arXiv subjects

Ali Hajilou

Publications and source records attributed to Ali Hajilou.

16 recordsLinked to original sources

arXiv2026

Spatial Wilson Loops and Energy Loss for Heavy Quarks in Magnetized HQCD Model

We investigate the effective potential and the string tension for the spatial Wilson loop (SWL) in hot dense QGP with two types of anisotropy, i.e. external magnetic field and spatial anisotropy, employing a holographic approach for the heavy quark model. In this approach, the string is extended in the 5th, holographic direction and has a turning point either on a dynamical wall (DW) configuration or on the horizon configuration in the 5th direction. We obtain the magnetic catalysis behavior for a phase transition between DW and horizon configuration of the string. The structure of the phase diagram does not depend on the boundary conditions choice for the dilaton field. Inclusion of the external magnetic field and spatial anisotropy enhance the string tension in the horizon configuration, namely drag force. For the spatially isotropic case $\nu = 1$ at different magnetic field values the string tension is proportional to $T^2$ and is qualitatively consistent with lattice results. However, for the anisotropic case, $\nu = 4.5$, it deviates from the quadratic term.

hep-th↗

arXiv2025

Jet Quenching in Holographic QCD as an Indicator of Phase Transitions in Anisotropic Regimes

In this paper, we employ the gauge/gravity duality to study jet quenching (JQ) phenomena in the quark-gluon plasma. For this purpose, we implement holographic QCD models constructed from an Einstein-Maxwell-dilaton gravity at finite temperature and finite chemical potential for light and heavy quarks. The models capture both the confinement and deconfinement phases of QCD and the first-order phase transitions. We calculate the JQ parameter in different models and compare them with the experimental data obtained in heavy-ions studies. In particular, we investigate how JQ, as a function of temperature $T$, chemical potential $\mu$, and magnetic field $c_B$, serves as a probe for identifying first-order phase transitions within the $(T,\mu,c_B)$ parameter space of holographic QCD. Particular attention is paid to the dependence of JQ on the parameter $\nu$, which characterizes longitudinal versus transverse anisotropy relative to the heavy-ion collision axis. By analyzing the dependence of the JQ parameters on these thermodynamic variables, we map critical regions associated with phase boundaries. We compare our findings to earlier studies of the running coupling constant's behavior within the gauge-gravity duality framework. This approach provides new insights into the interplay between non-perturbative dynamics and phase structure in strongly coupled systems.

hep-th↗

arXiv2025

Beta-Functions and RG flows for Holographic QCD with Heavy and Light Quarks: Isotropic case

In a previous paper [arXiv:2402.14512v3], we investigated the dependence of the running coupling constant on temperature and chemical potential for holographic models of the light and heavy quarks, supported by an Einstein-dilaton-Maxwell action. In this paper, we study the dependence of the corresponding $\beta$-functions on the temperature and the chemical potential. As in the previous paper, we give special attention to the behavior of the $\beta$-functions near the 1st order phase transitions. We consider different types of boundary conditions for the dilaton. Only one of the possible boundary conditions yields results that agree with lattice calculations at zero chemical potential. The corresponding $\beta$-functions are negative and exhibit jumps at the 1st order phase transitions. We also show that the RG fluxes are invariant with respect to the choice of the boundary conditions and that our exact solutions for the light and heavy quarks are unstable, as expected, given their negative dilaton potentials.

hep-th↗

arXiv2025

Holographic QCD Running Coupling for Heavy Quarks in Strong Magnetic Field

We investigate the influence of a magnetic field on the running coupling constant for a heavy-quark model in a bottom-up holographic approach. To achieve this, we employ a magnetized Einstein-Maxwell-dilaton background that captures the essential features of heavy quark dynamics. Similar to the light-quark model, the running coupling $\alpha$ for heavy quarks decreases in the presence of a strong external magnetic field at fixed temperature and chemical potential. The key distinction between the light and heavy quark models lies in the locations of their respective phase transitions. However, near the 1st order phase transitions, the behavior of $\alpha$ is analogous for both cases: $\alpha$ exhibits jumps that depend on temperature, chemical potential, and magnetic field strength.

hep-th↗

arXiv2024

Holographic QCD Running Coupling for Light Quarks in Strong Magnetic Field

We consider running coupling constant in holographic model with external magnetic field supported by Einstein-dilaton-three-Maxwell action. We obtain a significant dependence of the running coupling constant $α$ on chemical potential, temperature and magnetic field. We use the boundary condition that ensures the agreement with lattice calculations of string tension between quarks at zero chemical potential. The location of the 1st order phase transitions in $(μ, T)$-plane does not depend on the dilaton boundary conditions. We observe that running coupling $α$ decreases with increasing magnetic field for the fixed values of chemical potential and temperature. At the 1st order phase transitions the functions $α$ undergo jumps depending on temperature, chemical potential and magnetic field.

hep-th↗

arXiv2024

Magnetic Catalysis for Heavy Quarks in Anisotropic Holographic Model

We consider a twice anisotropic five-dimensional holographic model supported by Einstein-dilaton-three-Maxwell action that was constructed in the paper [arXiv:2011.07023]. Although, that model reproduced some essential features of the ``heavy quarks'', but did not describe the magnetic catalysis (MC) phenomenon expected from lattice results for the Quark-Gluon Plasma (QGP) with heavy quarks. We study MC phenomenon as well as typical properties of the heavy quarks phase diagram contains magnetic field as a new parameter by improving the holographic model, i.e. modifying the ``heavy quarks'' warp factor and the coupling function for the Maxwell field. Considering spatial anisotropy decreases the transition temperature for all values of the magnetic field for heavy quarks model.

hep-th↗

arXiv2024

Beta-Function Dependence on Running Coupling in Holographic QCD Models

We study the dependence of beta-function on running coupling constant in holographic models supported by Einstein-dilaton-Maxwell action for light and heavy quarks. Although, in the previous paper [arXiv:2402.14512], we considered different types of the dilaton boundary conditions, but since the behavior of $β$-function as a function of running coupling does not depend significantly on the boundary condition, we chose one. The corresponding $β$-functions are negative and monotonically decreasing functions, and have jumps on the 1-st order phase transitions for both light and heavy quarks. In addition, we compare our holographic results for $β$-function as a function of running coupling with perturbation results that obtained within 2-loop calculations.

hep-th↗

arXiv2024

Running Coupling for Holographic QCD with Heavy and Light Quarks: Isotropic case

We consider the running coupling constant in holographic models supported by Einstein-dilaton-Maxwell action for heavy and light quarks. To obtain the dependence of the running coupling constant $\alpha$ on temperature and chemical potential we impose boundary conditions on the dilaton field that depend on the position of the horizon. We use two types of boundary conditions: a simple boundary condition with the dilaton field vanishing at the horizon and a boundary condition that ensures an agreement with lattice calculations of string tension between quarks at zero chemical potential. The location of the 1st order phase transitions in $(\mu,T)$-plane does not depend on the dilaton boundary conditions for light and heavy quarks. At these phase transitions, the function $\alpha$ undergoes jumps depending on temperature and chemical potential. We also show that for the second boundary conditions the running coupling decreases with a temperature increase, and the dependence on temperature and chemical potential both for light and heavy quarks is actually specified in QGP phase by functions of one variable, demonstrating in this sense auto-model behavior.

hep-th↗

arXiv2024

Magnetic Catalysis in Holographic Model with Two Types of Anisotropy for Heavy Quarks

In our previous paper [arXiv:2011.07023] we have constructed a twice anisotropic five-dimensional holographic model supported by Einstein-dilaton-three-Maxwell action that reproduced some essential features of the "heavy quarks" model. However, that model did not describe the magnetic catalysis (MC) phenomena expected from lattice results for the QGP made up from heavy quarks. In this paper we fill this gap and construct the model that improves the previous one. It keeps typical properties of the heavy quarks phase diagram, and meanwhile possesses the MC. The deformation of previous model includes the modification of the "heavy quarks" warp factor and the coupling function for the Maxwell field providing the non-trivial chemical potential.

hep-th↗

arXiv2023

Meson Excitation Time as a Probe of Holographic Critical Point

We study the time evolution of expectation value of Wilson loop as a non-local observable in a strongly coupled field theory with a critical point at finite temperature and nonzero chemical potential, which is dual to an asymptotically AdS charged black hole via gauge/gravity duality. Due to inject of energy into the plasma, the temperature and chemical potential increase to finite values and the plasma experiences an out-of-equilibrium process. By defining meson excitation time $t_{ex}$ as a time at which the meson falls into the final excited state, we investigate the behavior of $t_{ex}$ near the critical point as the system evolves towards the critical point. We observe that by increasing the interquark distance the dynamical critical exponent increases smoothly. Also, we obtain for slow quenches different values of the dynamical critical exponent, although for fast quenches our result for the dynamical critical exponent is in agreement with the one that is reported for studying the quasi-normal modes. Consequently, this indicates that in this model for fast quenches and small values of interquark distances the gauge invariant Wilson loop is a good non-local observable to probe the critical point.

hep-th↗

arXiv2023

Analytically Approximation Solution to $R^{2}$ Gravity

In this paper, we obtain analytical approximate black hole solutions in the framework of $f(R)$ gravity and the absence of a cosmological constant. In this area, we apply the equations of motion of the theory to a spherically symmetric spacetime with one unknown function and derive black hole solutions without any constraints on the Ricci scalar. To do so, first, we obtain the near horizon and asymptotic solutions and then use both of them to obtain a complete solution by utilizing a continued-fraction expansion. Finally, we investigate the stability of the solutions by employing the thermodynamics and quasi-normal modes.

gr-qc↗

arXiv2022

More on Boundary Conditions for Warped AdS3 in GMG

In this paper, we study the Aggrawal, Ciambelli, Detournay, and Somerhausen (ACDS) boundary conditions ref[42] for Warped AdS$_3$ (WAdS$_3$) in the framework of General Massive Gravity (GMG) in the quadratic ensemble. We construct the phase space, the asymptotic structure, and the asymptotic symmetry algebra. We know that the global surface charges are finite, but not integrable, and also we find the conditions to make them integrable. In addition, to confirm that the phase space has the same symmetry as that of Warped Conformal Field Theory (WCFT), we compare the bulk entropy of Warped BTZ (WBTZ) black holes with the number of states belonging to a WCFT.

hep-th↗

arXiv2022

Meson Potential Energy in a Non-Conformal Holographic Model

We study the meson potential energy in a non-conformal model at both zero and finite temperature via gauge/gravity duality. This model consists of five-dimensional Einstein gravity coupled to a scalar field with a non-trivial potential. Interestingly, at both zero and finite temperature we find that the relative meson potential energy can be considered as a measure of non-conformality of the theory. At zero temperature we show that parameters of the Cornell potential, i.e. Coulomb strength parameter $κ$ and constant $C$ depends on the energy scale $Λ$ that breaks conformal symmetry and the difference between the number of degrees of freedom of UV and IR fixed points $ΔN$ while QCD string tension $σ_s$ just depends on the $Λ$. At finite temperature we see that there is a melting length $l_{m\ell}$ where beyond that the meson dissociates in the plasma and by increasing $Λ$ the value of $l_{m\ell}$ increases while its value decreases by increasing the temperature.

hep-th↗

arXiv2021

Chiral transition in the probe approximation from an Einstein-Maxwell-dilaton gravity model

We refine an earlier introduced 5-dimensional gravity solution capable of holographically capturing several qualitative aspects of (lattice) QCD in a strong magnetic background such as the anisotropic behaviour of the string tension, inverse catalysis at the level of the deconfinement transition or sensitivity of the entanglement entropy to the latter. Here, we consistently modify our solution of the considered Einstein-Maxwell-dilaton system to not only overcome an unphysical flattening at large distances in the quark-antiquark potential plaguing earlier work, but also to encapsulate inverse catalysis for the chiral transition in the probe approximation. This brings our dynamical holographic QCD model yet again closer to a stage at which it can be used to predict magnetic QCD quantities not directly computable via lattice techniques.

hep-th↗

arXiv2020

Anisotropic string tensions and inversely magnetic catalyzed deconfinement from a dynamical AdS/QCD model

We extend previous work on dynamical AdS/QCD models by introducing an extra ingredient under the form of a background magnetic field, this to gain insight into the influence such field can have on crucial QCD observables. Therefore, we construct a closed form analytic solution to an Einstein-Maxwell-dilaton system with a magnetic field. We specifically focus on the deconfinement transition, reporting inverse magnetic catalysis, and on the string tension, reporting a weaker/stronger confinement along/perpendicular to the magnetic field. The latter, being of importance to potential modelling of heavy quarkonia, is in qualitative agreement with lattice findings.

hep-th↗

arXiv2018

Meson Excitation at Finite Chemical Potential

We consider a probe stable meson in the holographic quark-gluon plasma at zero temperature and chemical potential. Due to the energy injection into the plasma, the temperature and chemical potential are increased to arbitrary finite values and the meson is also excited. Excitation time tex is the time at which the meson falls into the final excited state. We study the effect of various parameters of theory on the excitation time and observe that for larger values of final temperature and chemical potential the excitation time increases. Furthermore, our outcomes show that the more stable mesons are excited sooner.

hep-th↗