We consider planar codimension-one defects and interfaces in $$ \mathcal{N} $$ = 4 supersymmetric Yang-Mills (SYM) theory, realized by the D3/D5-brane intersection. Working in the probe limit, where the number of D5-branes is small compared to the number of D3-branes, we obtain analytic results for the holographic entanglement entropy of a ball-shaped region centered on the defect. A defect renormalization group flow is triggered by giving the defect hypermultiplets a mass, which corresponds to separating the D3- and D5-branes. Along this flow the entanglement C-function decreases monotonically. We also allow the D5-branes to carry worldvolume flux corresponding to dissolved D3-branes, in which case the setup describes an interface between two copies of $$ \mathcal{N} $$ = 4 SYM theory with different gauge groups, where an RG flow is triggered by placing one side of the interface onto the Coulomb branch. Here we again find monotonic behavior of the entanglement C-function, although its interpretation as a measure of effective degrees of freedom is problematic. We investigate possible alternative measures of degrees of freedom.

Finland

Niko Jokela

Germany

Jani Kastikainen

Italy

José Penín

Sweden

Ronnie Rodgers

Helime Ruotsalainen

Entanglement C-functions of defects and interfaces in $$ \mathcal{N} $$ = 4 supersymmetric Yang-Mills theory

We revisit the holographic description of heavy-light mesons in the D3/D7 system at zero temperature, analyzing the dynamics of the coupled probe D7-branes through the non-Abelian Dirac-Born-Infeld action. Distinct quark masses are realized by separating the flavor branes, producing holographic flavor hierarchies. We refine the calculation made in previous works: we impose Hermiticity on the induced metric and fix the expansion of the determinant for matrix-valued fields. Implementing these improvements yields modified fluctuation equations and quantitatively different meson spectra: the scalar modes become heavier while the vector modes become lighter, removing the degeneracy reported in the literature. At finite ’t Hooft coupling, we also observe a qualitatively different dependence of the vector modes on the quark masses. The resulting prescription provides a consistent, broadly applicable framework for incorporating non-Abelian flavor dynamics into holographic models and can be readily extended to situations away from the vacuum.

Spain

Carlos Hoyos

Andrea Olzi

Subtleties of non-Abelian D-brane actions and their effect on holographic heavy-light meson spectra

<p>We present evidence for a spatially modulated instability within the deconfined quark-gluon plasma phase of QCD, based on robust predictions from holographic models. In these models, which are accurately fitted to lattice data, the instability is driven by the Chern-Simons term mandated by the flavor anomalies of QCD. Such an instability occurs universally across holographic models at surprisingly low densities, within the crossover region amenable to lattice and experimental studies, therefore inviting further explorations of inhomogeneous phases in this region.</p>

Poland

Netherlands

Tuna Demircik

China

South Korea

Matti Järvinen

Aleksi Piispa

Is holographic quark-gluon plasma homogeneous?

The nucleation of bubbles during a first-order phase transition has recently been explored using holographic duality, which can provide an important complement to standard perturbative methods. These computations typically require finding static and spatially inhomogeneous saddle points, known as critical bubbles, which correspond in the gravitational dual to solutions of nonlinear partial differential equations. A computationally simpler alternative is to use the gravitational dual to derive the effective action of the boundary theory in a derivative expansion, and then solve the resulting lower-dimensional equations of motion. Once the effective action, typically truncated at two derivatives, is obtained, the holographic theory can be set aside, and bubble solutions can be found from ordinary differential equations. In this paper, we test this approach in a simple holographic setup: a scalar field in the probe limit in a black brane background, with nonlinear multi-trace boundary conditions. We compute critical bubble solutions both from the effective action and by solving the scalar field equation of motion directly in the gravity theory, and find good agreement between the two methods.

Oscar Henriksson

Xin Li

Testing the effective action approach to bubble nucleation in holography

Entanglement entropy has proven to be a powerful tool for probing renormalization group (RG) flows in quantum field theories, with c-functions derived from it serving as candidate measures of the effective number of degrees of freedom. While the monotonicity of such c-functions is well established in many settings, notable exceptions occur in theories with a mass scale. In this work, we investigate entanglement c-functions in the context of holographic RG flows, with a particular focus on flows across dimensions induced by circle compactifications. We argue that in spacetime dimensions $\textit{d}$ ≥ 4, standard constructions of c-functions, which rely on higher derivatives of the entanglement entropy of either a ball or a cylinder, generically lead to non-monotonic behavior. Working with known dual geometries, we argue that the non-monotonicity stems not from any pathology or curvature singularity, but from a transition in the holographic Ryu-Takayanagi surface. In compactifications from four to three dimensions, we propose a modified construction that restores monotonicity in the infrared, although a fully monotonic ultraviolet extension remains elusive. Furthermore, motivated by entanglement entropy inequalities, we conjecture a bound on the cylinder entanglement c-function, which holds in all our examples.

United Kingdom

Carlos Nunez

Javier Subils

On entanglement c-functions in confining gauge field theories

<p>We reinterpret the Nieh-Yan (NY) anomaly using holography, finding that the U(1) axial symmetry remains unbroken and the axial current coupling to an external gauge field is conserved. Instead, the anomaly arises from a breakdown of Hodge duality relations between fermion bilinears due to symmetry constraints on one-form currents. We show that the axial response associated with the NY term is distinct from the chiral vortical effect and exhibits a characteristic <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>T</mi><mn>2</mn></msup></math> dependence. Torsion-induced axial transport decouples spin and axial charge dynamics, thereby clarifying the physical significance of the NY anomaly and motivating further field-theoretic and holographic studies.</p>

José Manuel Penín

Thermal response of the Nieh-Yan term

We investigate the reconstruction of asymptotically anti-de Sitter (AdS) bulk geometries from boundary entanglement entropy data for ball-shaped entangling regions. By deriving an explicit inversion formula, we relate variations in entanglement entropy to deviations of the bulk metric about a fixed background. Applying this formula, we recover the Schwarzschild-AdS spacetime in the low-temperature regime to first order. We further extend our analysis to include deformations of the bulk geometry with nontrivial dependence on boundary directions, and propose an iterative reconstruction scheme aimed at recovering the full spacetime starting close to a conformal fixed point. We do this by building on recent advances in the mathematics of inverse problems by introducing the higher-order linearization method as a new tool in the context of holographic bulk reconstruction.

Tony Liimatainen

Miika Sarkkinen

Australia

Leo Tzou

Bulk metric reconstruction from entanglement data via minimal surface area variations

We study the dynamics of a first-order phase transition in a strongly coupled gauge theory at non-zero temperature and chemical potential, computing nucleation rates and wall speeds from first principles. The gauge theory is the four-dimensional superconformal SU($\textit{N}$) × SU($\textit{N}$) Klebanov-Witten theory, which at low temperatures displays an instability to forming scalar condensates that higgses the theory. The computation is made possible by utilizing the gravity dual, type IIB string theory on asymptotically AdS × $\textit{T}$ spacetimes. The instability is detected through the nucleation and subsequent localization of D-branes in the bulk, which in the probe limit is amenable to calculations. The nucleation rates suggest a preference for greater asymmetry between the two gauge groups as the temperature is lowered beyond its critical value. The wall speed stays below the speed of sound of the conformal plasma across all parameter values and exhibits nonlinear dependence on the energy difference between the two phases.

Julia Junttila

Dynamics of a Higgs phase transition in the Klebanov-Witten theory

We investigate quark-gluon plasma at nonzero density by using two holographic models for QCD based on either brane or Einstein-Maxwell-dilaton actions. We determine the parameters of these models through a systematic statistical fitting procedure to lattice QCD data, which encompasses the equation of state (through the entropy density) and the two lowest baryon number susceptibilities. The predictions from the two models for the higher-order susceptibilities and the equation of state at nonzero density are strikingly similar. In particular, both models suggest the presence of a critical point on the <math><mrow><mo>(</mo><msub><mrow><mi>μ</mi></mrow><mrow><mi>B</mi></mrow></msub><mo>,</mo><mi>T</mi><mo>)</mo></mrow></math>-phase diagram near <math><msub><mi>μ</mi><mi>B</mi></msub><mo>/</mo><mi>T</mi><mo>≈</mo><mn>6</mn></math>. The results for the equation of state are in agreement with lattice data for higher-order susceptibilities and experimental data for the cumulants of the net-proton number from the beam energy scan program at the Relativistic Heavy-Ion Collider.

Refining holographic models of the quark-gluon plasma

Modern hydrodynamic simulations of core-collapse supernovae and neutron-star mergers require knowledge not only of the equilibrium properties of strongly interacting matter, but also of the system’s response to perturbations, encoded in various transport coefficients. Using perturbative and holographic tools, we derive here an improved weak-coupling and a new strong-coupling result for the most important transport coefficient of unpaired quark matter, its bulk viscosity. These results are combined in a simple analytic pocket formula for the quantity that is rooted in perturbative quantum chromodynamics at high densities but takes into account nonperturbative holographic input at neutron-star densities, where the system is strongly coupled. This expression can be used in the modeling of unpaired quark matter at astrophysically relevant temperatures and densities.

Mexico

Jesús Cruz Rojas

Tyler Gorda

Norway

Aleksi Kurkela

Risto Paatelainen

Saga Säppi

Aleksi Vuorinen

Estimate for the Bulk Viscosity of Strongly Coupled Quark Matter Using Perturbative QCD and Holography