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Anton Rebhan

Publications and source records attributed to Anton Rebhan.

At least 37 records · Page 2Linked to original sources
arXiv2021

Hadronic light-by-light contribution to the muon $g-2$ from holographic QCD with massive pions

We extend our previous calculations of the hadronic light-by-light scattering contribution to the muon anomalous magnetic moment in holographic QCD to models with finite quark masses and a tower of massive pions. Analysing the role of the latter in the Wess-Zumino-Witten action, we show that the Melnikov-Vainshtein short-distance constraint is satisfied solely by the summation of contributions from the infinite tower of axial vector meson contributions. There is also an enhancement of the asymptotic behavior of pseudoscalar contributions when their infinite tower of excitations is summed, but this leads only to subleading contributions for the short-distance constraints on light-by-light scattering. We also refine our numerical evaluations, particularly in the pion and $a_1$ sector, which corroborates our previous findings of contributions from axial vector mesons that are significantly larger than those adopted for the effects of axials and short-distance constraints in the recent White Paper on the Standard Model prediction for $(g-2)_μ$.

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arXiv2021

Pseudoscalar transition form factors and the hadronic light-by-light contribution to the anomalous magnetic moment of the muon from holographic QCD

We revisit the predictions for the pseudoscalar-photon transition form factors in bottom-up and top-down holographic QCD models which only use the pion decay constant and the $ρ$ meson mass as input. We find remarkable agreement with the available experimental data for the single-virtual $π^0$ form factor that have recently been extended to lower momenta by BESIII, down to 0.3 GeV$^2$. The bottom-up models moreover turn out to be roughly consistent with recent experimental results obtained by BaBar for the double-virtual $η'$ form factor at large momenta as well as with a recent lattice extrapolation for the double-virtual $π^0$ form factor. Calculating the pion pole contribution to the hadronic light-by-light scattering in the anomalous magnetic moment of the muon, we find that the bottom-up models in question span the range $a_μ^{π^0}=6.1(4)\cdot 10^{-10}, which is somewhat lower than estimated previously by approximating these holographic predictions through simple interpolators, and in remarkably good agreement with recent results based on a dispersive approach or lattice simulations.

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arXiv2021

Central exclusive diffractive production of axial-vector $f_{1}$ mesons in proton-proton collisions

Exclusive production of axial-vector $f_{1}(1285)$ meson in proton-proton collisions via pomeron-pomeron fusion within the tensor-pomeron approach is discussed. Two ways to construct the pomeron-pomeron-$f_{1}$ coupling are presented. We adjust the parameters of our model to the WA102 experimental data and compare with predictions of the Sakai-Sugimoto model. Predictions for LHC experiments are given.

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arXiv2020

Axial vector transition form factors in holographic QCD and their contribution to the muon $g-2$

Whereas the theoretical results for the dominant contributions to hadronic light-by-light scattering coming from pseudoscalar meson exchange have converged over the past years, the various published estimates of the contribution due to axial vector meson exchange differ wildly. Since holographic AdS/QCD models have proved to provide rather good models of singly and doubly virtual pion transition form factors, which reproduce remarkably well the known low-energy data and also the asymptotic leading-order pQCD behavior, it is of interest to consider their predictions for axial vector transition form factors. Indeed, we find that these reproduce also very well existing data from the L3 experiment for $f_1\toγγ^*$. Including the full infinite tower of axial vector mesons of the AdS/QCD models we moreover show that the Melnikov-Vainshtein short-distance constraint can be satisfied, while almost all existing models for hadronic light-by-light scattering fail to incorporate it. The contribution to $g-2$, which is dominated by the first few resonances, turns out to be significantly larger than estimated previously.

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arXiv2020

Central exclusive diffractive production of axial-vector $f_{1}(1285)$ and $f_{1}(1420)$ mesons in proton-proton collisions

We present a study of the central exclusive diffractive production of the $f_{1}(1285)$ and $f_{1}(1420)$ resonances in proton-proton collisions. The theoretical results are calculated within the tensor-pomeron approach. Two pomeron-pomeron-$f_{1}$ tensorial couplings labeled by $(l,S) = (2,2)$ and $(4,4)$ are derived. We adjust the model parameters (coupling constants, cutoff constant) to the WA102 experimental data taking into account absorption effects. Both the $(l,S) = (2,2)$ and $(4,4)$ couplings separately allow one to describe the WA102 differential distributions. We compare these predictions with those of the Sakai-Sugimoto model, where the pomeron-pomeron-$f_{1}$ couplings are determined by the mixed axial-gravitational anomaly of QCD. We derive an approximate relation between the pomeron-pomeron-$f_{1}$ coupling constants of this approach and the $(l,S) = (2,2)$ and $(4,4)$ couplings. Then we present our predictions for the energies available at the RHIC and LHC. The total cross sections and several differential distributions are presented. Analysis of the distributions in the azimuthal angle $ϕ_{pp}$ between the transverse momenta of the outgoing protons may be used to disentangle $f_{1}$- and $η$-type resonances contributing to the same final channel. We find for the $f_{1}(1285)$ a total cross section $\sim 38$ $μ$b for $\sqrt{s} = 13$ TeV and a rapidity cut on the $f_{1}$ of $|{\rm y_{M}}| < 2.5$. We predict a much larger cross section for production of $f_{1}(1285)$ than for production of $f_{2}(1270)$ in the $π^{+}π^{-}π^{+}π^{-}$ decay channel for the LHC energies. This opens a possibility to study the $f_{1}(1285)$ meson in experiments planned at the LHC.

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arXiv2020

Hydrodynamic attractor of a hybrid viscous fluid in Bjorken flow

The nonequilibrium evolution in a boost-invariant Bjorken flow of a hybrid viscous fluid model containing two interacting components with different viscosities, such that they represent strongly and weakly self-coupled sectors, is shown to be characterized by a hydrodynamic attractor which has an early-time behavior that is reminiscent of the so-called bottom-up thermalization scenario in heavy-ion collisions. The hydrodynamization times for the two sectors can differ strongly, with details depending on the curve realized on the two-dimensional attractor surface, which might account for different scenarios for small and large systems in nuclear collisions. The total system behaves like a single viscous fluid with a dynamically determined effective shear viscosity.

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arXiv2019

Witten-Veneziano mechanism and pseudoscalar glueball-meson mixing in holographic QCD

We revisit the $U(1)_A$ anomaly in the holographic model of low-energy QCD by Witten, Sakai, and Sugimoto, presenting a new and direct derivation of the Witten-Veneziano mechanism for generating the mass of the $η'$ through an anomalous mixing of the Ramond-Ramond $C_1$ field with the singlet component of the pseudoscalar mesons. The latter turns out to have a kinetic mixing with the normalizable modes of the $C_1$ field representing pseudoscalar glueballs, yielding additional vertices for their production and their decay that dominate over those of the unmixed case considered previously in the Witten-Sakai-Sugimoto model. The leading channel is predicted to be decay into two vector mesons, followed in importance by decay into three pseudoscalar mesons. The issue of production of pseudoscalar glueballs in radiative $J/ψ$ decays and in double diffractive processes is also discussed briefly.

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arXiv2018

A broad pseudovector glueball from holographic QCD

We study the decay of a pseudovector glueball with quantum numbers $J^{PC}=1^{+-}$, the lightest glueball with spin different from zero or two, in the top-down holographic Witten-Sakai-Sugimoto model. This glueball is dual to the string-theoretic Kalb-Ramond tensor field, which completely fixes all its couplings by gauge invariance and $D$-brane anomaly inflow. The dominant decay channels are determined by a Chern-Simons-like action for the flavor branes; couplings from the Dirac-Born-Infeld action turn out to be subdominant. While the resulting width is parametrically of order $N_c^{-2}λ^{-1}$ for $N_c$ colors and 't Hooft coupling $λ$, when extrapolated to $N_c=3$ and with $λ$ matched to experimental data, the prediction is that the pseudovector glueball is an extremely broad resonance.

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arXiv2018

Hybrid Fluid Models from Mutual Effective Metric Couplings

Motivated by a semi-holographic approach to the dynamics of quark-gluon plasma which combines holographic and perturbative descriptions of a strongly coupled infrared and a more weakly coupled ultraviolet sector, we construct a hybrid two-fluid model where interactions between its two sectors are encoded by their effective metric backgrounds, which are determined mutually by their energy-momentum tensors. We derive the most general consistent ultralocal interactions such that the full system has a total conserved energy-momentum tensor in flat Minkowski space and study its consequences in and near thermal equilibrium by working out its phase structure and its hydrodynamic modes.

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arXiv2018

Time evolution of a toy semiholographic glasma

We extend our previous study of a toy model for coupling classical Yang-Mills equations for describing overoccupied gluons at the saturation scale with a strongly coupled infrared sector modeled by AdS/CFT. Including propagating modes in the bulk we find that the Yang-Mills sector loses its initial energy to a growing black hole in the gravity dual such that there is a conserved energy-momentum tensor for the total system while entropy grows monotonically. This involves a numerical AdS simulation with a backreacted boundary source far from equilibrium.

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arXiv2017

Predictions for production and decay of the pseudoscalar glueball from the Witten-Sakai-Sugimoto model

The Witten-Sakai-Sugimoto model, a top-down gauge/gravity model of large-$N_c$ low-energy QCD based on type-IIA string theory and $N_c$ D4 branes with chiral quarks added by probe D8 branes, involves only one dimensionless parameter, the 't Hooft coupling at its cutoff (Kaluza-Klein) scale. Although this cutoff scale is around 1 GeV, the model has turned out to be surprisingly predictive also quantitatively, reproducing masses of vector and axial vector mesons, their decay rates, as well as the anomalous mass of the η' meson, all within 10-30% errors. Using it as a guide for glueball signatures, we have argued that it indicates that the meson $f_0(1710)$ may be a nearly pure glueball, with rather specific predictions for the still-to-be-measured 4πand ηη' decays. Here we present our new predictions for the decay pattern of the (very narrow) pseudoscalar glueball, which is closely related to the U(1)$_A$ problem that the model is in fact handling correctly.

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arXiv2017

Holographic QCD predictions for production and decay of pseudoscalar glueballs

The top-down holographic Witten-Sakai-Sugimoto model for low-energy QCD, augmented by finite quark masses, has recently been found to be able to reproduce the decay pattern of the scalar glueball candidate f0(1710) on a quantitative level. In this Letter we show that this model predicts a narrow pseudoscalar glueball heavier than the scalar glueball and with a very restricted decay pattern involving eta or eta' mesons. Production should be either in pairs or in association with eta(') mesons. We discuss the prospect of discovery in high-energy hadron collider experiments through central exclusive production by comparing with eta' pair production.

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arXiv2016

Semi-Holography for Heavy Ion Collisions: Self-Consistency and First Numerical Tests

We present an extended version of a recently proposed semi-holographic model for heavy-ion collisions, which includes self-consistent couplings between the Yang-Mills fields of the Color Glass Condensate framework and an infrared AdS/CFT sector, such as to guarantee the existence of a conserved energy-momentum tensor for the combined system that is local in space and time, which we also construct explicitly. Moreover, we include a coupling of the topological charge density in the glasma to the same of the holographic infrared CFT. The semi-holographic approach makes it possible to combine CGC initial conditions and weak-coupling glasma field equations with a simultaneous evolution of a strongly coupled infrared sector describing the soft gluons radiated by hard partons. As a first numerical test of the semi-holographic model we study the dynamics of fluctuating homogeneous color-spin-locked Yang-Mills fields when coupled to a homogeneous and isotropic energy-momentum tensor of the holographic IR-CFT, and we find rapid convergence of the iterative numerical procedure suggested earlier.

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arXiv2016

Glueball Decay Rates in the Witten-Sakai-Sugimoto Model

We revisit and extend previous calculations of glueball decay rates in the Sakai-Sugimoto model, a holographic top-down approach for QCD with chiral quarks based on D8 probe branes in Witten's holographic model of nonsupersymmetric Yang-Mills theory. The rates for decays into two pions, two vector mesons, four pions, and the strongly suppressed decay into four pi0 are worked out quantitatively, using a range of the 't Hooft coupling which closely reproduces the decay rate of rho and omega mesons and also leads to a gluon condensate consistent with QCD sum rule calculations. The lowest holographic glueball, which arises from a rather exotic polarization of gravitons in the supergravity background, turns out to have a significantly lower mass and larger width than the two widely discussed glueball candidates f0(1500) and f0(1710). The lowest nonexotic and predominantly dilatonic scalar mode, which has a mass of 1487 MeV in the Witten-Sakai-Sugimoto model, instead provides a narrow glueball state, and we conjecture that only this nonexotic mode should be identified with a scalar glueball component of f0(1500) or f0(1710). Moreover the decay pattern of the tensor glueball is determined, which is found to have a comparatively broad total width when its mass is adjusted to around or above 2 GeV.

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arXiv2015

Constraints on the $ηη'$ decay rate of a scalar glueball from gauge/gravity duality

Predictions of glueball decay rates in the holographic Witten-Sakai-Sugimoto model for low-energy QCD can be uniquely extended to include finite quark masses up to an as yet undetermined parameter in the coupling of glueballs to the nonanomalous part of the pseudoscalar mass terms. The assumption of a universal coupling of glueballs to mass terms of the full nonet of pseudoscalar mesons leads to flavor asymmetries in the decay rates of scalar glueballs that agree well with experimental data for the glueball candidate $f_0(1710)$ and implies a vanishing decay rate into $ηη'$ pairs, for which only upper bounds for the $f_0(1710)$ meson are known at present from experiment. Relaxing this assumption, the holographic model gives a tight correlation between the decay rates into pairs of pseudo-Goldstone bosons of same type and $ηη'$ pairs. If $Γ(G\to KK)/Γ(G\toππ)$ is kept within the range reported currently by the Particle Data Group for the $f_0(1710)$ meson, the rate $Γ(G\toηη')/Γ(G\toππ)$ is predicted to be $\lesssim 0.04$. The corresponding situation for $f_0(1500)$ is also discussed, which however is found to be much less compatible with the interpretation of a largely unmixed glueball.

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arXiv2015

Glueball decay patterns in top-down holographic QCD

We discuss our results on scalar glueball decay in the top-down holographic Witten-Sakai-Sugimoto model for low-energy QCD and compare with available experimental data, which appear to disfavor the glueball candidate $f_0(1500)$ but seem to be perfectly consistent with interpreting $f_0(1710)$ as a nearly unmixed glueball. The holographic model moreover makes definite predictions for future experiments.

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arXiv2015

Glueball Decay in the Witten-Sakai-Sugimoto Model and Finite Quark Masses

We discuss recent results on the calculation of glueball decay rates in the Witten-Sakai-Sugimoto model, which favor the $f_0(1710)$ meson as a glueball candidate. The flavor asymmetric decay of $f_0(1710)$ is frequently attributed to a putative chiral suppression in glueball decays, which is however questionable in view of the large constituent quark masses induced by chiral symmetry breaking. We find that this can be explained by what we call nonchiral enhancement when finite quark masses are included in the holographic model, with good quantitative agreement with experimental data for $f_0(1710)$. Assuming the latter to indeed be a nearly pure glueball, the model makes essentially parameter-free and thus falsifiable predictions for its decay rates involving vector mesons and an upper limit on the $ηη'$ decay rate.

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