arXiv subjects

Etsuko Itou

Publications and source records attributed to Etsuko Itou.

At least 19 recordsLinked to original sources

arXiv2026

Equation of state of QCD(-like) theory using Lattice Monte Carlo simulations

The equation of state (EoS) of strongly interacting matter at low temperature and high baryon density is a central ingredient in the physics of compact stars, but it is still difficult to determine directly from first-principles QCD calculations because of the sign problem. Two-colour QCD (QC$_2$D) provides a useful and controllable laboratory: for an even number of fundamental flavours the fermion determinant is real and positive, while the theory shares the nonperturbative properties with three-colour QCD at least zero chemical-potential regime. In this short review we summarize recent lattice progress on dense QC$_2$D, with emphasis on the phase structure, the emergence of superfluidity, the Bose--Einstein-condensation (BEC) to Bardeen--Cooper--Schrieffer(BCS) crossover, and thermodynamic quantities. A particularly interesting outcome is that the sound velocity in the cold dense superfluid regime can exceed the conformal value $c_{\rm s}^2/c^2=1/3$. This behaviour, now seen in the simulations for several QCD-like theories, gives a stiff strongly interacting matter. It is expected to offer insight into at least some aspects of the physics realized inside neutron stars.

nucl-th↗

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.

hep-lat↗

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}.

hep-lat↗

arXiv2025

Lattice results for the equation of state in dense QCD-like theories

We review recent progress in Monte Carlo simulations of dense two-color QCD (QC$_2$D), focusing on the phase diagram, the equation of state, and the sound velocity in the low-temperature regime. In three-color QCD at finite density, especially at low temperatures, the notorious sign problem makes lattice Monte Carlo simulations intractable. In contrast, QC$_2$D is free from this issue due to the pseudoreality of the quark representation. Recent independent lattice studies have revealed unexpected phenomena through first-principles calculations of the phase structure and thermodynamics. A particularly notable finding is that the sound velocity exceeds the so-called conformal (holography) bound, $ c_s^2/c^2 \le 1/3 $, which had not been observed in QCD-like theories at finite temperature. In this review, we focus primarily on results from a series of works by our group~\cite{Iida:2019rah, Iida:2020emi,Iida:2022hyy, Ishiguro:2021yxr, Iida:2024irv}, along with related studies in dense QC$_2$D and three-color QCD with isospin chemical potential. We discuss the possibility and physical implications of conformal bound violation even for three-color dense QCD, together with insights from effective model analyses and recent observations of neutron stars.

hep-lat↗

arXiv2025

Monte Carlo study on Heisenberg model with local dipolar interaction

Aharony and Fisher showed that non-local dipolar effects in magnetism destabilize the Heisenberg fixed point in real ferromagnets, leading to a new fixed point, called the dipolar fixed point. The non-perturbative nature of the new fixed point, however, has not been uncovered for many decades. Inspired by the recent understanding that the dipolar fixed point is scale-invariant but not conformal invariant, we perform the Monte Carlo simulation of the local Heisenberg-dipolar model on the lattice of $40^3$ by introducing the local cost function parameterized by a parameter $λ$ and study its critical exponents, which should become identical to the dipolar fixed point of Aharony and Fisher in the infinite coupling limit $λ= \infty$. We find that the critical exponents become noticeably different from those of the Heisenberg fixed point for a finite coupling constant $λ=8$ (e.g. $ν=0.601(2)(^{+0}_{-2})$ in the local Heisenberg-dipolar model while $ν=0.712(1)(^{+3}_{-0})$ in the Heisenberg model), and the spin correlation function has a feature that it becomes divergence-free, implying the lack of conformal invariance.

hep-th↗

arXiv2025

New computational methods in lattice gauge theory -- quantum computation and tensor networks

Lattice QCD calculations have been conducted using large-scale classical computers based on the Lagrangian formalism of field theory for the past 40 years. On the other hand, the advent of quantum computers has brought increasing attention to first-principles computational methods based on the Hamiltonian formalism compatible with these machines. In this talk, we discuss recent results on how to calculate hadronic properties using the Hamiltonian formalism.

hep-lat↗

arXiv2025

Computing theta-dependent mass spectrum of the 2-flavor Schwinger model in the Hamiltonian formalism

We compute the $θ$-dependent mass spectrum of the 2-flavor Schwingr model using the tensor network (DMRG) in the Hamiltonian formalism. The pion and the sigma meson are identified as stable particles of the model for nonzero $θ$ whereas the eta meson becomes unstable. The meson masses are obtained from the one-point functions, using the meson operators defined by diagonalizing the correlation matrix to deal with the operator mixing. We also compute the dispersion relation directly by measuring the energy and momentum of the excited states, where the mesons are distinguished by the isospin quantum number. We confirmed that the meson masses computed by these methods agree with each other and are consistent with the calculation by the bosonized model. Our methods are free from the sign problem and show a significant improvement in accuracy compared to the conventional Monte Carlo methods. Furthermore, at the critical point $θ= π$, the mesons become almost massless, and the one-point functions reproduce the expected CFT-like behavior.

hep-lat↗

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.

hep-lat↗

arXiv2024

Scale setting and hadronic properties in light quark sector with $(2+1)$-flavor Wilson fermions at the physical point

We report scale setting and hadronic properties for our new lattice QCD gauge configuration set (HAL-conf-2023). We employ $(2+1)$-flavor nonperturbatively improved Wilson fermions with stout smearing and the Iwasaki gauge action on a $96^4$ lattice, and generate configurations of 8,000 trajectories at the physical point. We show the basic properties of the configurations such as the plaquette value, topological charge distribution and their auto-correlation times. The scale setting is performed by detailed analyses of the $Ω$ baryon mass. We calculate the physical results of quark masses, decay constants of pseudoscalar mesons and single hadron spectra in light quark sector. The masses of the stable hadrons are found to agree with the experimental values within a sub-percent level.

hep-lat↗

arXiv2024

End-to-end complexity for simulating the Schwinger model on quantum computers

The Schwinger model is one of the simplest gauge theories. It is known that a topological term of the model leads to the infamous sign problem in the classical Monte Carlo method. In contrast to this, recently, quantum computing in Hamiltonian formalism has gained attention. In this work, we estimate the resources needed for quantum computers to compute physical quantities that are challenging to compute on classical computers. Specifically, we propose an efficient implementation of block-encoding of the Schwinger model Hamiltonian. Considering the structure of the Hamiltonian, this block-encoding with a normalization factor of $\mathcal{O}(N^3)$ can be implemented using $\mathcal{O}(N+\log^2(N/\varepsilon))$ T gates. As an end-to-end application, we compute the vacuum persistence amplitude. As a result, we found that for a system size $N=128$ and an additive error $\varepsilon=0.01$, with an evolution time $t$ and a lattice spacing a satisfying $t/2a=10$, the vacuum persistence amplitude can be calculated using about $10^{13}$ T gates. Our results provide insights into predictions about the performance of quantum computers in the FTQC and early FTQC era, clarifying the challenges in solving meaningful problems within a realistic timeframe.

quant-ph↗

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.

hep-lat↗

arXiv2024

DMRG study of the theta-dependent mass spectrum in the 2-flavor Schwinger model

We study the $θ$-dependent mass spectrum of the massive $2$-flavor Schwinger model in the Hamiltonian formalism using the density-matrix renormalization group(DMRG). The masses of the composite particles, the pion and sigma meson, are computed by two independent methods. One is the improved one-point-function scheme, where we measure the local meson operator coupled to the boundary state and extract the mass from its exponential decay. Since the $θ$ term causes a nontrivial operator mixing, we unravel it by diagonalizing the correlation matrix to define the meson operator. The other is the dispersion-relation scheme, a heuristic approach specific to Hamiltonian formalism. We obtain the dispersion relation directly by measuring the energy and momentum of the excited states. The sign problem is circumvented in these methods, and their results agree with each other even for large $θ$. We reveal that the $θ$-dependence of the pion mass at $m/g=0.1$ is consistent with the prediction by the bosonized model. We also find that the mass of the sigma meson satisfies the semi-classical formula, $M_σ/M_π=\sqrt{3}$, for almost all region of $θ$. While the sigma meson is a stable particle thanks to this relation, the eta meson is no longer protected by the $G$-parity and becomes unstable for $θ\neq 0$.

hep-lat↗

arXiv2024

Chemical potential (in)dependence of hadron scatterings in the hadronic phase of QCD-like theories and its applications

We formulate a method for calculating the hadron-hadron scattering amplitudes at nonzero chemical potential ($μ$) in the hadronic phase at zero temperature, where the baryon number symmetry remains to be violated. Although it is widely believed that the physical quantities do not change even if we turn on a small $μ$ at zero temperature, the shape of correlation functions for a single hadron depends on $μ$. Then, the dispersion relation of the single hadron is modified to $E({\bf p},μ) = \sqrt{{\bf p}^2+m^2}-μn_{O}$. Here, $m$ and $n_O$ denote the hadron mass at $μ=0$ and the quantum number, respectively. From this relation, it is possible that the effective mass of the hadron depends on $μ$. We extend the HAL QCD method at $μ=0$ to the case of $μ\ne 0$, which allows us to extract the scattering phase shifts via the interaction potential. We have found that the interaction potential can depend on $μ$ only through the effective mass while the scattering phase shifts, obtained by solving the Schrödinger equation with the interaction potential, are independent of $μ$. We also numerically analyze the S-wave scatterings of two pions with isospin $I=2$ and two scalar diquarks within the framework of QC$_{2}$D at nonzero quark chemical potential. While the lattice is not exactly set to zero temperature, the $μ$-independence can be observed. Furthermore, we improve the results for the S-wave scatterings of two hadrons obtained above by taking the $μ$-independence for granted. Thanks to the asymmetric property of the correlation functions for diquarks at $μ\neq0$, we can access a long-$τ$ regime and can reduce the systematic error coming from inelastic contributions.

hep-lat↗

arXiv2024

Three ways of calculating mass spectra for the 2-flavor Schwinger model in the Hamiltonian formalism

We propose three independent methods to compute the hadron mass spectra of gauge theories in the Hamiltonian formalism. The determination of hadron masses is one of the key issues in QCD, which has been precisely calculated by the Monte Carlo method in the Lagrangian formalism. We confirm that the mass of hadrons can be calculated by examining correlation functions, the one-point function, or the dispersion relation in Hamiltonian formalism. These methods are suitable for quantum computation and tensor network approaches. The methods are demonstrated with the tensor network (DMRG) in the 2-flavor Schwinger model, which shares important properties with QCD. We show that the numerical results are consistent with each other and with the analytic prediction of the bosonization technique. We also discuss their efficiency and potential applications to other models.

hep-lat↗

arXiv2023

Mass spectrum of spin-one hadrons in dense two-color QCD: Novel predictions by extended linear sigma model

We construct an extended version of the linear sigma model in such a way as to describe spin-$1$ hadrons as well as spin-$0$ hadrons in two-color QCD (QC$_2$D) by respecting the Pauli-Gürsey $SU(4)$ symmetry. Within a mean-field approximation, we therefrom examine a mass spectrum of the spin-$1$ hadrons at finite quark chemical potential ($μ_q$) and zero temperature. Not only mean fields of scalar mesons and scalar-diquark baryons but also of vector mesons and vector-diquark baryons are incorporated. As a result, we find that, unless all of those four types of mean fields are taken into account, neither lattice result for the critical $μ_q$ that corresponds to the onset of baryon superfluidity nor for $μ_q$ dependence of the pion mass can be reproduced. We also find that a slight suppression of the $ρ$ meson mass in the superfluid phase, which was suggested by the lattice simulation, is reproduced by subtle mixing effects between spin-$0$ and spin-$1$ hadrons. Moreover, we demonstrate the emergence of an axialvector condensed phase and possibly of a vector condensed phase by identifying the values of $μ_q$ at which the corresponding hadron masses vanish. The possible presence of iso-triplet $1^-$ diquarks that may be denoted by a tensor-type quark bilinear field is also discussed.

hep-ph↗

arXiv2023

Speed of sound exceeding the conformal bound in dense 2-color QCD

We review recent works on the Monte Carlo simulations of dense two-color QCD (QC$_2$D) by focusing on the phase diagram, the equation of state, and the sound velocity at nonzero quark chemical potential. A possible upper bound of the sound velocity is known as the conformal bound, namely, $c_s^2/c^2 \leq 1/3$. The sound velocity is below the bound at least in the case of finite-temperature QCD. However, our recent work~\cite{Iida:2022hyy} shows the breaking of this bound in dense QC$_2$D. This phenomenon was previously unknown from any lattice calculations. We also discuss recent related works including lattice studies on QCD at nonzero isospin chemical potential, some effective model analyses, and an analysis based on recent neutron star observations. These works also suggest the breaking of the conformal bound.

hep-lat↗

arXiv2023

Quantum Simulation of Finite Temperature Schwinger Model via Quantum Imaginary Time Evolution

We study the Schwinger model at finite-temperature regime using a quantum-classical hybrid algorithm. The preparation of thermal state on quantum circuit presents significant challenges. To address this, we adopt the Thermal Pure Quantum (TPQ) state approach and apply the Quantum Imaginary Time Evolution (QITE) algorithm to implement the necessary imaginary time evolution. We first compute the chiral condensate in the massless Schwinger model, verifying its consistency with the analytical solution. We then simulate the massive Schwinger model with non-zero topological $θ$-term to investigate the temperature and $θ$-dependence of the chiral condensate. Our method works well even at non-zero $θ$ regime, while the conventional lattice Monte Carlo method suffers from the sign problem in this system.

hep-lat↗

arXiv2023

Calculating composite-particle spectra in Hamiltonian formalism and demonstration in 2-flavor QED$_{1+1\text{d}}$

We consider three distinct methods to compute the mass spectrum of gauge theories in the Hamiltonian formalism: (1) correlation-function scheme, (2) one-point-function scheme, and (3) dispersion-relation scheme. The first one examines spatial correlation functions as we do in the conventional Euclidean Monte Carlo simulations. The second one uses the boundary effect to efficiently compute the mass spectrum. The third one constructs the excited states and fits their energy using the dispersion relation with selecting quantum numbers. Each method has its pros and cons, and we clarify such properties in their applications to the mass spectrum for the 2-flavor massive Schwinger model at $m/g=0.1$ and $θ=0$ using the density-matrix renormalization group (DMRG). We note that the multi-flavor Schwinger model at small mass $m$ is a strongly coupled field theory even after the bosonizations, and thus it deserves to perform the first-principles numerical calculations. All these methods mostly agree and identify the stable particles, pions $π_a$ ($J^{PG}=1^{-+}$), sigma meson $σ$ ($J^{PG}=0^{++}$), and eta meson $η$ ($J^{PG}=0^{--}$). In particular, we find that the mass of $σ$ meson is lighter than twice the pion mass, and thus $σ$ is stable against the decay process, $σ\to ππ$. This is consistent with the analytic prediction using the WKB approximation, and, remarkably, our numerical results are so close to the WKB-based formula between the pion and sigma-meson masses, $M_σ/M_π=\sqrt{3}$.

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