Quark flavors in holographic QCD and the critical point

• Matti Jarvinen

Room: APC Laboratory give an overview of recent progress in implementing quark flavors and the strange quark mass in the holographic V-QCD model. The model, including its flavor dependence, is pinned down by a detailed comparison with (N_f=2+1) flavor lattice QCD results for the equation of state and quark number susceptibilities at small chemical potentials. Taking into account the flavor dependence and the strange quark mass significantly affects the results at finite density. I discuss implications for neutron star physics and the critical point in the QCD phase diagram as a function of temperature and baryon chemical potential. In beta equilibrium, the critical point is found at much smaller density than in earlier constructions. If one instead imposes constraints mimicking conditions relevant for heavy-ion collisions, the critical point disappears entirely.

Near-extremal holographic correlators

• Blaise Gouteraux

Room: APC Laboratory Near-extremal black holes with an AdS2 throat are of great interest in string theory and in holography due to their ubiquity as classical gravitational saddles. The emergent SL(2,R) symmetry associated to the throat plays an important role in their low-temperature physics. In this talk, I will describe recent progress on analytically computing holographic correlators in black hole spacetimes with a near-extremal AdS2xR2 near-horizon geometry, focusing on current-current and shear correlators. By improving on previous matching calculations, I will show how it is possible to obtain analytical approximations to the correlators that interpolate between the hydrodynamic (frequencies small compared to the temperature) and the non-hydrodynamic low-temperature regimes. The expressions we obtain capture the hydrodynamic poles, the gapped poles controlled by the SL(2,R) symmetry of the AdS2 throat and the successive collisions with them. I will also comment on the appearance of zero temperature gapless poles in the spectrum and their relation to the SL(2,R) spectrum.

Effective and holographic description of fluctuations in first order phase transitions

• Carlos Hoyos Badajos

Room: APC Laboratory When fluctuations are suppressed in first order phase transitions, bubble nucleation may be too slow to complete the transition, and the system may explore configurations beyond stable and metastable states. The system can exhibit qualitative differences in thermodynamic and dynamic properties depending on the strength of the coupling, or more precisely, on the number of degrees of freedom involved in the transition. We propose an effective theory coupling a fluid to a scalar order parameter to account for the observed behaviour. The effective theory is a generalization of fluid-scalar models used to describe the dynamics of bubbles in cosmological phase transitions. We test the validity of the effective theory against microscopic results using holography.

Flavor current correlators at finite isospin density: non-Abelian hydrodynamics and quasinormal modes

• Andrea Olzi

Room: APC Laboratory We study flavor current correlators in strongly coupled matter at finite quark and isospin chemical potentials using holography. Finite isospin density qualitatively modifies the low-energy dynamics of charged currents, combining diffusion with flavor precession. We analyze the corresponding quasinormal-mode spectrum and its interplay with the infrared modes associated with the emergent AdS_2 region of the near-extremal geometry. Finally, we discuss how this structure can be incorporated into an extended hydrodynamic description of the current correlators beyond the conventional hydrodynamic regime.

Self-sustained, out-of-equilibrium inflation

• Jorge Casalderrey Solana

Room: APC Laboratory We use holography to study dS-invariant states of non-conformal, strongly coupled quantum field theories in four-dimensional de Sitter space. We show that out-of-equilibrium effects can sustain exponential inflation within the regime of validity of semiclassical gravity, $H \ll M \ll M_{\text{sp}}$, where $H$ is the Hubble parameter, $M$ is the QFT characteristic scale, $M_{\text{sp}} = M_p / N$ is the species scale, $M_p$ is the Planck scale, and $N^2$ is the number of matter fields. In the holographic description, the required fine-tuning scales only logarithmically with $M_{\text{sp}} / H$, and the resulting solutions feature apparent horizons whose growing area reflects a continuous increase in comoving entropy density. Extending this framework to dynamical gravity, we demonstrate that this inflationary state emerges as the late-time attractor of an initial Friedmann--Lema\^itre--Robertson--Walker universe. Finally, we present preliminary results on a mechanism that terminates this inflationary epoch after a large number of e-foldings.

Gravitational Duals from Equations of State

• Pau Sole Villaro

Room: APC Laboratory

Non-conformal obstructions to bubble expansion

• Mikel Sanchez Garitaonandia

Room: APC Laboratory The hydrodynamics of expanding bubbles plays an important role in the study of gravitational wave emission from first-order phase transitions. It determines the possible expansion types and provides an estimate of the energy available for gravitational wave emission. By interpolating between electroweak- and QCD-like transitions, I will investigate the hydrodynamics of expanding bubbles in transitions with non-conformal thermodynamics. In doing so, I will uncover a new set of obstructions to bubble expansion and a new class of bubble solutions, both of which are absent in electroweak-like transitions, and I will discuss their impact on the space of solutions and the kinetic energy fraction.

Dense QCD in Neutron Stars: Twin Stars, Maximum Compactness, and Curvature

• Christian Ecker

Room: APC Laboratory I will discuss three recent model-agnostic studies of dense QCD in neutron stars, combining constraints from nuclear theory, perturbative QCD, and multimessenger observations. First, I will show that twin-star solutions are strongly disfavored by current data. The few surviving cases require highly fine-tuned EOS behavior and do not support the standard interpretation of twin stars as a robust signature of deconfinement. I will then ask how compact a neutron star can be, showing that the maximum-mass star is also the most compact one and that realistic neutron stars satisfy an approximate upper bound of 1/3, determined by perturbative-QCD constraints. Finally, I will discuss how dense-matter properties are reflected directly in spacetime geometry, including the appearance of negative Ricci curvature and its relation to the trace anomaly.

From pQCD to neutron-star cores: recent advances in equation-of-state inference

• Aleksi Vuorinen

Room: APC Laboratory I will discuss recent developments in high-order perturbative QCD calculations at high baryon density. These efforts have now reached the full next-to-next-to-next-to-leading order (N3LO) for the pressure of cold and dense QED matter and are approaching the same order for cold quark matter. The latter result is expected to substantially improve theoretical control over the high-density regime of the neutron-star-matter equation of state (EoS), with direct implications for model-agnostic EoS inference and for the fate of quark-matter cores inside massive neutron stars.

Tour de Holography: From c-theorems to spacetime reconstruction

• Niko Jokela

Room: APC Laboratory

Nuclear chiral density wave from field theory and holography

• Andreas Schmitt

Room: APC Laboratory Cold and dense QCD matter may break rotational invariance spontaneously. One anisotropic candidate phase is the chiral density wave, where the chiral condensate oscillates between scalar and pseudoscalar sectors. I will present results from a nucleon-meson model that allows for a chiral phase transition. Taking into account the conditions in the interior of neutron stars and constraints from astrophysical data this model predicts the chiral density wave to be absent from neutron star cores. I will compare these results to a calculation within holography, where the nuclear chiral density wave has been constructed recently for the first time.

Bubble Dynamics in Superheated Transitions

• Aldo Lorenzo Cotrone

Room: APC Laboratory First order phase transitions in superheated systems could be relevant in cosmological contexts and in neutron star mergers. I will discuss the bubble dynamics in a chiral symmetry restoring superheated transition in the Witten-Sakai-Sugimoto holographic model. I will show how to estimate the bubble nucleation rate and the bubble terminal velocity at zero chemical potential, and conclude with some results about the finite chemical potential case.

Learning quark-gluon plasma properties holographically with neural ODEs

• Song He

Room: APC Laboratory We use neural ODEs and state-of-the-art lattice QCD thermodynamics to calibrate a holographic Einstein-Maxwell-Dilaton model with Gauss-Bonnet corrections. The model quantitatively reproduces the equation of state at zero and finite baryon chemical potential and predicts the phase structure in the T-\mu_B plane, a temperature-dependent, and a peaked, while remaining consistent with thermodynamic constraints.

Holographic baryons in V-QCD and Tachyon-Chern-Simons action

• Jean-Loup Raymond

Room: APC Laboratory Chern-Simons terms are necessary to reproduce global flavor anomalies in holographic models of QCD. They are also required to model holographic baryons. We will present how the Chern-Simons action must be generalized in the presence of a bulk tachyon field (dual to the quark bilinear), extending earlier work restricted to flavor-independent quark masses. We will then extend recent work about massless-quarks V-QCD baryons and show that a single baryon compatible with generic quark masses is represented in the gravity dual by a point-like defect. The Tachyon-Chern-Simons action allows us to identify the baryon number with the generalized bulk instanton number. If time permits, we will show that this action produces a Wess-Zumino-Witten term in the boundary effective field theory for the pions and that the baryon current is identified to the boundary Skyrme current.

Beyond Quasinormal Modes: A Nonlinear Late‑Time Attractor for Black Branes and Viscous Fluids

• HaoTian Sun

Room: APC Laboratory