arXiv subjects

Si-wen Li

Publications and source records attributed to Si-wen Li.

At least 19 recordsLinked to original sources

arXiv2026

Holographic correlation functions of fermions in anisotropic plasma

By using the gauge-gravity duality, we study the holographic fermionic correlation functions in strongly coupled anisotropic plasmas. Starting from the isotropic black AdS background, we revisit the prescription for computing the retarded Green\textquoteright s function of a probe Dirac fermion and then generalize the formulas with respect to the anisotropic geometries. The method is applied to three distinct holographic models that capture different physical origins of anisotropy: axion-induced, magnetic-field-induced and unquenched-flavor-induced. Numerical results for the holographic correlation functions reveal direction-dependent corrections, negative dips in the imaginary part signalling vacuum instabilities (axion and magnetic field), Landau levels in the fermionic dispersion (magnetic field), and a momentum-independent pseudogap indicating an incoherent metallic phase (flavors). Our results complement and go beyond the hard thermal loop approximation, providing non-perturbative insights into fermionic excitations in strongly coupled anisotropic plasmas relevant for heavy-ion collisions and certain condensed matter systems.

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arXiv2026

Low-energy hadronic physics in holographic $\mathrm{QCD_{3}}$ with anisotropy

Using the gauge-gravity duality, we construct the anisotropic D3/D7 approach as a three-dimensional QCD-like theory, then investigate systematically the hadronic mass spectra, the dragging terms and the lowest hadronic interactions in the presence of the anisotropy in holography. Our derivation illustrates the dragging terms in the effective action are very necessary for an anisotropic theory since they are the key roles to affect the transport properties of the dual theory. And the numerical results in addition imply the hadronic system will become unstable if the anisotropy is much larger than its confinement energy scale. It agrees with that the confining phase in this approach becomes unstable if the anisotropy is sufficiently large in our previous works with this model. Therefore, this work is constructive to understand the anisotropy in the gauge field theory.

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arXiv2025

Memory effects in a dynamical decoupling process

We establish a simple quantitative relationship between the environmental memory effects and the characteristics in a dynamical decoupling process. In contrast to previous works, our measures of non-Markovianity are tailored and extended to evaluate the strength of memory effects in dynamical decoupling. We find that if each kick commutes with the dynamical map of the uncontrolled system, then the change of the final dynamical map or the final state brought by the control (called the "effect of control") is upper (lower) bounded by the summation (difference) of the strengths of memory effects with and without control. We propose sufficient conditions for the commutation relation for parity kicks and illustrate our finding with a dissipative quantum Rabi model by numerical simulations where one or many cycles of parity kicks are implemented on the qubit. Besides, the results show that under certain conditions, the effect of control or the increase of performance by the control may be simply proportional to the strength of memory effects with or without control.

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arXiv2025

Electromagnetic instability of vacuum with instantons in the holographic plasma

Using the gauge-gravity duality, we study the electromagnetic instability of vacuum with instantons in holographic plasma. The model we employ is the D(-1)-D3 brane system in which the D(-1)-branes correspond to the instantons in holography. To take into account the flavored quarks, the coincident probe D7-branes as flavors are embedded into the bulk geometry so that the effective electromagnetic Lagrangian with flavors corresponds to the action of the D7-branes according to gauge-gravity duality. We numerically evaluate the vacuum decay rate, the critical electric field and the V-A curve of the vacuum by using the D7-brane action with various values of the electromagnetic field. It implies the particles in the plasma acquire an effective mass in the presence of instantons as it is expected in the quantum field theory, and the plasma trends to become insulating when the electric field is small. This work reveals the relation between electromagnetic and instantonic properties of the vacuum in the plasma.

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arXiv2025

Holographic spectroscopy of fermion with instantons

Using the gauge-gravity duality, we investigate the fermionic spectroscopy in the D(-1)-D3 brane system. The background geometry of this system described by IIB supergravity includes a black (deconfined) and bubble (confined) D3-brane which corresponds respectively to a deconfined and a confined gauge theory in holography. The charge of the D(-1) brane as the D-instanton gives the gluon condensate in this model. To simplify the holographic setup, we first reduce briefly the ten-dimensional supergravity background produced by D(-1)-D3-branes to an equivalently five-dimensional background. Then the fermionic spectrum in the confined case is obtained by decomposing the fermion with dimensional reduction. In addition, by using the standard method for computing the Green function in the AdS/CFT dictionary, we derive the equations for the fermionic correlation functions and solve them numerically with the infalling boundary condition. Our numerical results in the deconfined case illustrate that the fermionic correlation function as spectral function includes two branches of the dispersion curves whose behavior is very close to the results obtained from the method of hard thermal loop. And the effective mass generated by the medium effect of fermion splits into two values due to the spin-dependent interactions induced by instantons. In the confined case, the holographic correlation function indicates several separated dispersion curves which illustrates consistently the onset mass in the fermionic spectrum we obtained. Therefore, this work in holography demonstrates the instantonic configuration is very influential to the fermion in QCD.

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arXiv2025

Worldvolume fermion as baryon with homogeneous instantons in holographic $\mathrm{QCD}_{3}$

(ArXiv version) We investigate holographically the effective theory of the worldvolume fermion on the flavor branes in the D3/D7 model with homogeneously smeared D(-1)-branes. As a top-down approach in gauge-gravity duality, the D(-1)-branes are instantons and violate the CP symmetry in the dual theory. In the confined geometry, we introduce a baryon vertex as a D5-brane wrapped on $S^{5}$, then identify the fermionic fluxes produced by the $N_{c}$ open strings on the D7-brane as a baryonic operator. Afterwards, we study the spectrum and the holographic correlation function of the baryonic fermion on the D7-branes. Remarkably, the fermionic spectrum is in agreement with the dispersion curves obtained from the confined correlation function, and the mass ratio of the lowest baryon and meson in our model is close to the associated experimental data. Moreover, the effective interaction terms of the holographic baryon and meson are derived and all the coupling constants take order of $N_{c}^{1/2}$ agreeing with the evaluation from the large $N_{c}$ field theory. In the deconfined geometry, the holographic correlation function is also evaluated numerically while the fermion on D7-brane is identified to plasmino instead of baryon. The dispersion curves from the deconfined correlation function basically covers the results from the hard thermal loop approximation and may imply the instanton-induced interaction with spin. Overall, this work constructs a holographic theory about baryonic fermion and mesonic boson with instantons or CP violation.

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arXiv2025

Chaos in the holographic matrix models for meson and baryon

In recent years, the investigation of chaos has become a bridge connecting gravity theory and quantum field theory, especially within the framework of gauge-gravity duality. In this work, we study holographically the chaos in the matrix models for meson and baryon, which are derived from the $\mathrm{D4}/\mathrm{D6}/\overline{\mathrm{D6}}$ approach as a top-down holographic model for QCD. Since these matrix models can be simplified into coupled oscillator models with special parameters, we analyze the chaos in the resultant coupled oscillators. In the analysis of the classical chaos, we calculate numerically the orbits on the Poincaré section, the Lyapunov exponent as a function of the total energy and derive the large $N_{c}$ behavior analytically, then discuss the possible phase structure both in the mesonic and baryonic matrix models. These analyses suggest that chaos might serve as an order parameter to detect the gauge theory with spontaneous breaking or restoration of symmetry. Besides, in the analysis of the quantum chaos, we demonstrate the numerical calculation of the OTOCs and analytically derive their large $N_{c}$ behavior by using the perturbation method in quantum mechanics. The numerical calculation illustrates there is a critical temperature, as a critical energy scale, that the OTOC begins to saturate, which covers qualitatively the classical analysis of the Lyapunov exponent. And the large $N_{c}$ analytics indicates the OTOCs are suppressed by the growth of $N_{c}$. Overall, the investigation of chaos in this work may be helpful to identify common features shared by the matrix models, hadronic physics, gauge theory, quantum mechanics, and gravity theory.

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arXiv2025

Spin polarization of holographic baryon in strongly coupled fluid

The spin polarization for baryon in a hydrodynamic medium has been extensively studied in the weakly coupled regime using quantum kinetic theory. As a first study of this problem in the strongly coupled regime, we investigate holographically the spectral function of a probe baryon in the fluid-gravity background. This is done by carefully performing gradient expansion of the Dirac equation in the fluid-gravity background. Different contributions in the expansion are understood in terms of density matrices of the probe baryon and the medium. The resulting spectral functions indicate that the holographic baryonic is polarized as responses to fluid acceleration, shear stress and vorticity. The structures of the responses are similar to those found in weakly coupled studies.

hep-th↗

arXiv2024

The worldvolume fermion as baryon in holographic QCD with instanton

In this work, we investigate the worldvolume fermion on the flavor brane in the D0-D4/D8 model which is holographically equivalent to the four-dimensional QCD with instantons, or equivalently with a theta angle. The action for the worldvolume fermion is obtained by the T-duality rules in string theory and we accordingly derive its effectively five-dimensional, canonical four-dimensional forms by using the systematical dimensional reduction and decomposition of spinor. Afterwards, we employ the AdS/CFT dictionary in order to evaluate the two-point correlation function as the spectral function for the worldvolume fermion and interpret the fermion as baryon by analyzing its quantum number with the baryon vertex in holography. In this sense, the interacted action involving the worldvolume fermion and gauge field on the flavor brane are finally derived in holography which describes the various interaction of meson and baryon with instantons in large-N limit. Therefore, this work provides a holographic picture to describe baryon and its interactions based on string theory, in particular, in the presence of instantons or a theta angle.

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arXiv2024

Out-of-time-order correlator as a detector of baryonic phase structure in holographic QCD with instanton

We study the out-of-time-order correlators (OTOC) of Skyrmion as baryon in the D0-D4/D8 model which is expected to be holographically dual to QCD with instantons as D0-branes or with a non-zero theta angle. Baryon states are identified to the excitations of the Skyrmion which are described by a holographic quantum mechanical system in this model. By employing the definition of OTOC in quantum mechanics, we derive the formulas and demonstrate explicitly the numerical calculations of the OTOC. Our calculation illustrates the quantum OTOC with imaginary Lyapunov coefficient indicates the possibly metastable baryonic status in the presence of the instanton while the classical OTOC can not, thus it reveals the instantonic or theta-dependent features of QCD are dominated basically by its quantum properties. Furthermore, the OTOC also implies the baryonic phase becomes really chaotic with real Lyapunov exponent if the instanton charge increases sufficiently which agrees with the unstable baryon spectrum presented in this model. In this sense, we believe the OTOC may be treated as a tool to detect the baryonic phase structure of QCD.

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arXiv2024

Correlation function of flavored fermion in holographic QCD

By using the gauge-gravity duality, we investigate the correlation function of flavored fermion in the \mathrm{D}_{p}/\mathrm{D}_{p+4} model as top-down approaches of holographic QCD for p=4,3. The bulk spinor, as the source of the flavored fermion in QCD, is identified to the worldvolume fermion on the flavor \mathrm{D}_{p+4}-branes and the standard form of its action can be therefore obtained by the T-duality rules in string theory. Keeping this in hand, we afterwards generalize the prescription for two-point correlation function in AdS/CFT dictionary into general D-brane backgrounds and apply it to the case of p=4,3, i.e. the D4/D8 and D3/D7 approach respectively. Resultantly, our numerical calculation with the bubble background always displays discrete peaks in the correlation functions which imply the bound states created by the flavored fermions as the confinement in QCD. With the black brane background, the onshell condition illustrated by the correlation function covers basically the dispersion curves of fermion obtained by the hard thermal loop approximation in the hot medium. Finally, we interpret the flavored fermions in the bubble background as baryons by taking into account a baryon vertex, then find the two-point correlation function is able to fit the lowest baryon spectrum. In this sense, we conclude remarkably that our top-down approach in this work could reveal the fundamental properties of QCD both in the confined and deconfined phase.

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arXiv2024

Out-of-time-order correlators of Skyrmion as baryon in holographic QCD

As the out-of-time-order correlator (OTOC) is a measure of quantum chaos and an important observable in the context of AdS/CFT, we investigate the OTOC of holographic Skyrmion which is described by an analytical quantum mechanical system from the D4/D8 model (as the holographic QCD). By employing the OTOC defined in quantum mechanics, we derive the formulas and demonstrate the numerical calculations of the OTOC explicitly which is also available for the general case with central force field. Our numerical evaluation illustrates the behaviors of the OTOC with large $N_{c}$, however the expected exponential growth of OTOC is not obtained. Besides, we also take a look at the classical limit of the OTOC and analyze the associated behaviors. At the end of this work, we additionally study the OTOC with three-dimensional Coulomb potential, as another example for the central force field, to support our analyses of the general properties of quantum OTOC.

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arXiv2023

The D4/D8 model and holographic QCD

As a top-down holographic approach, the D4/D8 model is expected to be the holographic version of QCD since it almost includes all the elementary features of QCD based on string theory. In this manuscript, we review the fundamental properties of the D4/D8 model with respect to the D4-brane background, embedding of flavor branes and holographic quark, gluon, meson, baryon and glueball with various symmetries, then we also take a look at some interesting applications and developments based on this model.

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arXiv2022

Corrections to the instanton configuration as baryon in holographic QCD

In this work, we first derive the corrections to the instanton configuration of the flavored gauge field in the D4-D8 model with generic flavor numbers. Then, as the instanton configuration on the D8-branes represents equivalently baryon in this model, keeping our corrections in hand, we systemically study the spectrum of baryon, heavy-light baryon or heavy-light meson and find it is possible to fit the experimental data with the meson data in this model. Besides, we briefly outline how to include the interaction of glueball and heavy-light meson or baryon with our corrections, evaluate numerically the decay rate of the heavy-light meson or baryonic matter involving glueball. Since it is possible to fit all the spectra with same choice of the parameters to the experimental data, we believe our corrections improve the framework of D4-D8 model and the corrected instanton configuration is also useful to investigate other properties of baryon in holography.

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arXiv2022

Holographic Schwinger effect and electric instability with anisotropy

According to the gauge-gravity duality, we systematically study the Schwinger effect and electric instability with anisotropy in a top-down holographic approach. The anisotropic black brane and bubble (soliton) background in IIB supergravity are employed and the dual theories in these backgrounds are expected to be anisotropic theory at finite temperature and anisotropic theory with confinement respectively. Then performing the potential analysis, we find due to the anisotropy, the potential barrier behaves oppositely with parallel and perpendicular electric fields, and this behavior agrees with the previous study about the quark potential with anisotropy in this system. Afterwards, we evaluate the pair production rate by solving the equation of motion for a fundamental string numerically which reveals the consistent behavior with the potential analysis. Furthermore, the probe D7-brane as flavor is introduced into the bulk in order to investigate the electric instability. The vacuum decay rate can be obtained by evaluating the imaginary part of the D7-brane action which again agrees with our potential analysis. Solving the associated constraint of gauge field strength on the flavor brane, we finally obtain the V-A curve displaying the distinct behavior of the conductivity in parallel and perpendicular direction which is in agreement with some bottom-up and phenomenologically holographic approaches in anisotropic fluid. Accordingly, we believe this work may be remarkable to study the electric features in strongly coupled anisotropic system.

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arXiv2022

Holographic $\mathrm{QCD}_{3}$ and Chern-Simons theory from anisotropic supergravity

Based on the gauge-gravity duality, we study the three-dimensional QCD ($\mathrm{QCD}_{3}$) and Chern-Simons theory by constructing the anisotropic black D3-brane solution in IIB supergravity. The deformed bulk geometry is obtained by performing a double Wick rotation and dimension reduction which becomes an anisotropic bubble configuration exhibiting confinement in the dual theory. And its anisotropy also reduces to a Chern-Simons term due to the presence of the dissolved D7-branes or the axion field in bulk. Using the bubble geometry, we investigate the the ground-state energy density, quark potential, entanglement entropy and the baryon vertex according to the standard methods in the AdS/CFT dictionary. Our calculation shows that the ground-state energy illustrates degenerate to the Chern-Simons coupling coefficient which is in agreement with the properties of the gauge Chern-Simons theory. The behavior of the quark tension, entanglement entropy and the embedding of the baryon vertex further implies strong anisotropy may destroy the confinement. Afterwards, we additionally introduce various D7-branes as flavor and Chern-Simons branes to include the fundamental matter and effective Chern-Simons level in the dual theory. By counting their orientation, we finally obtain the associated topological phase in the dual theory and the critical mass for the phase transition. Interestingly the formula of the critical mass reveals the flavor symmetry, which may relate to the chiral symmetry, would be restored if the anisotropy increases greatly. As all of the analysis is consistent with characteristics of quark-gluon plasma, we therefore believe our framework provides a remarkable way to understand the features of Chern-Simons theory, the strong coupled nuclear matter and its deconfinement condition with anisotropy.

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arXiv2022

Three-dimensional Yang-Mills Chern-Simons theory from D3-brane background with D-instantons

By constructing the configuration of D3-branes with D(-1)-branes as D-instantons, we study the three-dimensional Yang-Mills Chern-Simons theory in holography. Due to the presence of the D-instantons, the D7-branes with discrepant embedding functions are able to be introduced in order to include the fundamental fermions (as flavors) and the Chern-Simons term (at very low energy) in the dual theory. The vacuum structure at zero temperature is studied in the soliton background and it illustrates the topological phase transition in the presence of instantons. Moreover, since the confinement/deconfinement phase transition could be holographically identified as the Hawking-Page transition in the bulk, we accordingly calculate the critical temperature of the deconfinement phase transition by collecting the bulk onshell action as the thermodynamical free energy. On the other hand, we evaluate the difference of the entanglement entropy in slab configuration by using the RT formula since the confinement may also be characterized by the entanglement entropy. Altogether we find the behavior of the critical temperature is in qualitative agreement with the behavior of the critical length determined by the entanglement entropy which implies the entanglement entropy could indeed be a character of the confinement in our setup and the D3-D(-1) system would be a remarkable approach to study the three-dimensional gauge theory.

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arXiv2021

Holographic Schwinger effect in the confining background with D-instanton

Using the gauge-gravity duality, we study the holographic Schwinger effect by performing the potential analysis on the confining D3- and D4-brane background with D-instantons then evaluate the pair production/decay rate by taking account into a fundamental string and a single flavor brane respectively. The two confining backgrounds with D-instantons are obtained from the black D(-1)-D3 and D0-D4 solution with a double Wick rotation. The total potential and pair production/decay rate in the Schwinger effect are calculated numerically by examining the NG action of a fundamental string and the DBI action of a single flavor brane all in the presence of an electric field. In both backgrounds our numerical calculation agrees with the critical electric field evaluated from the DBI action and shows the potential barrier is increased by the presence of the D-instantons, thus the production/decay rate is suppressed by the D-instantons. Our interpretation is that particles in the dual field theory could acquire an effective mass through the Chern-Simons interaction or the theta term due to the presence of D-instantons so that the pair production/decay rate in Schwinger effect is suppressed since it behaves as $e^{-m^{2}}$. Our conclusion is in agreement with the previous results obtained in the deconfined D(-1)-D3 background at zero temperature limit and from the approach of the flavor brane in the D0-D4 background. In this sense, this work may be also remarkable to study the phase transition in Maxwell-Chern-Simons theory and observable effects by the theta angle in QCD.

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