arXiv subjects

Hiwa A. Ahmed

Publications and source records attributed to Hiwa A. Ahmed.

12 recordsLinked to original sources

arXiv2026

Medium effect on spin alignment of strange and charm vector mesons

Understanding the spin alignment of vector mesons in relativistic heavy-ion collisions requires a nonperturbative description of their spin-dependent in-medium properties. We investigate this problem within a unified four-flavor soft-wall holographic framework that combines an anisotropic Einstein--Maxwell--dilaton background at finite temperature, baryon chemical potential, and angular velocity. Spin alignment is determined from the medium-induced splitting of the spin-resolved vector-current spectral functions through an instantaneous freeze-out prescription. We systematically study the strange and charm vector mesons $K^{*}$, $ϕ$, $D^{*}$, $D_s^{*}$, and $J/ψ$ and the dependence of their spin alignment on transverse momentum, rapidity, temperature, baryon chemical potential, and angular velocity. We find that the heavy charm vector mesons $D^{*}$, $D_s^{*}$, and $J/ψ$ mesons exhibit $ρ_{00}>1/3$ at low transverse momentum, whereas the light strange vector mesons $K^{*}$ and $ϕ$ exhibit the opposite low-momentum behavior and angular distributions with $ρ_{00}<1/3$. We trace this flavor-dependent separation to the different locations of the vacuum mass shell relative to the thermally shifted longitudinal and transverse spectral peaks. The results qualitatively reproduce several trends observed at low and intermediate transverse momentum. Spin alignment is insensitive to baryon chemical potential and only weakly affected by angular velocity. These results establish an equilibrium holographic baseline for vector-meson spin alignment across flavor sectors and help delineate the regimes in which additional mechanisms, such as nonequilibrium evolution, fluctuations, and hard production, become important.

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arXiv2026

Effects of Born-Infeld Electrodynamics on Chiral Symmetry Restoration and Meson Susceptibilities in Holographic QCD

Within a holographic QCD framework, we numerically investigate chiral symmetry breaking and the associated phase transition at finite temperature and chemical potential. The model is constructed on a nonlinear charged Born-Infeld black hole background. The chiral condensate, extracted from the asymptotic behavior of the bulk scalar field, serves as the primary order parameter. At zero chemical potential, we find a chiral crossover transition for physical quark masses with a pseudocritical temperature of $T_{pc}=0.1477$ GeV. In the chiral limit, the transition becomes first-order with a critical temperature of $T_{c}=0.1337$ GeV. A critical strange quark mass of $m_s=37$ MeV, at zero light quark mass, separates first- and second-order transition regions. For finite chemical potential ($μ$) and a physical strange mass ($m_s=95$ MeV) with massless light quarks, the transition remains second-order, with $T_c$ decreasing as $μ$ increases. These results are further supported by the behavior of meson susceptibilities $(χ_π-χ_σ)$, which exhibit a rapid thermal decay and convergence across the phase boundary. Introducing the Born-Infeld parameter $β$ shifts the second-order phase boundary to higher temperatures for smaller $β$ (stabilizing the chirally broken phase) but does not alter the transition order or introduce a critical endpoint within the studied range. Our findings are consistent with previous soft-wall model studies and highlight the significant role of nonlinear bulk electrodynamics in modifying the chiral phase diagram.

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arXiv2026

Probing the chiral and $U(1)$ axial symmetry restoration via meson susceptibilities in holographic QCD

We investigate the restoration patterns of chiral and $U(1)$ axial symmetries at finite temperature using a soft-wall holographic QCD model. The study employs two distinct parameter sets (Case I and Case II), both calibrated to reproduce a pseudocritical temperature $T_{\rm pc} \sim 155$ MeV and the physical pion mass. The temperature dependence of the light and strange quark condensates confirms a smooth chiral crossover transition, with pseudocritical temperatures of $T_{\rm pc}=0.157$ GeV and $T_{\rm pc}=0.154$ GeV for Cases I and II, respectively. The screening masses of chiral partner mesons ($π$-$σ$ and $η$-$a_0$) become degenerate near $T_{\rm pc}$, providing a clear signature of chiral symmetry restoration. Analysis of the corresponding meson susceptibilities further supports this conclusion. However, the indicator for $U(1)$ axial symmetry restoration, $χ_π- χ_{a_0}$, vanishes at a temperature $T \sim 0.190 $ GeV, which indicates a distinct restoration scale with chiral symmetry restoration scale within the present holographic framework. The temperature-dependent topological susceptibility $χ_{\rm top}^{1/4}$ is also computed, showing a sharp drop near $T_{\rm pc}$ and a subsequent slight decrease. While the model qualitatively captures established features of the chiral transition, the results highlight a limitation in the qualitative description of the $U(1)$ axial anomaly compared to LQCD in our work.

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arXiv2025

Gluon polarization contribution to the spin alignment of vector mesons from holography

We investigate the behaviour of vector mesons $ρ$, $ϕ$, and $J/Ψ$ in both non-rotating and rotating thermal media using the soft-wall holographic QCD model with four flavours. By incorporating anisotropic backgrounds derived from the Einstein-Maxwell-dilaton action, we incorporate rotational effects via a $U(1)$ gauge field, and the induced polarization of gluons is described by a rotation dependent dilation field. Spectral function analysis reveals that $ρ$ and $ϕ$ mesons exhibit broad peaks at lower temperatures, indicating their presence in the medium, while these peaks disappear at higher temperatures. Rotation delays this melting process, increasing the dissociation temperature. In contrast, the $J/Ψ$ meson, owing to its heavy charm quark content, demonstrating its resilience to thermal effects. We further explore the global spin alignment of these mesons in the event plane frame. For the $ϕ$ meson, the averaged $ρ_{00}$ over the full range of azimuthal angle shows weak temperature dependence at low transverse momentum ($p_T$) but significant suppression at high $p_T$, aligning with experimental observations. Rotation enhances $ρ_{00}$ at high $p_T$, a phenomenon attributed to angular momentum transfer via spin-orbit coupling. The $J/Ψ$ meson, however, displays insensitivity to temperature and rotation up to $p_T=5$ GeV, with a very small suppression observed at higher $p_T$, likely due to its heavy quark nature. Although $ρ$ meson spin alignment is not yet experimentally measured, it exhibits behaviour qualitatively similar to the $ϕ$ meson, with thermal fluctuations dampening alignment and rotation enhancing it.

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arXiv2024

The effect of charm quark on chiral phase transition in $N_f=2+1+1$ holographic QCD

We investigate the effect of 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, 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.

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arXiv2023

$D_{(s)}-$ mesons semileptonic form factors in the 4-flavor holographic QCD

We investigate semileptonic form factors of $D_{(s)}$ meson from a modified soft-wall 4-flavor holographic model. The model successfully reproduces the masses and decay constants of various mesons, including $ρ$, $K^*$, $D^*$, $D_s^*$, $a_1$, $K_1$, $f_1$, $D_1$,$D_{s1}$, $π$, $K$, $η$, $D$, and $D_s$. Moreover, we study the semileptonic decay processes $D^{+} \to (π, K, η) l^{+} ν_{l}$ and $D_{s}^{+} \to ( K, η) l^{+} ν_{l}$, associated with the vector meson exchange, as well as $D_{(s)}^{+} \to K^{} l^{+} ν_{l}$, associated with the vector and axial vector meson exchange. The form factors $f_{+}(q^{2})$ for $D \toπ$ and $D_{(s)}\to K$ decays agree excellently with experimental and lattice data, outperforming other theoretical approaches. The $f_{+}(q^{2})$ form factor for $D^{+} \to η$ is compatible with experimental data, while a slight discrepancy is observed for $D_{s}^{+} \to η$ at large $q^{2}$. Additionally, we predict the vector form factors $V(q^{2})$ and $A_{1}(q^{2})$ for $D \to K^{}$ and $D_{s} \to K^{}$ decays, respectively. The results agree well with other approaches and lattice data at maximum recoil ($q^{2}=0$).

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arXiv2023

The electromagnetic form factors in the $N_{f}=4$ holographic QCD

In this study, we employ a modified soft-wall holographic model with four flavors to investigate the meson spectra, decay constants, electromagnetic form factors, and charge radius of various mesons. We obtain the spectra for vector, axial vector, and pseudoscalar mesons. Decay constants are calculated and compared with experimental and lattice QCD data. The pion and kaon electromagnetic form factors are compared with the experimental data, and a good agreement is achieved for the kaon at low $Q^{2}$. For the charmed mesons, the electromagnetic form factors of the $D$ and $D_{s}$ and electric form factors of the $D^{*}$ and $D_{s}^{*}$ are well consistent with the lattice QCD data. Moreover, the electric, magnetic, and quadrupole form factors are predicted for the $ρ$, $K^{*}$, $a_1$, $K_1$, $D_1$, and $D_{s1}$ mesons. Furthermore, the charge radius of the vector, axial vector, and pseudoscalars, including the strange and charmed mesons, are computed.

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arXiv2022

Study of $\bar{B}^0_{(s)}$ decaying into $D^0$ and $π^{+}π^{-}$, $K^{+}K^{-}$, or $π^{0}η$ with final state interactions

We investigate the resonant contributions in the decays $\bar{B}^0_{(s)} \to D^0 π^{+}π^{-}$, $\bar{B}^0_{(s)} \to D^0 K^{+}K^{-}$, and $\bar{B}^{0} \rightarrow D^{0} π^{0}η$ by considering the final state interactions within the chiral unitary approach. The scalar resonances produced dynamically from the final state interactions of these decays are the ones of $f_{0}(500)$, $f_{0}(980)$, and $a_{0}(980)$. In addition, the vector mesons $ρ$ and $ϕ$ produced in the $p$-wave are also taken into account. From the invariant mass distributions of $B^{0}$ decays, one can see that the contributions of the states $f_{0}(500)$ and $a_{0}(980)$ are remarkably larger than the one of $f_{0}(980)$. However, for the case of $B^{0}_{s}$ decays only a clear structure closed to the $K^{+} K^{-}$ threshold appears, which corresponds to the $f_{0}(980)$ state and has no signal for the $f_{0}(500)$ resonance. The dominant contributions are from the vector meson $ϕ$ for the decay $\bar{B}^0_s \to D^0 K^{+}K^{-}$, where the results of the $K^{+} K^{-}$ invariant mass distributions are in agreement with the experimental data up to $1.1$ GeV. Moreover, the branching fractions and the ratios of branching fractions are also investigated, where some of them are consistent with the experimental measurements, and the branching fraction $\text{Br}(\bar{B}^{0} \rightarrow D^{0} a_{0}(980) [a_{0}(980) \to π^{0}η])=(6.08 \pm 0.72)\times 10^{-5}$ and the ratio of branching fractions $\text{Br}(\bar{B}^{0}\rightarrow D^0 f_{0}(980))/\text{Br}(\bar{B}^{0}\rightarrow D^0 a_{0}(980))=0.13 \pm 0.03$ are predicted.

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arXiv2021

Scalar resonances in the final state interactions of the decays $D^{0} \rightarrow π^{0}π^{0}π^{0}, π^{0}π^{0}η,π^{0}ηη$

We investigate the scalar resonances in the processes of $D^{0} \rightarrow π^{0}π^{0}π^{0}, π^{0}π^{0}η, π^{0}ηη$ decays based on the chiral unitary approach for the final state interaction. We start from singly Cabibbo-suppressed production diagrams which provide a primary quark pair to hadronize two pseudoscalar mesons in $D^{0}$ decays. The resonances $f_{0}(500)$, $f_{0}(980)$ and $a_{0}(980)$ are dynamically produced from the final state interactions of the meson pairs. In our results, the experimental data for the $π^{0}η$ invariant mass spectrum of the $D^{0} \rightarrow π^{0}ηη$ decay can be described well. We also make the predictions for the $π^{0}π^{0}$ invariant mass spectrum of the $D^{0} \rightarrow π^{0}π^{0}π^{0}$, where the $f_{0}(980)$ can be found, and for the $π^{0}π^{0}$, $π^{0}η$ invariant mass spectrum of the $D^{0} \rightarrow π^{0}π^{0}η$, where the $f_{0}(500)$, $f_{0}(980)$ and $a_{0}(980)$ appear. Furthermore, the branching ratios of each decay channel are predicted. We expect more accurate measurements of these decays to better understand the nature of the states $f_{0}(500)$, $f_{0}(980)$ and $a_{0}(980)$.

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arXiv2021

Is two-poles' $Λ(1405)$ one state or two?

To understand the nature of two poles for the $Λ(1405)$ state, we revisit the interactions of $\bar{K}N$ and $πΣ$ with their coupled channels, where two-poles structure is found in the second Riemann sheet. We also dynamically generate two poles in the single channel interaction of $\bar{K}N$ and $πΣ$, respectively. Moreover, we make a further study of two poles' properties by evaluating the couplings, the compositeness, the wave functions, and the radii for the interactions of four coupled channels, two coupled channels and the single channel. Our results show that the nature of two poles is unique. The higher-mass pole is a pure $\bar{K} N$ molecule, and the lower-mass one is a compositeness of mainly $πΣ$ with tiny component $\bar{K} N$. From our results, one can conclude that the $Λ(1405)$ state would be overlapped with two different states of the same quantum number.

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arXiv2020

Study $B^{0}_{(s)}$ decays into $ϕ$ and a scalar or vector meson

In the present work, we investigate the decays of $B^{0}_{s} \rightarrow ϕπ^{+}π^{-}$ and $B^{0} \rightarrow ϕπ^{+}π^{-}$ with the final state interactions based on the chiral unitary approach. In the final state interactions of the $π^+π^-$ with its coupled channels, we study the effects of the $ηη$ channel in the two-body interactions for the reproduction of the $f_{0}(980)$ state. Our results for the $π^+π^-$ invariant mass distributions of the decay $B^{0}_{s} \rightarrow ϕπ^{+}π^{-}$ describe the experimental data up to 1 GeV well, with the resonance contributions from the $f_{0}(980)$ and $ρ$. For the predicted invariant mass distributions of the $B^{0} \rightarrow ϕπ^{+}π^{-}$ decay, we found that the contributions from the $f_{0}(500)$ are significant except for the ones from the $f_{0}(980)$ state. With some experimental branching ratios as input to determine the production vertex factors, we make some predictions for the branching ratios of the other final decay channels, including the vector mesons, in the $B^0_{(s)}$ decays, where some of them are consistent with the experimental ones within the uncertainties.

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arXiv2020

Study the molecular nature of $σ$, $f_{0}(980)$, and $a_{0}(980)$ states

We investigate the characteristics of $σ$, $f_{0}(980)$, and $a_{0}(980)$ with the formalism of chiral unitary approach. With the dynamical generation of them, we make a further study of their properties by evaluating the couplings, the compositeness, the wave functions and the radii. We also research their properties in the single channel interactions, where the $a_{0}(980)$ can not be reproduced in the $K\bar{K}$ interactions with isospin $I=1$ since the potential is too weak. In our results, the states of $σ$ and $f_{0}(980)$ can be dynamically reproduced stably with varying cutoffs both in the coupled channel and the single channel cases. We find that the $πη$ components is much important in the coupled channel interactions to dynamically reproduce the $a_{0}(980)$ state, which means that $a_{0}(980)$ state can not be a pure $K\bar{K}$ molecular state. We obtain their radii as: $|\langle r^2 \rangle|_{f_0(980)} = 1.80 \pm 0.35$ fm, $|\langle r^2 \rangle|_σ = 0.68 \pm 0.05$ fm and $|\langle r^2 \rangle|_{a_0(980)} = 0.94 \pm 0.09$ fm. Based on our investigation results, we conclude that the $f_{0}(980)$ state is mainly a $K\bar{K}$ bound state, the $σ$ state a resonance of $ππ$ and the $a_{0}(980)$ state a loose $K\bar{K}$ bound state. From the results of the compositeness, they are not pure molecular states and have something non-molecular components, especially for the $σ$ state.

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