arXiv subjects

Daiki Suenaga

Publications and source records attributed to Daiki Suenaga.

At least 19 recordsLinked to original sources

arXiv2026

Meson mass spectrum in isospin QCD medium from the $N_f=2+1$ quark-meson model

We study phase structures, meson mass spectra, and sound velocities at finite temperature and density in QCD with isospin chemical potential ($\mu_I$). We employ the quark-meson model with $N_f=2+1$, incorporating the Kobayashi-Maskawa-'t Hooft type coupling to capture dynamical effects of the $U(1)_A$ axial anomaly. Within the mean-field approximation at quark one-loop, we analyze the onset of pion condensation, which manifests as a second-order phase transition at low temperatures and may exhibit a first-order behavior at higher temperatures within this approximation. We examine the corresponding mass spectra of scalar and pseudoscalar singlet-octet mesons, in which the $\pi_+$ mass is exactly massless in the superfluid phase due to its Nambu-Goldstone boson nature. The neutral pion mass is, meanwhile, found to exhibit a strictly linear growth with $\mu_I$ in the pion condensed phase. We also investigate the isothermal squared sound velocity and identify characteristic structures associated with the phase transitions. Furthermore, we highlight how enhanced $U(1)_A$ anomaly effects facilitate the pion condensate to generate a less-pronounced sound velocity peak in cold medium. Our findings are expected to provide future lattice simulations with useful information on meson mass spectra from symmetry aspects.

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arXiv2026

FRG analysis of dense two-color QCD within the linear sigma model

We investigate the phase structure, hadron masses, and topological susceptibility in the two-flavor and two-color QCD (QC$_2$D) medium, particularly focusing on the $U(1)_A$ axial anomaly effects. To this end, we employ the linear sigma model, and hadron fluctuations are incorporated through the functional renormalization group method. We establish in detail an effective potential that respects symmetries of QC$_2$D at finite quark chemical potential, $\mu_q$: $SU(2)_L\times SU(2)_R$ chiral, $U(1)$ baryon-number, parity and time-reversal symmetries. We find that the $U(1)_A$ anomaly couplings for mesons at finite temperature are enhanced with increasing $\mu_q$, while that of the baryons are not too sensitive to $\mu_q$. Despite the anomaly enhancement, we find that the topological susceptibility at larger $\mu_q$ is always suppressed regardless of the temperature, following chiral restoration. We also find that mass degeneracies of the chiral partners are well realized at higher temperatures and densities by the chiral restoration. Our findings are expected to provide useful information on properties of the $U(1)_A$ anomaly in medium for sign-problem-free lattice simulations of QC$_2$D.

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arXiv2026

Hadron spectra of finite-density QC$_2$D

We investigate the chemical-potential dependence of hadron spectra in two-color QCD using first-principles lattice simulations. We compute two-point correlation functions for all allowed hadronic operators by newly including the contributions from disconnected diagrams, and extract the corresponding effective masses. In the meson sector, the mass hierarchy in the hadronic phase (normal vacuum) is found to be $m_\pi \lesssim m_{\eta} < m_\sigma \mathrm{(noisy)} < m_\rho \sim m_\omega \ll m_{a_1}$, which is similar to that in three-color QCD. In the superfluid phase, this hierarchy is modified, and with increasing density it changes to $m_\sigma \mathrm{(noisy)} < m_{a_1} < m_\rho < m_\pi \sim m_{\eta} \mathrm{(noisy)} \ll m_{\omega} \mathrm{(noisy)}$. In the diquark sector, the ordering remains as $m_{NG} \lesssim m_{I=0, S} < m_{I=1, AV} < m_{I=0, PS} \lesssim m_{I=0, V}$ in both phases, and the Nambu--Goldstone mode associated with spontaneous breaking of $U(1)_B$ is confirmed to be nearly massless. Furthermore, by comparing correlators for chiral partners, we find indications of chiral symmetry restoration at high density.

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arXiv2026

Inverse Excitation Hierarchy in Doubly-Heavy Tetraquarks within the Diquark Model

We investigate the $T_{cc}$ tetraquark, treating it as a bound state of a heavy diquark and a light antidiquark. Using the Silvestre-Brac potential and solving the Schr\"odinger equation via the Gaussian Expansion Method, we find that the excitation energy between the heavy diquark and light antidiquark is unexpectedly larger than that between the two light anti-quarks within the anti-diquark -- contrary to the naive expectation where the former is smaller than the latter. We trace this inversion of the mass hierarchy to the centrifugal force acting on the light degree of freedom. Applying the same framework to other systems ($T_{bb}, \Lambda_b, \Lambda_c$) yields qualitatively identical behavior, demonstrating the robustness of the mechanism. These results provide new insights into diquark dynamics and the mass structure of exotic hadrons.

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arXiv2025

Speed of sound exceeding the conformal bound in dense QCD-like theories

We investigated the phase structure and the equation of state (EoS) for dense two-color QCD at low temperatures using the lattice Monte Carlo simulations. A rich phase structure below the pseudo-critical temperature $T_c$ as a function of quark chemical potential has been revealed. In a high-density regime, we can see a superfluid phase, where the diquark condensate takes a non-zero expectation value. We have newly found that the speed of sound exceeds the conformal bound, which is the value of the relativistic free theory. This talk is based on Refs.~\cite{Iida:2022hyy, Iida:2024irv, Itou:2025vcy}.

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arXiv2025

Thermal quarks and Polyakov loops in two-color dense QCD

We study confinement and deconfinement in dense two color QCD by analyzing the dynamics of thermal quarks and gluons. The Polyakov loop is used as a probe of the relevant thermal excitations, distinguishing quark and hadron dominated regimes in dense matter. To describe the Polyakov loop, we adopt both lattice informed phenomenological models and the massive Yang Mills framework. After calibrating these models at zero density, we investigate in medium modifications of the Polyakov loops and gluon propagators at finite temperature and density. Diquark gaps control the screening at zero temperature, whereas the screening due to thermal quarks is sensitive to the Polyakov loop. Inclusion of the Polyakov loop helps to reproduce lattice data at low temperature, suggesting that thermal excitations are predominantly hadronic rather than uncorrelated quarks.

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arXiv2025

Roles of $U(1)$ axial anomaly effects in cold and dense two-color QCD with $2+2$ flavors

We explore phase structures and hadron mass spectra in cold and dense two-color QCD with $2+2$ flavors where the sign problem disappears. We particularly focus on $U(1)$ axial anomaly effects. We employ an $N_f=2+2$ linear sigma model based on the $SU(8)$ Pauli-G\"{u}rsey symmetry to describe negative-parity as well as positive-parity hadrons, for which low-energy excitations are appropriately described particularly in the baryon superfluid phase with light diquark condensates. As a result, the strange chiral condensate is found to be enhanced in the superfluid phase owing to the flavor-mixing structure of $U(1)$ axial anomaly effects. We also confirm this enhancement by means of the Nambu--Jona-Lasinio model. Besides, our present analysis predicts the existence of a novel superfluid phase where heavy diquarks condense in dense regime. The topological susceptibility with $2+2$ flavors from the viewpoints of the anomaly effects and chiral-symmetry restoration is investigated. Furthermore, we derive a complete inverse mass hierarchy for negative-parity diquarks with sufficient anomaly effects. Our findings are expected to provide future lattice simulations with useful information on flavor-symmetry violation from the $U(1)$ axial anomaly aspects in cold and dense two-color QCD medium.

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arXiv2025

Enhancement of axial anomaly effects in hot two-color QCD: FRG approach in the linear sigma model

We investigate the thermal properties of hadrons in two-color quantum chromodynamics (QC$_2$D) using the functional renormalization group (FRG) method, with particular focus on modifications of the $U(1)$ axial anomaly effects. The hadrons are described by a linear sigma model (LSM) based on the Pauli-G\"{u}rsey $SU(4)$ symmetry, which incorporates both low-lying $0^\pm$ mesons and diquark baryons. We find that all quartic couplings are comparably suppressed when physical values of the pion mass and decay constant are taken as inputs, for which a reasonably smooth chiral symmetry restoration at finite temperature is reproduced. Consequently, mass differences among chiral partners remain small. Despite these tiny mass differences, mass degeneracies of chiral partners in the hot medium are clearly demonstrated, consistent with chiral symmetry restoration. Moreover, we find that the couplings responsible for the $U(1)$ axial anomaly are enhanced upon entering the finite temperature regime. Baryonic fluctuations also provide sizable contributions to these enhancements. Finally, the fate of the topological susceptibility in the hot QC$_2$D medium is examined.

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arXiv2025

Fate of $Σ_c$, $Ξ_c'$ and $Ω_c$ baryons at high temperature with chiral restoration

Masses of the singly heavy baryons (SHBs), composed of a heavy quark and a light diquark, are studied from the viewpoints of heavy-quark spin symmetry (HQSS) and chiral-symmetry restoration at finite temperature. We consider the light diquarks with spin-parity $J^P=0^\pm$ and $1^\pm$. Medium corrections to the SHBs are provided through the diquarks whereas the heavy quark is simply regarded as a spectator. The chiral dynamics of the diquark are described by the Nambu-Jona-Lasinio (NJL) model having (pseudo)scalar-type and (axial)vector-type four-point interactions and the six-point ones responsible for the $U(1)_A$ axial anomaly. The divergences are handled by means of the three-dimensional proper-time regularization with both ultraviolet and infrared cutoffs included, in order to eliminate unphysical imaginary parts. As a result, the mass degeneracies between the parity partners of all the SHBs are predicted in accordance with the chiral restoration. In particular, the HQS-doublet SHBs exhibit clear mass degeneracies due to the absence of the direct anomaly effects. We also predict a mass degeneracy of $Σ_c$ and $Ω_c$ above the pseudocritical temperature $T_{\rm pc}$ of chiral restoration, which results in a peculiar mass hierarchy for positive-parity HQS-doublet SHBs where $Ξ_c'$ becomes heavier than $Ω_c$ Besides, it is found that the decay width of $Σ_c\toΛ_cπ$ vanishes above $T_{\rm pc}$ reflecting a closing of the threshold. The predicted modifications of masses and decay widths of the SHBs are expected to provide future heavy-ion collision experiments and lattice simulations with useful information on chiral dynamics of the diquarks.

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arXiv2025

Anisotropic pressure and novel first-order phase transition in SU(3) Yang-Mills theory on $\mathbb{T}^2\times\mathbb{R}^2$

We investigate the thermodynamic behavior and phase diagram of $SU(3)$ Yang-Mills theory on $\mathbb{T}^2 \times \mathbb{R}^2$ in Euclidean spacetime using an effective model. In our approach, the Polyakov loops along the compactified directions are treated as dynamic variables, and the model is calibrated to match lattice simulation results for thermodynamic observables on $\mathbb{T}^2 \times \mathbb{R}^2$. Our analysis reveals a novel first-order phase transition in the deconfined phase that ends at critical points, which appear to belong to the two-dimensional $Z_2$ universality class. This transition is driven by the interplay between the two Polyakov loops, introduced via a cross-term in the Polyakov-loop potential.

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arXiv2025

Novel first-order phase transition and critical points on $SU(3)$ Yang-Mills theory in $\mathbb{T}^2\times\mathbb{R}^2$

We investigate the thermodynamics and phase structure of $SU(3)$ Yang-Mills theory on $\mathbb{T}^2\times\mathbb{R}^2$ with anisotropic spatial volumes in Euclidean spacetime in lattice numerical simulations and an effective model. In lattice simulations, the energy-momentum tensor defined through the gradient flow is used for the analysis of the stress tensor on the lattice. It is found that a clear pressure anisotropy is observed only at a significantly shorter spatial extent compared with the free scalar theory. We then study the thermodynamics obtained on the lattice in an effective model that incorporates two Polyakov loops along two compactified directions as dynamical variables. The model is constructed to reproduce thermodynamics measured on the lattice. The model analysis indicates the existence of a novel first-order phase transition and critical points as its endpoints. We argue that the interplay of the Polyakov loops induces the first-order transition.

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arXiv2025

Chiral effective model of cold and dense two-color QCD: The linear sigma model approach

This review is devoted to summarizing recent developments of the linear sigma model (LSM) in cold and dense two-color QCD (QC$_2$D), in which lattice simulations are straightforwardly applicable thanks to the disappearance of the sign problem. In QC$_2$D, both theoretical and numerical studies derive the presence of the so-called baryon superfluid phase at sufficiently large chemical potential ($μ_q$), where diquark condensates govern the ground state. The hadron mass spectrum simulated in this phase shows that the mass of an iso-singlet ($I=0$) and $0^-$ state is remarkably reduced, but such a mode cannot be described by the chiral perturbation theory. Motivated by this fact, I invent the LSM constructed upon the linear representation of chiral symmetry, or more precisely the Pauli-Gürsey symmetry. Then, it is shown that my LSM successfully reproduces the low-lying hadron mass spectrum in a broad range of $μ_q$ simulated on the lattice. As applications of the LSM, topological susceptibility and sound velocity in cold and dense QC$_2$D are evaluated to compare with lattice results. Besides, generalized Gell-Mann-Oakes-Renner relation and hardon mass spectrum in the presence of a diquark source are analyzed. I also introduce an extended version of the LSM incorporating spin-$1$ hadrons.

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arXiv2024

Phase and equation of state of finite density QC$_2$D at lower temperature

We investigate the phase structure and the equation of state (EoS) for dense two-color QCD at low temperatures, $T = 40$ MeV ($32^4$ lattice) and $T = 80$ MeV ($16^4$ lattice). A rich phase structure below the pseudo-critical temperature $T_c$ as a function of quark chemical potential $μ$ has been revealed. By performing $T = 40$ MeV simulations, essentially similar results to the previous ones at $T = 80$ MeV are obtained, but several finer understandings are achieved. Breaking of the conformal bound is also confirmed thanks to smaller statistical errors. This talk is mainly based on Refs.~\cite{Iida:2022hyy, Iida:2024irv}. It also includes related studies and subsequent developments that were not mentioned in the original papers.

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arXiv2024

Novel first-order phase transition and critical points in SU(3) Yang-Mills theory with spatial compactification

We investigate the thermodynamics and phase structure of $SU(3)$ Yang-Mills theory on $\mathbb{T}^2\times\mathbb{R}^2$ in Euclidean spacetime in an effective-model approach. The model incorporates two Polyakov loops along two compactified directions as dynamical variables, and is constructed to reproduce thermodynamics on $\mathbb{T}^2\times\mathbb{R}^2$ measured on the lattice. The model analysis indicates the existence of a novel first-order phase transition on $\mathbb{T}^2\times\mathbb{R}^2$ in the deconfined phase, which terminates at critical points that should belong to the two-dimensional $Z_2$ universality class. We argue that the interplay of the Polyakov loops induced by their cross term in the Polyakov-loop potential is responsible for the manifestation of the first-order transition.

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arXiv2024

Lattice study on finite density QC$_2$D towards zero temperature

We investigate the phase structure and the equation of state (EoS) for dense two-color QCD (QC$_2$D) at low temperature ($T = 40$ MeV, $32^4$ lattice) for the purpose of extending our previous works~\cite{Iida:2019rah, Iida:2022hyy} at $T=80$ MeV ($16^4$ lattice). Indeed, a rich phase structure below the pseudo-critical temperature $T_c$ as a function of quark chemical potential $μ$ has been revealed, but finite volume effects in a high-density regime sometimes cause a wrong understanding. Therefore, it is important to investigate the temperature dependence down to zero temperature with large-volume simulations. By performing $32^4$ simulations, we obtain essentially similar results to the previous ones, but we are now allowed to get a fine understanding of the phase structure via the temperature dependence. Most importantly, we find that the hadronic-matter phase, which is composed of thermally excited hadrons, shrinks with decreasing temperature and that the diquark condensate scales as $\langle qq \rangle \propto μ^2$ in the BCS phase, a property missing at $T=80$ MeV. From careful analyses, furthermore, we confirm a tentative conclusion that the topological susceptibility is independent of $μ$. We also show the temperature dependence of the pressure, internal energy, and sound velocity as a function of $μ$. The pressure increases around the hadronic-superfluid phase transition more rapidly at the lower temperature, while the temperature dependence of the sound velocity is invisible. Breaking of the conformal bound is also confirmed thanks to the smaller statistical error.

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arXiv2024

Isospin QCD as a laboratory for dense QCD

QCD with the isospin chemical potential, $μ_I$, is a useful laboratory to delineate the microphysics in dense QCD. To study the quark-hadron-continuity we use a quark-meson model that interpolates hadronic and quark matter physics at microscopic level. The equation of state is dominated by mesons at low density but taken over by quarks at high density. We extend our previous studies with two-flavors to the three-flavors case to study the impact of the strangeness which may be brought by kaons $(K_+, K_0) = (u\bar{s}, s\bar{d})$ and the U$_A$(1) anomaly. In the normal phase the excitation energies of kaons are reduced by $μ_I$ in the same way as hyperons in nuclear matter at finite baryon chemical potential. Once pions condense, kaon excitation energies increases as $μ_I$ does. Moreover, strange quarks become more massive through the U$_A$(1) coupling to the condensed pions. Hence at zero and low temperature the strange hadrons and quarks are highly suppressed. The previous findings in two-flavor models, sound speed peak, negative trace anomaly, gaps insensitve to $μ_I$, persist in our three-flavor model and remain consistent with the lattice results to $μ_I \sim 1$ GeV. We discuss the non-perturbative power corrections and quark saturation effects as important ingredients to understand the crossover equations of state measured on the lattice.

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arXiv2024

Sound velocity peak induced by the chiral partner in dense two-color QCD

Recently, the peak structure of the sound velocity was observed in the lattice simulation of two-color and two-flavor QCD at the finite quark chemical potential. The comparison with the chiral perturbation theory (ChPT) result was undertaken, however, the ChPT failed in reproducing the peak structure. In this study, to extend the ChPT framework, we incorporate contributions of the $σ$ meson, that is identified as the chiral partner of pions, on top of the low-energy pion dynamics by using the linear sigma model (LSM). Based on the LSM we derive analytic expressions of the thermodynamic quantities as well as the sound velocity within a mean-field approximation. As a result, we find that those quantities are provided by sums of the ChPT results and corrections, where the latter is characterized by a mass difference between the chiral partners, the $σ$ meson and pion. The chiral partner contributions are found to yield a peak in the sound velocity successfully. We furthermore show that the sound velocity peak emerges only when $m_σ>\sqrt{3}m_π$ and $μ_q > m_π$, with $m_{σ(π)}$ and $μ_q$ being the $σ$ meson (pion) mass and the quark chemical potential, respectively. The correlation between the sound velocity peak and the sign of the trace anomaly is also addressed.

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arXiv2024

QCD Kondo effect for single heavy quark in chiral-symmetry broken phase

We consider the quantum chromodynamics (QCD) Kondo effect for a single heavy quark in quark matter composed of light quarks with chiral symmetry breaking. Introducing several spinor structures in QCD Kondo condensates, i.e., particle-projected condensate, antiparticle-projected condensate, and normal condensate without projection, we calculate the attractive energy gained by the heavy quark within the mean-field approximation in the path-integral formalism. We show that the normal condensate is favored at low density and the particle-projected condensate is favored at high density, when the light quark has a nonzero mass. We interpret such a density-dependent transition between the two condensates in terms of the Kondo resonances.

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