arXiv subjects

Kazem Bitaghsir Fadafan

Publications and source records attributed to Kazem Bitaghsir Fadafan.

At least 37 records · Page 2Linked to original sources

arXiv2018

Diffusivities bounds in the presence of Weyl corrections

In this paper, we investigate the behavior of the thermoelectric DC conductivities in the presence of Weyl corrections with momentum dissipation in the incoherent limit. Moreover, we compute the butterfly velocity and study the charge and energy diffusion with broken translational symmetry. Our results show that the Weyl coupling $γ$, violates the bounds on the charge and energy diffusivity. It is also shown that the Weyl corrections violate the bound on the DC electrical conductivity in the incoherent limit.

hep-th↗

arXiv2018

A Holographic Description of Colour Superconductivity

The difficulty of describing the gauge dependent bi-quark condensate in the QCD colour superconducting phase has made it hard to construct a holographic dual of the state. To side step this problem, we argue that near the chiral restoration transition in the temperature-chemical potential plane, the strongly coupled gluons are likely completely gapped so that the colour quantum numbers of the quarks can be thought of below that gap as global indices. A standard AdS-superconductor model can then be used to analyze the fermionic gap formation. We investigate the role of four-fermion interactions, which might be used to include the gapped QCD interactions, on the vacuum and metastable vacua of the model. It turns out to be easiest to simply relate the standard interaction of the holographic superconductor to the strength of the gapped gluons. The result is a holographic description of the QCD colour superconducting phase diagram. We take a first look at how quark mass enters and causes a transition between the colour flavour locked phase and the 2SC phase.

hep-ph↗

arXiv2018

Holographic Gauged NJL Model: the Conformal Window and Ideal Walking

We study the holographic Dynamic AdS/QCD description of a SU($N_c$) non-abelian gauge theory with $N_f$ fermions in the fundamental representation which also have Nambu-Jona-Lasinio interactions included using Witten's multi-trace prescription. In particular here we study aspects of the dynamics in and near the conformal window of the gauge theory as described by the two loop running of the gauge theory. If the number of flavours is such that the IR fixed point lies with the anomalous dimension, $γ$, of the quark bilinear above one then chiral symmetry breaking occurs. Here we display a spiral in the mass - quark condensate plane describing a sequence of unstable excited states of the vacuum. An attractive NJL operator enhances the vacuum condensate but only an infinitely repulsive NJL interaction switches off the condensation completely. When $N_f$ changes so that the IR fixed point falls below one (the conformal window region) there is a numerical discontinuity in the phase structure with condensation only occurring with a super critical NJL interaction. In the conformal window, the running of $γ$ to a non-trivial IR fixed point is similar to walking dynamics, although chiral symmetry breaking is not triggered. In the "Ideal Walking" scenario, chiral symmetry is broken in that IR conformal regime by the NJL interaction, but the change in $γ$ enhances the UV condensate. That enhancement of the condensate is shown in an analytic model with a sharp change in $γ$ and we show equivalent numerical results for the case of the two loop running. In the model the $σ$ becomes massless as the gauge theory running becomes near conformal and we show it is possible to realize a light higgs-like state in Ideal Walking models.

hep-ph↗

arXiv2018

Jets in a strongly coupled anisotropic plasma

In this paper, we study the dynamics of the light quark jet moving through the static, strongly coupled $\mathcal{N}=4$, anisotropic plasma with and without charge. The light quark is presented by a point-like initial condition falling string in the context of the AdS/CFT. We calculate the stopping distance of the light quark in the anisotropic medium and compare it with its isotropic value. By studying the falling string in the beam direction and transverse direction, we find that the jet quenching increases in both directions. Although, the enhancement of quenching is larger in the beam direction. Also, the suppression of stopping distance is more prominent when the anisotropic plasma have the same temperature as the isotropic plasma.

hep-th↗

arXiv2017

Linearized Holographic Isotropization at Finite Coupling

We study holographic isotropization of an anisotropic homogeneous non-Abelian strongly coupled plasma in the presence of Gauss-Bonnet corrections. It was verified before that one can linearize Einstein's equations around the final black hole background and simplify the complicated setup. Using this approach, we study the expectation value of the boundary stress tensor. Although we consider small values of the Gauss-Bonnet coupling constant, it is found that finite coupling leads to significant increasing of the thermalization time. By including higher order corrections in linearization, we extend the results to study the effect of the Gauss-Bonnet coupling on the entropy production on the event horizon.

hep-th↗

arXiv2016

Holographic energy loss in non-relativistic backgrounds

In this paper, we study some aspects of energy loss in non-relativistic theories from holography. We analyze the energy lost by a rotating heavy point particle along a circle of radius $l$ with angular velocity $ω$ in theories with general dynamical exponent $z$ and hyperscaling violation exponent $θ$. It is shown that this problem provides a novel perspective on the energy loss in such theories. A general computation at zero and finite temperature is done and it is shown that how the total energy loss rate depends non-trivially on two characteristic exponents $(z,θ)$. We find that at zero temperature there is a special radius $l_c$ where the energy loss is independent of different values of $(θ,z)$. Also at zero temperature, there is a crossover between a regime in which the energy loss is dominated by the linear drag force and by the radiation because of the acceleration of the rotating particle. We find that the energy loss of the particle decreases by increasing $θ$ and $z$. We note that, unlike in the zero temperature, there is no special radius $l_c$ at finite temperature case.

hep-th↗

arXiv2016

Conductivity bound from dirty black holes

We propose a lower bound of the dc electrical conductivity in strongly disordered, strongly interacting quantum field theories using holography. We study linear response of black holes with broken translational symmetry in Einstein-Maxwell-dilaton theories of gravity. Using the generalized Stokes equations at the horizon, we derive the lower bound of the electrical conductivity for the dual two dimensional disordered field theory.

hep-th↗

arXiv2015

Entropic destruction of a moving heavy quarkonium

Recently it has been shown that the peak of the quarkonium entropy at the deconfinement transition is related to the emergent entropic force which destructs the quarkonium. Using the AdS/CFT correspondence, we consider dissociation of a moving heavy quarkonium by entropic force. We find that the entropic force destructs the moving quarkonium easier than the static case which is expected from perturbative weakly coupled plasma. By considering the Maxwell charge, we study the effect of medium on the destruction of heavy quarkonium. It is shown that the quarkonium dissociates easier in the medium.

hep-ph↗

arXiv2015

Energy loss at zero temperature from extremal black holes

Using the AdS/CFT correspondence, we probe the extremal black holes by studying the energy loss of a moving heavy point particle in a strongly-coupled boundary field theory at zero temperature and finite charge density. We first consider the extremal Reissner-Nordstr$\rm\ddot{o}$m-AdS (AdSRN) background black hole in the bulk and find an analytic solution of drag force which depends on the finite charge density of the boundary field theory. By studying the near horizon geometry of trailing string in the bulk, we show the IR divergency does not show the logarithmic behavior. We study also the stochastic behavior of the probe and find the Langevin diffusion coefficients. This study is extended to the extremal backgrounds with vanishing entropy density in appropriate extremal limits.

hep-th↗

arXiv2015

On Holographic Non-relativistic Schwinger Effect

Using the AdS/CFT correspondence, we study the Schwinger effect in strongly coupled theories with an anisotropic scaling symmetry in time and spatial direction. We consider Lifshitz and hyperscaling violation theories and use their gravity duals. It is shown that the shape of the potential barrier depends on the parameters of theory. One concludes that the production rate for the pair creation of particle and anti-particle will not be the same as the relativistic case.

hep-th↗

arXiv2015

The Imaginary Potential and Thermal Width of Moving Quarkonium from Holography

We study the effect of finite 't Hooft coupling corrections on the imaginary potential and the thermal width of a moving heavy quarkonium from the AdS/CFT correspondence. To study these corrections, we consider $\mathcal{R}^4$ terms and Gauss$-$Bonnet gravity. We conclude that the imaginary potential of a moving or static heavy quarkonium starts to be generated for smaller distances of quark and antiquark. Similar to the case of static heavy quarkonium, it is shown that by considering the corrections the thermal width becomes effectively smaller. The results are compared with analogous calculations in a weakly coupled plasma.

hep-th↗

arXiv2015

Thermal Width of Quarkonium from Holography

From the AdS/CFT correspondence, the effects of charge and finite 't Hooft coupling correction on the thermal width of a heavy quarkonium are investigated. To study the charge effect, we consider Maxwell charge which is interpreted as quark medium. In the case of finite 't Hooft coupling corrections, $\mathcal{R}^4$ terms and Gauss-Bonnet gravity have been considered, respectively. It is shown that these corrections affect the thermal width. It is also argued that by decreasing the 't Hooft coupling, the thermal width becomes effectively smaller. Interestingly, this is similar to analogous calculations in weakly coupled plasma.

hep-th↗

arXiv2013

The Imaginary Part of the Static Potential in Strongly Coupled Anisotropic Plasma

Using the gauge/gravity duality we study the imaginary part of the static potential associated to the thermal width in finite temperature strongly coupled anisotropic plasma. We firstly derive the potential for a generic anisotropic background. Then we apply our formulas to a theory where the anisotropy has been generated by a space dependent axion term. We find that using our method there exist a peculiar turning point in the imaginary part of the potential, similar to the one appearing in the real part. The presence of anisotropy leads to decrease of the imaginary potential, where larger decrease happens along the anisotropic direction when the temperature is kept fixed. When the entropy density is fixed, increase happens along the parallel direction while along the transverse plane we observe a decrease. To estimate the thermal width we use an approximate extrapolation beyond the turning point and we find a decrease in presence of the anisotropy, independently of the comparison scheme used.

hep-th↗

arXiv2012

Energy loss in a strongly coupled anisotropic plasma

We study the energy loss of a rotating infinitely massive quark moving, at constant velocity, through an anisotropic strongly-coupled N=4 plasma from holography. It is shown that, similar to the isotropic plasma, the energy loss of the rotating quark is due to either the drag force or radiation with a continuous crossover from drag-dominated regime to the radiation dominated regime. We find that the anisotropy has a significant effect on the energy loss of the heavy quark, specially in the crossover regime. We argue that the energy loss due to radiation in anisotropic media is less than the isotropic case. Interestingly this is similar to analogous calculations for the energy loss in weakly coupled anisotropic plasma.

hep-th↗

arXiv2012

Strange metals at finite 't Hooft coupling

In this paper, we consider the AdS-Schwarzshild black hole in light-cone coordinates which exhibits non-relativistic z=2 Schrodinger symmetry. Then, we use the $AdS/CFT$ correspondence to investigate the effect of finite-coupling corrections to two important properties of the strange metals which are the Ohmic resistivity and the inverse Hall angle. It is shown that the Ohmic resistivity and inverse Hall angle are linear and quadratic temperature dependent in the case of $\mathcal{R}^4$ corrections, respectively. While in the case of Gauss-Bonnet gravity, we find that the inverse Hall angle is quadratic temperature dependent and the Ohmic conductivity can never be linear temperature dependent.

hep-th↗

arXiv2010

Drag force in asymptotically Lifshitz spacetimes

We calculated drag force for asymptotically Lifshitz space times in (d + 2)-dimensions with arbitrary dynamical exponent $z$. We find that at zero and finite temperature the drag force has a non-zero value. Using the drag force calculations, we investigate the DC conductivity of strange metals.

hep-th↗

arXiv2010

Static Length Scales of N=6 Chern-Simons Plasma

Using gravity description, we compute various static length scales of N=6 Chern Simons plasma in a strongly coupled regime. For this, we consider the CP3 compactification of the type IIA supergravity down to four dimensions, and identify all the low-lying bosonic modes up to masses corresponding to the operator dimension 3 together with all the remaining CP3 invariant modes. We find the true mass gap, the Debye screening mass and the corresponding dual operators to be probed in the field theory side.

hep-th↗

arXiv2010

Charge effect and finite 't Hooft coupling correction on drag force and Jet Quenching Parameter

The effects of charge and finite 't Hooft coupling correction on drag force and jet quenching parameter are investigated. To study charge effect and finite 't Hooft coupling correction, we consider Maxwell charge and Gauss-Bonnet terms, respectively. The background is Reissner-Nordström-AdS black brane solution in Gauss-Bonnet gravity. It is shown that these corrections affect drag force and jet quenching parameter. We find an analytic solution of drag force in this background which depends on Gauss-Bonnet coupling and charge. We set Gauss-Bonnet coupling to be zero and find drag force in the case of Reissner-Nordström-AdS background. Also we discuss the relaxation time of a moving heavy quark in this gravity background.

hep-th↗