The Sakai-Sugimoto holographic model is famous for implementing the approximate chiral symmetry of QCD and reproducing the Chiral Lagrangian in a top-down approach. In this manuscript, we revisit the model in a formalism that is somewhat different from the original work by Sakai and Sugimoto: we start by identifying boundary degrees of freedom and splitting the bulk equations of motion into dynamical ones and constraints. The former are then solved to all orders in derivatives of the boundary fields. The constraints are left unsolved, leaving the dynamical degrees of freedom off-shell. This approach enables us to systematically derive the effective action of the boundary theory. The derived effective action is very rich in physics: it contains an U($\textit{N}$ ) multiplet of massless pseudoscalars interacting (via trilinear and higher terms) with towers of massive (axial-)vector mesons. In contrast to the previous studies, our effective action is non-local. The original Chiral Lagrangian is recovered as its local expansion in small $\textit{π}$-meson momenta (derivative expansion). We particularly zoom in on the values of four derivative terms couplings, the low energy constants, and compare those with the ones reported in the literature.

Israel

Michael Lublinsky

Timofey Solomko

Sakai-Sugimoto model in an off-shell: Chiral Lagrangian to all orders

We study the energy-momentum tensor (EMT) of a baryon in a top-down holographic quantum chromodynamics (QCD). In holographic QCD, the baryons are represented as solitons in a 5D gauge theory. We obtain the soliton solution by solving the equations of motion numerically. Using this result, the EMT and related quantities such as the mass, mean square radii, and the D-term (druck term) are computed. The evaluated D-term is about <inline-formula><tex-math id='TM0001' notation='LaTeX'>$-2.05$</tex-math></inline-formula>, whose absolute value is significantly larger than that in a previous work.

Japan

Shigeki Sugimoto

Taichi Tsukamoto

Energy-Momentum Tensor and D-Term of Baryons in Top-down Holographic QCD

We revisit the chiral spectra on charged 1- and 5-branes in the 10d non-supersymmetric $$ \mathfrak{sp}(16) $$ string theory (also known as the Sugimoto model), and verify that they consistently cancel the anomaly inflow induced by a Green-Schwarz mechanism in the bulk. By analyzing the $$ \mathfrak{sp}(16) $$ representations arising from quantizing the fermion zero modes on these branes as well as uncharged 4-branes, we find compelling evidence that the global structure of the gauge group is Sp(16)/$\textit{ℤ}$ . We further comment on a possible duality to non-supersymmetric heterotic strings, and explore bottom-up anomaly inflow constraints for 10d effective Sp(16)/$\textit{ℤ}$ gauge theories coupled to gravity.

Italy

Vittorio Larotonda

Ling Lin

Anomaly inflow and gauge group topology in the 10d Sugimoto string theory

We investigate the high-energy fixed-angle scattering of pions and $\textit{ρ}$-mesons in a bottom-up holographic QCD model. To this end, we generalise the approach of Polchinski and Strassler [1] to write an ansatz for meson scattering amplitudes based on superstring scattering amplitudes in asymptotically AdS space. We demonstrate that our generalisation of the Polchinski-Strassler proposal is necessary to describe $\textit{ρ}$-meson scattering consistently with the Nambu-Goldstone boson equivalence theorem. Our results for pion and $\textit{ρ}$-meson scattering amplitudes are in agreement with the constituent counting rule found in QCD. Moreover, our proposal for 2-to-2 scattering amplitudes provides a method for computing scattering angle dependence.

United Kingdom

Adi Armoni

Bartosz Pyszkowski

South Korea

Dorin Weissman

High-energy fixed-angle meson scattering and the constituent counting rule in holographic QCD

We consider the vector and the pseudovector glueball in the top-down holographic model of large-<math><mrow><msub><mrow><mi>N</mi></mrow><mrow><mi>c</mi></mrow></msub></mrow></math> QCD of Witten and their decays into ordinary mesons described by the D8 brane construction due to Sakai and Sugimoto. At leading order, the relevant interactions are determined exclusively by the Chern-Simons action of the D8 branes and are thus rigidly connected to the chiral anomaly and the Wess-Zumino-Witten terms. As found in a previous study of the pseudovector glueball, which we revisit and complete, the resulting decay widths are surprisingly large, implying that both the pseudovector and the vector glueball are very broad resonances, with a conspicuous dominance of decays into <math><msub><mi>a</mi><mn>1</mn></msub><mi>ρ</mi></math> and <math><msub><mi>K</mi><mn>1</mn></msub><mo>(</mo><mn>1400</mn><mo>)</mo><msup><mi>K</mi><mo>*</mo></msup></math> in the case of the vector glueball. We also obtain a certain weak mixing of vector glueballs with ordinary vector mesons, but we conclude that it does not provide an explanation for the so-called <math><mi>ρ</mi><mi>π</mi></math> puzzle in charmonium decays.

Austria

Florian Hechenberger

Josef Leutgeb

Anton Rebhan

Spin-1 glueballs in the Witten-Sakai-Sugimoto model

We consider dimensional reduction of cigar geometries which are obtained by a Wick rotation of black hole solutions. Originally the cigar geometry is smooth around the tip, but after the dimensional reduction along the Euclidean time direction, there appears an end-of-the-world brane (ETW brane). We derive the tension of the brane by two methods: bulk equations of motion and boundary equations of motion.

Yu-ki Suzuki

End of the world branes from dimensional reduction

We consider the Witten-Sakai-Sugimoto model in the approximation of smeared instantons at finite density via a homogeneous ansatz, which is known to be discontinuous in order to be able to contain a nonvanishing baryon density. The discontinuity at the infrared tip of the bulk spacetime gives rise to subtleties of discarding boundary terms that are normally discarded in the literature. We propose a reason for discarding this boundary term, by scrutinizing the currents and topological properties of the model. Along the way, we find a very effective and simple condition to compute the point of thermodynamic equilibrium.

China

Lorenzo Bartolini

Sven Bjarke Gudnason

Boundary terms in the Witten-Sakai-Sugimoto model at finite density

We utilize the top-down holographic QCD model, the Witten-Sakai-Sugimoto model, in a hybrid setting with the SLy4, soft chiral EFT and stiff chiral EFT equations of state to describe neutron stars with high precision. In particular, we employ a calibration that bootstraps the nuclear matter by fitting the Kaluza-Klein scale and the ’t Hooft coupling such that the physical saturation density and physical symmetry energy are achieved. We obtain static stable neutron star mass-radius data via the Tolman-Oppenheimer-Volkov equations that yield sufficiently large maximal masses of neutron stars to be compatible with the recently observed PSR-J0952-0607 data as well as all other known radius and tidal deformation constraints.

Sven Gudnason

Neutron stars in the Witten-Sakai-Sugimoto model

We calculate radiative decay rates of mesons and glueballs in the top-down holographic Witten-Sakai-Sugimoto model with finite quark masses. After assessing to what extent this model agrees or disagrees with experimental data, we present its predictions for so far undetermined decay channels. Contrary to widespread expectations, we obtain sizeable two-photon widths of scalar, tensor, and pseudoscalar glueballs, suggesting in particular that the observed two-photon rate of the glueball candidate <math><msub><mi>f</mi><mn>0</mn></msub><mo>(</mo><mn>1710</mn><mo>)</mo></math> is not too large to permit a glueball interpretation, but could be even much higher. We also discuss the so-called exotic scalar glueball, which in the Witten-Sakai-Sugimoto model is too broad to match either of the main glueball candidates <math><msub><mi>f</mi><mn>0</mn></msub><mo>(</mo><mn>1500</mn><mo>)</mo></math> and <math><msub><mi>f</mi><mn>0</mn></msub><mo>(</mo><mn>1710</mn><mo>)</mo></math> but might be of interest with regard to the alternative scenario of the so-called fragmented scalar glueball. Employing the exotic scalar glueball for the latter, much smaller two-photon rates are predicted for the ground-state glueball despite a larger total width; relatively large two-photon rates would then apply to the excited scalar glueball described by the predominantly dilatonic scalar glueball. In either case, the resulting contributions to the muon <math><mi>g</mi><mo>−</mo><mn>2</mn></math> from hadronic light-by-light scattering involving glueball exchanges are small compared to other single meson exchanges, of the order of <math><mo>≲</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>12</mn></mrow></msup></math>.

Radiative meson and glueball decays in the Witten-Sakai-Sugimoto model

Abstract The D-term is one of the conserved charges of hadrons defined as the forward limit of the gravitational form factor D(t). We calculate the nucleon’s D-term in a holographic quantum chromodynamics model in which the nucleon is described as a soliton in five dimensions. We show that the form factor D(t) is saturated by the exchanges of infinitely many 0$^{++}$ and 2$^{++}$ glueballs dual to transverse-traceless metric fluctuations on the Wick-rotated AdS$_{7}$ black hole geometry. We refer to this phenomenon as “glueball dominance,” in perfect analogy to the vector meson dominance of the electromagnetic form factors. However, the value at vanishing momentum transfer D(t = 0) can be interpreted as due to the exchange of pairs of pions and infinitely many vector and axial-vector mesons without any reference to glueballs. We find that the D-term is slightly negative as a result of a cancellation between the isovector and isoscalar meson contributions.