics. We investigate the analytical structure of the Borel-transformed series and find the proper relation between the poles and nonhydro modes.

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arXiv2017

Dilepton production rate in a hot and magnetized quark-gluon plasma

The differential multiplicity of dileptons in a hot and magnetized quark-gluon plasma, $Δ_{B}\equiv dN_{B}/d^{4}xd^{4}q$, is derived from first principles. The constant magnetic field $B$ is assumed to be aligned in a fixed spatial direction. It is shown that the anisotropy induced by the $B$ field is mainly reflected in the general structure of photon spectral density function. This is related to the imaginary part of the vacuum polarization tensor, $\mbox{Im}[Π^{μν}]$, which is derived in a first order perturbative approximation. As expected, the final analytical expression for $Δ_{B}$ includes a trace over the product of a photonic part, $\mbox{Im}[Π^{μν}]$, and a leptonic part, ${\cal{L}}_{μν}$. It is shown that $Δ_{B}$ consists of two parts, $Δ_{B}^{\|}$ and $Δ_{B}^{\perp}$, arising from the components $(μ,ν)=(\|,\|)$ and $(μ,ν)=(\perp,\perp)$ of $\mbox{Im}[Π^{μν}]$ and ${\cal{L}}_{μν}$. Here, the transverse and longitudinal directions are defined with respect to the direction of the $B$ field. Combining $Δ_{B}^{\|}$ and $Δ_{B}^{\perp}$, a novel anisotropy factor $ν_{B}$ is introduced. Using the final analytical expression of $Δ_{B}$, the possible interplay between the temperature $T$ and the magnetic field strength $eB$ on the ratio $Δ_{B}/Δ_{0}$ and $ν_{B}$ is numerically studied. Here, $Δ_{0}$ is the Born approximated dilepton multiplicity in the absence of external magnetic fields. It is, in particular, shown that for each fixed $T$ and $B$, in the vicinity of certain threshold energies, $Δ_{B}\gg Δ_{0}$ and $Δ_{B}^{\perp}\gg Δ_{B}^{\|}$. The latter anisotropy may be interpreted as one of the microscopic sources of the macroscopic anisotropies, reflecting themselves, e.g., in the elliptic asymmetry factor $v_{2}$ of dileptons.

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arXiv2016

Magnetized plasminos in cold and hot QED plasmas

The complete quasi-particle spectrum of a magnetized electromagnetic plasma is systematically explored at zero and nonzero temperatures. To this purpose, the general structure of the one-loop corrected propagator of magnetized fermions is determined, and the dispersion relations arising from the pole of this propagator are numerically solved. It turns out that in the lowest Landau level, where only one spin direction is allowed, the spectrum consists of one positively (negatively) charged fermionic mode with positive (negative) spin. In contrast, in higher Landau levels, as an indirect consequence of the double spin degeneracy of fermions, the spectrum consists of two massless collective modes with left- and right-chiralities. The mechanism through which these new collective excitations are created in a uniform magnetic field is similar to the production mechanism of dynamical holes (plasminos) at finite temperature and zero magnetic fields. Whereas cold magnetized plasminos appear for moderate magnetic fields and for all positive momenta of propagating fermions, hot magnetized plasminos appear only in the limit of weak magnetic fields and soft momenta.

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arXiv2015

On the contribution of plasminos to the shear viscosity of a hot and dense Yukawa-Fermi gas

Using the standard Green-Kubo formalism, we determine the shear viscosity $η$ of a hot and dense Yukawa-Fermi gas. In particular, we study the effect of particle and plasmino excitations on thermal properties of the fermionic part of the shear viscosity, and explore the effects of thermal corrections to particle masses on bosonic and fermionic shear viscosities, $η_b$ and $η_f$. It turns out that the effects of plasminos on $η_f$ become negligible with increasing (decreasing) temperature (chemical potential).

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arXiv2014

On the contribution of plasminos to the shear viscosity of a hot and dense Yukawa-Fermi gas

We determine the shear viscosity of a hot and dense Yukawa-Fermi gas, using the standard Green-Kubo relation, according to which the shear viscosity is given by the retarded correlator of the traceless part of viscous energy-momentum tensor. We approximate this retarded correlator using a one-loop skeleton expansion, and express the bosonic and fermionic shear viscosities, $η_{b}$ and $η_{f}$, in terms of bosonic and fermionic spectral widths, $Γ_{b}$ and $Γ_{\pm}$. Here, the subscripts $\pm$ correspond to normal and collective (plasmino) excitations of fermions. We study, in particular, the effect of these excitations on thermal properties of $η_{f}[Γ_{\pm}]$. To do this, we determine first the dependence of $Γ_{b}$ and $Γ_{\pm}$ on momentum $p$, temperature $T$, chemical potential $μ$ and $ξ_{0}\equiv m_{b}^{0}/m_{f}^{0}$, in a one-loop perturbative expansion in the orders of the Yukawa coupling. Here, $m_{b}^{0}$ and $m_{f}^{0}$ are $T$ and $μ$ independent bosonic and fermionic masses, respectively. We then numerically determine $η_{b}[Γ_{b}]$ and $η_{f}[Γ_{\pm}]$, and study their thermal properties. It turns out that whereas $Γ_{b}$ and $Γ_{+}$ decrease with increasing $T$ or $μ$, $Γ_{-}$ increases with increasing $T$ or $μ$. This behavior qualitatively changes by adding thermal corrections to $m_{b}^{0}$ and $m_{f}^{0}$, while the difference between $Γ_{+}$ and $Γ_{-}$ keeps increasing with increasing $T$ or $μ$. Moreover, $η_{b}$ ($η_{f}$) increases (decreases) with increasing $T$ or $μ$. We show that the effect of plasminos on $η_{f}$ becomes negligible with increasing (decreasing) $T$ ($μ$).

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arXiv2012

Local electric current correlation function in an exponentially decaying magnetic field

The effect of an exponentially decaying magnetic field on the dynamics of Dirac fermions in 3+1 dimensions is explored. The spatially decaying magnetic field is assumed to be aligned in the third direction, and is defined by {\mathbf{B}}(x)=B(x){\mathbf{e}}_{z}, with B(x)=B_{0}e^{-ξ x/\ell_{B}}. Here, ξ is a dimensionless damping factor and \ell_{B}=(eB_{0})^{-1/2} is the magnetic length. As it turns out, the energy spectrum of fermions in this inhomogeneous magnetic field can be analytically determined using the Ritus method. Assuming the magnetic field to be strong, the chiral condensate and the \textit{local} electric current correlation function are computed in the lowest Landau level (LLL) approximation and the results are compared with those arising from a strong homogeneous magnetic field. Although the constant magnetic field B_{0} can be reproduced by taking the limit of ξ-> 0 and/or x-> 0 from B(x), these limits turn out to be singular once the quantum corrections are taken into account.

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