Soft-wall Holographic Model Of QCD Research Articles

Effect of charm quark on chiral phase transition in Nf=2+1+1 holographic QCD

We investigate the effect of the charm quark on the chiral phase transition of light quarks at finite temperature based on the four-flavor soft-wall holographic QCD model. In the massless limit, we find that the thermal chiral phase transition is of the second order in the four-quark-flavor system. In the case with the massive charm quark and the massless light and strange quarks, the order of the phase transition changes to the first order. This is due to the quark-flavor symmetry breaking which is associated with the violation of the U(1) axial symmetry. Once the light and strange quarks get massive, the explicit chiral symmetry breaking becomes eminent, and then the crossover phase transition is realized at the physical quark masses. We also map the order of the phase transition on a phase diagram in the quark-mass plane where the light- and strange-quark masses are degenerate but differ from the value of the charm-quark mass. This phase diagram is an extension of the conventional Columbia plot to the four-quark-flavor system. The critical exponents related to the chiral phase transition are also addressed. Published by the American Physical Society 2024

π0,η,η′ two-photon transition form factors in the holographic soft-wall model and contributions to (g−2)μ

We compute the two-photon transition form factors of the light pseudoscalar π0, η and η′ mesons in a soft-wall holographic model of QCD with a solution of the U(1)A problem. We compare the results with the experimental data in different ranges of photon virtualities. The obtained transition form factors are used to determine the π0, η and η′ pole terms in the hadronic light-by-light scattering contribution to the anomalous magnetic moment of the muon.

Chiral phase transition of QCD with Nf = 2 + 1 flavors from holography

Chiral phase transition for three-flavor Nf = 2 + 1 QCD with mu = md ≠ ms is investigated in a modified soft-wall holographic QCD model. Solving temperature dependent chiral condensates from equations of motion of the modified soft-wall model, we extract the quark mass dependence of the order of chiral phase transition in the case of Nf = 2 + 1, and the result is in agreement with the “Columbia Plot”, which is summarized from lattice simulations and other non-perturbative methods. First order phase transition is observed around the three flavor chiral limit mu/d = 0, ms = 0, while at sufficient large quark masses it turns to be a crossover phase transition. The first order and crossover regions are separated by a second order phase transition line. The second order line is divided into two parts by the mu/d = ms line, and the ms dependence of the transition temperature in these two parts are totally contrast, which might indicate that the two parts are governed by different universality classes.

A dynamical soft-wall holographic QCD model for chiral symmetry breaking and linear confinement

A self-consistent holographic QCD model is proposed which can realize both chiral symmetry breaking and confinement, two most important phenomena of QCD. It is pointed out that the model can accommodate both Regge spectra of hadrons and the linear potential between quarks. The model is formulated in the framework of graviton-dilaton-scalar system, where the dilaton field is dual to the dimension-2 gluon condensate and can lead to the linear confinement, while the scalar field corresponds to the quark anti-quark condensate and can explain the property of chiral dynamics.

Phase structure in a dynamical soft-wall holographic QCD model

We consider the Einstein-Maxwell-dilaton system with an arbitrary kinetic gauge function and a dilaton potential. A family of analytic solutions is obtained by the potential reconstruction method. We then study its holographic dual QCD model. The kinetic gauge function can be fixed by requesting the linear Regge spectrum of mesons. We calculate the free energy to obtain the phase diagram of the holographic QCD model and interpret our result as the heavy quarks system by comparing the recent lattice QCD simulation. We finally obtain the equations of state in our model.

Anomalous AV*V vertex in the soft-wall holographic model of QCD

We discuss the vertex function of two vector and one axial-vector operators in the soft-wall holographic model of QCD. When one of the two vector currents represents an on-shell soft photon, such a vertex is described by two structure functions wL and wT, which are usually calculated through triangular loop diagrams. We evaluate these functions in the soft-wall model of holographic QCD (HQCD) and compare the outcome to the QCD findings.

Temperature and quark density effects on the chiral condensate: an AdS/QCD study

We investigate the dependence of the chiral condensate $<\bar qq>$ on the temperature and quark density using the soft-wall holographic model of QCD, adopting geometries with black holes at finite temperature and quark chemical potential $\mu$. We find that, for $\mu$ below a critical value, increasing the temperature the condensate decreases and vanishes at a temperature $\tilde T\simeq 210$ MeV (at $\mu=0$). An analogous behaviour is observed increasing the chemical potential at fixed temperature. These results agree with the findings obtained by other methods. We also comment on the robustness of the results if geometries not involving black holes are adopted at low temperature, and an Hawking-Page transition is implemented.

In-medium hadronic spectral functions through the soft-wall holographic model of QCD

We study the scalar glueball and vector meson spectral functions in a hot and dense medium by means of the soft-wall holographic model of QCD. Finite temperature and density effects are implemented through the AdS/RN metric. We analyse the behaviour of the hadron masses and widths in the $(T,\mu)$ plane, and compare our results with the experimental ones and with other theoretical determinations.

AnomalousAV*Vvertex function in the soft-wall holographic model of QCD

We consider the vertex function of two vector and one axial-vector currents using the soft-wall holographic model of QCD with the Chern-Simons term. Two structure functions ${w}_{L}$ and ${w}_{T}$ describe such a vertex in the special case in which one of the two vector currents corresponds to an on-shell soft photon. We briefly review the QCD results for these functions, obtained from triangular loop diagrams with quarks having mass ${m}_{q}=0$ or ${m}_{q}\ensuremath{\ne}0$, we compute ${w}_{L}$ and ${w}_{T}$ in the soft-wall model and compare the outcome to the QCD findings. We also calculate and discuss the two-point ${\ensuremath{\Pi}}_{VV}\ensuremath{-}{\ensuremath{\Pi}}_{AA}$ correlation function, together with a few low-energy constants, which turn out to be close to the QCD results. Finally, we comment on a relation proposed by Son and Yamamoto between ${w}_{T}$ and ${\ensuremath{\Pi}}_{VV}\ensuremath{-}{\ensuremath{\Pi}}_{AA}$.

Holographic approach to finite temperature QCD: The case of scalar glueballs and scalar mesons

We study scalar glueballs and scalar mesons at $T \neq 0$ in the soft wall holographic QCD model. We find that, using the Anti-de Sitter-Black Hole metric for all values of the temperature, the masses of the hadronic states decrease and the widths become broader when T increases, and there are temperatures for which the states disappear from the scalar glueball and scalar meson spectral functions. However, the values of the temperatures in correspondence of which such phenomena occur are low, of the order of 40-60 MeV. A consistent holographic description of in-medium effects on hadron properties should include the Hawking-Page transition, which separates the phase with the Anti-de Sitter metric at small temperatures from the phase with Anti-de Sitter-Black Hole metric at high temperatures.

Holographic hadro-quarkonium

We consider the recently suggested model for some resonances near the open charm threshold as bound states of charmonium inside excited light mesons. It is argued in the soft-wall holographic model of QCD that such states of heavy quarkonium necessarily exist at sufficiently large spin of the light meson. The bound state is provided by the dilaton exchange through the 5D bulk. We also argue that the decay of such bound systems into mesons with open heavy flavors due to splitting of the heavy quarkonium can be treated as semiclassical tunneling and is suppressed. This behavior is in agreement with the known relative suppression of the decay of the discussed charmonium-like resonances into channels with D mesons.