arXiv subjects

Masayasu Harada

Publications and source records attributed to Masayasu Harada.

At least 19 recordsLinked to original sources

arXiv2026

Study of pion condensate in nuclear matter based on a Parity Doublet Model

In this work, we investigate the possibility of pion condensate in isospin-asymmetric nuclear matter within a parity doublet model at zero temperature, treating the baryon and isospin chemical potentials as independent variables. We include the $σ$, $π$, $ω$, and $ρ$ mesons and determine their mean fields from the gap equations. We introduce a novel subtraction scheme to calculate the thermodynamic potential in the no-sea approximation, in which the effective isospin chemical potential is retained in the subtraction term to remove the divergence. We find that the presence of baryons suppresses pion condensate, and that transition to the pion condensation phase is of second order. We further find that the phase boundary depends systematically on the chiral-invariant mass $m_0$: a smaller $m_0$ requires a larger critical isospin chemical potential for pion condensate. The presence of pion condensate also shifts the nuclear Liquid-Gas phase boundary toward larger baryon chemical potentials. Finally we study the existence of pion condensation phase in neutron star matter and find that pion condensate is unlikely to occur.

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arXiv2026

Effects of the $a_0(980)$ Meson on Finite Nuclei in a Parity Doublet Model

We investigate nuclear structure within the parity doublet model (PDM), with particular emphasis on the role of the isovector-scalar $a_0(980)$ meson. We adopt the models with and without the $a_0$ meson and fit their parameters to nuclear matter saturation and ground state properties of selected nuclei, subject to constraints from the vacuum. The resulting parameter sets reproduce the nuclear matter saturation properties and provide a good description of binding energies and charge radii of selected nuclei. Our fits show that a large value of the chiral invariant mass of nucleon is preferred by the fitting: the best fitted value is $m_0 = 689 \pm 71 $ MeV with $a_0$ and $m_0 = 689 \pm 69 $ MeV without $a_0$. We find that the effects of the $a_0$ meson on the binding energies and charge radii are negligibly small. It is difficult to clarify the effect of $a_0$ meson to neutron skin thickness of Sn, Pb, and Ca isotopes, due to the large uncertainty of the predictions. We note that our prediction of $^{208} \text{Pb}$ is consistent with the measurement by PREX-II collaboration, while that of $^{48} \text{Ca}$ cannot explain the CREX result.

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arXiv2026

Two-pion exchange potential in $D^{()}D^{()}$ system

Motivated by the recent HAL QCD lattice results on the $D$ - $D^*$ potential at nearly physical point $m_π=146.4$ MeV, we theoretically examine effects from the two-pion exchange in (semi-)long-range parts of the $DD^*$ system. We employ the framework of heavy-meson chiral perturbation theory with heavy-quark spin symmetry. The pion exchanges up to next-to-next-to-leading order (N$^2$LO) are taken into account. In order to separate the (semi-)long-range contributions, we apply the dispersion-relation method to one-loop diagrams. As a result, we find that the lattice data on the $D$ - $D^*$ potential in the regime of $0.5\, {\rm fm} \lesssim r$ is reproduced by adjusting the unknown $O(p^2)$ couplings. In particular, it turns out that contributions from isospin-independent N$^2$LO triangle diagrams play a central role in reproducing the lattice data of the form $V^{DD^*}\sim \left({\rm e}^{-m_πr}/r\right)^2$ in $1.0\, {\rm fm} \lesssim r\lesssim 2.0\, {\rm fm}$. We also present predictions of other $D^{(*)}$ - $D^{(*)}$ potentials, where the similar two-pion exchange tail is predicted. Our findings provide useful information on the (semi-)long-range regime of the $D$ - $D^*$ potential focusing on two-pion exchanges.

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arXiv2026

A quark-diquark model for parity doublet structure of baryons

The chiral invariant mass of baryons is a phenomenological input of parity doublet models, and its microscopic origin remains an open question. We propose that the chiral invariant mass and the parity doublet structure originate from the diquarks: the scalar ($0^+$) and pseudoscalar ($0^-$) diquarks form a parity doublet whose invariant mass is generated by gluon dynamics rather than by the quark condensate. We construct a three-flavor chiral quark--diquark model in which a quark and a diquark are bounded into a baryon through a chiral-invariant four-body interaction whose structure is reduced from one-gluon exchange. It is shown that the quark--diquark structure automatically yields the two chiral representations and the mirror assignment of the parity doublet model, and the composite baryons acquire chiral invariant masses even for massless quarks. We find that the octet baryon spectrum and the nucleon sigma terms are reproduced very well with a minimal set of parameters. Furthermore, after chiral symmetry restoration, the model predicts a distinctive inverted mass hierarchy: the nucleon remains relatively heavy, whereas the $Σ$ and $Ξ$ baryon become lighter than the nucleon. This inverse mass ordering may therefore provide a novel, experimentally testable signature towards chiral symmetry restoration.

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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 Schrödinger 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 ($T_{bb}, Λ_b, Λ_c$) 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

Origin of hadron mass from gravitational D-form factor and neutron star measurements

Clarifying the origin of hadron mass is one of the fundamental problems in particle physics, relevant from hadronic scales to astrophysical observations. At low energies, this issue is reflected in the decomposition of the hadron mass into chiral-variant and -invariant components. In this letter, we propose a method to extract the chiral invariant mass from the gravitational $D$-form factor under the assumption of the lightest-sigma meson dominance. Focusing on the nucleon, we show that a sizable chiral invariant mass is required to reproduce lattice QCD data, consistent with neutron star constraints.

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arXiv2025

A Study of $T_{cc}(3875)^+$ Nature : Compact v.s. Molecule

A central question in exotic-hadron physics is their internal structure whether these states are loosely bound hadronic molecules or compact multiquark configurations. To shed light on this issue, we develop a model that incorporates mixing between hadronic-molecular and compact multiquark components. We then apply this framework to the specific case of the $T_{cc}(3875)^+$ and analyze the peak structure in the $D^0D^0π^+$ invariant-mass spectrum reported by LHCb. We find that a scenario based on a predominantly compact tetraquark provides the best fitted solution which can explain the $T_{cc}(3875)^+$. We also find that the model admits two more solutions of comparable quality, both of which imply that the $T_{cc}(3875)^+$ is a molecular state: (1) the $T_{cc}(3875)^+$ is a $D^{*+}D^0$ molecule and there is a $D^{*0}D^+$ molecular state in addition; (2) the $T_{cc}(3875)^+$ is a $D^{*0}D^+$ molecule and an aditional $D^{*+}D^0$ molecular state is found below $D^0D^0π^+$ threshold. These molecular states are not simple $I = 0$ states, but mixtures of $I = 0$ and $I = 1$ states. We show that all three scenarios are also consistent with the experimentally observed near-threshold $D^0D^0$ and $D^0D^+$ invariant-mass distributions.

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arXiv2025

Implication of neutron star observations to the origin of nucleon mass

We investigate the implications of neutron star observations for understanding the origin of nucleon mass using a framework that combines three complementary approaches: the equation of state based on parity doublet structure for hadronic matter below $2n_0$, the Nambu-Jona-Lasinio (NJL) model for quark matter above $5n_0$, and a model-independent analysis of the intermediate density region based on fundamental physical principles. By systematically exploring parameter spaces and comparing theoretical predictions with recent observational constraints, we establish constraints on the chiral invariant mass. Our results suggest that more than a half of the nucleon mass originates from sources beyond spontaneous chiral symmetry breaking, challenging conventional understanding of nucleon mass generation. These constraints arise solely from fundamental physical principles and observational data, independent of specific assumptions about the nature of the quark-hadron transition, providing robust insights into the microscopic origin of hadron masses.

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arXiv2025

Chiral Invariant Mass Constraints from HESS J1731 347 in an Extended Parity Doublet Model with Isovector Scalar Meson

The recent discovery of a central compact object (CCO) within the supernova remnant HESS J1731-347, with mass $0.77^{+0.20}_{-0.17} \ M_\odot $ and radius $10.4^{+0.86}_{-0.78}$ km is the lightest and smallest compact object ever observed. We identify it as an ultra-light Neutron star (NS) and constrain the chiral invariant mass of nucleon $m_0$ from the observational data of NS using an extended parity doublet model with including the isovector scalar meson $a_0(980)$. We study the higher order asymmertic matter properties such as the symmetry incompressibility $K_{sym}$ and the symmetry skewness $Q_{sym}$ in the presence of $a_0$ meson. We find that $K_{sym}$ and $Q_{sym}$ is sensitive to the chiral invariant mass of nucleon $m_0$ in the presence of $a_0$ meson. We show that the equation of state in the present model satisfies all observational constraints within $2σ$ credible region including the HESS J1731-347 observation, as well as the constraint from $K_{sym}$ when $740 \,\text{ MeV} \lesssim m_0 \lesssim 860 \,\text{ MeV}$ for $L_0 = $ 57.7 MeV. Yet, the $1σ$ constraint from neutron stars appears to be not fully compatible with the constraint from $K_{sym}$ from the present model.

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arXiv2024

Quarkyonic matter with chiral symmetry restoration

We present a novel unified approach to describe the dense symmetric nuclear matter by combining the quarkyonic matter framework with the parity doublet model. This integration allows for a consistent treatment of the transition from hadronic to quark degrees of freedom while incorporating chiral symmetry restoration effects. Our model introduces a chiral invariant mass for both baryons and constituent quarks, enabling a smooth crossover between hadronic and quark matter in symmetric nuclear matter. We derive the equation of state (EOS) for this hybrid system and investigate its thermodynamic properties. The model predicts a gradual onset of quark degrees of freedom at high densities while maintaining aspects of confinement.

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arXiv2024

Mass and decay width of $T_{ccs}$ from symmetries

We analyze the mass and width of the doubly heavy tetraquark $T_{ccs}$ composed of a heavy diquark and a light-quark cloud with strangeness with assuming that a color antitriplet heavy diquark is a dominant component of the doubly charmed tetraquarks $T_{cc}$ and $T_{ccs}$. We construct an effective Lagrangian for masses of heavy hadrons based on the superflavor symmetry between the doubly heavy tetraquarks and the singly heavy baryons by including the terms that simultaneously break the heavy-quark and light-flavor symmetries, and predict the mass of $T_{ccs}$ as $M(T_{ccs}) = 4047\pm11$\,MeV. The comparison of this prediction with future experimental observation will give a clue to understand the color structure of the heavy diquark. We also predict the mass of $Ω_{cc}$ as $M(Ω_{cc}) = 3706^{+14}_{-15}\,$MeV. We next calculate the decay width of $T_{ccs}$, based on solely the light-flavor symmetry, as $Γ(T_{ccs}) = 42\pm 24$\,MeV.

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arXiv2024

Exploring the first-order phase transition in neutron stars using the parity doublet model and NJL-type quark model

We investigate the possibility and impacts of a first-order phase transition from hadronic matter to quark matter in neutron stars (NSs) using two specific models: the parity doublet model (PDM) for the hadronic phase and the Nambu-Jona-Lasinio (NJL) type model for the quark phase. By combining these models, we construct hybrid equations of state (EOSs) that capture the transition between the two phases. We explore the parameter space of both models to identify the conditions under which a first-order phase transition can occur and study its effects on NS properties. We identify the suitable parameter space and constrain the onset density of the first-order phase transition. For $m_0$ = 500 MeV -- the chiral invariant mass in PDM, the phase transition occurs between 1.9$n_0$ and 2.95$n_0$ and ends between 2.1$n_0$ and 3.6$n_0$. Increasing $m_0$ to 600 MeV shifts the phase transition to higher densities, occurring between 2.9$n_0$ and 4.1$n_0$ and ending between 3.4$n_0$ and 4.6$n_0$.

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arXiv2024

Nuclear matter and finite nuclei: recent studies based on Parity Doublet Model

In this review, we summarize recent studies on nuclear matter and finite nuclei based on parity doublet models. We first construct a parity doublet model (PDM), which includes the chiral invariant mass $m_0$ of nucleons together with the mass generated by the spontaneous chiral symmetry breaking. We then study the density dependence of the symmetry energy in the PDM, which shows that the symmetry energy is larger for smaller chiral invariant mass. Then, we investigate some finite nuclei by applying the Relativistic Continuum Hartree-Bogoliubov (RCHB) theory to the PDM. We present the root-mean-square deviation (RMSD) of the binding energies and charge radii, and show that $m_0$ = 700 MeV is preferred by the nuclear properties. Finally, we modify the PDM by adding the iso-vector scalar meson $a_0(980)$ and show that the inclusion of the $a_0(980)$ enlarges the symmetry energy of the infinite nuclear matter.

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arXiv2024

Reconciling the HESS J1731-347 constraints with Parity doublet model

The recent discovery of a central compact object (CCO) within the supernova remnant HESS J1731-347, characterized by a mass of approximately $0.77^{+0.20}_{-0.17} M_{\odot}$ and a radius of about $10.4^{+0.86}_{-0.78}$ km, has opened up a new window for the study of compact objects. This CCO is particularly intriguing because it is the lightest and smallest compact object ever observed, raising questions and challenging the existing theories. To account for this light compact star, a mean-field model within the framework of parity doublet structure is applied to describe the hadron matter. Inside the model, part of the nucleon mass is associated with the chiral symmetry breaking while the other part is from the chiral invariant mass $m_0$ which is insensitive to the temperature/density. The value of $m_0$ affects the nuclear equation of state for uniform nuclear matter at low density and exhibits strong correlations with the radii of neutron stars. We point out that HESS J1731-347 can be explained as the lightest neutron star for $m_0 \simeq 850$\,MeV.

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arXiv2024

Parity doublet model for baryon octets: ground states saturated by good diquarks and the role of bad diquarks for excited states

Parity doublet model is an effective chiral model that includes the chiral variant and invariant masses of baryons. The chiral invariant mass has large impacts on the density dependence of models which can be constrained by neutron star observations. In the previous work, models of two-flavors have been considered up to a few times nuclear saturation density, but in such dense region it is also necessary to consider hyperons. With the chiral invariant masses baryons can stay massive in extreme environments (e.g., neutron stars) where the chiral symmetry restoration takes place. In this work, we generalize the previous $\mbox{SU(2)}_L \times \mbox{SU(2)}_R$ parity models of nucleons to $\mbox{SU(3)}_L \times \mbox{SU(3)}_R$ models of the baryon octet, within the linear realization of the chiral symmetry. The major problem in constructing such models has been too many candidates for the chiral representations of baryons. Motivated by the concepts of diquarks and the mended symmetry, we choose the $(3_L, \bar{3}_R) + (\bar{3}_L, 3_R)$, $(3_L, 6_R) + (6_L, 3_R)$ and $(1_L, 8_R) + (8_L, 1_R)$ representations and use quark diagrams to constrain the possible types of Yukawa interactions. The masses of the baryon octets for positive and negative baryons up to the first excitations are successfully reproduced. As expected from the diquark considerations, the ground state baryons are well dominated by $(3_L, \bar{3}_R) + (\bar{3}_L, 3_R)$ and $(1_L, 8_R) + (8_L, 1_R)$ representations, while the excited states require $(3_L, 6_R) + (6_L, 3_R)$ representations. Important applications of our model are the chiral restoration for strange quarks at large density and the continuity of diquarks from hadronic to quark matter. We also address the problem of large Yukawa couplings which are enhanced in three-flavor construction.

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arXiv2023

Fate of the $ρ$-$a_1$ mixing in dilepton production

We investigate the effect of chiral mixing on dilepton production by combining the in-medium spectral function in the chiral effective field theory with the state-of-the-art fluid dynamical simulations. We compare the spectral functions with different chiral symmetry restoration scenarios. We find that the scenario with proper chiral symmetry restoration that takes into account the degenerate $ρ$ and $a_1$ mesons leads to an increase of the yield in the window of $1.1<M<1.4$ GeV. Whereas, the low-temperature theorem of chiral mixing extrapolated toward a chiral crossover, often used in the literature, leads to a substantial overestimate at $M=1.2$ GeV.

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arXiv2023

Neutron star matter based on a parity doublet model including the $a_0(980)$ meson

We study the effect of the isovector-scalar meson $a_0$(980) on the properties of nuclear matter and the neutron star (NS) matter by constructing a parity doublet model with including the $a_0$ meson based on the chiral SU(2)$_L\times$SU(2)$_R$ symmetry. We also include the $ω$-$ρ$ mixing contribution to adjust the slope parameter at the saturation. We find that, when the chiral invariant mass of nucleon $m_0$ is smaller than about 800 MeV, the existence of $a_0$(980) enlarges the symmetry energy by strengthening the repulsive $ρ$ meson coupling. On the other hand, for large $m_0$ where the Yukawa coupling of $a_0$(980) to nucleon is small, the symmetry energy is reduced by the effect of $ω$-$ρ$ mixing. We then construct the equation of state (EoS) of a neutron star matter to obtain the mass-radius relation of NS. We find that, in most choices of $m_0$, the existence of $a_0$(980) stiffens the EoS and makes the radius of NS larger. We then constrain the chiral invariant mass of nucleon from the observational data of NS, and find that $580 \,\text{ MeV} \lesssim m_0 \lesssim 860 \,\text{ MeV} $ for $L_0=57.7$ MeV.

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arXiv2023

QCD vacuum and baryon masses

To study a possible role of the quantum chromodynamics (QCD) vacuum in nuclear and hadron physics, we evaluate a physical quantity in a candidate of the QCD vacuum. In this study we adopt the Copenhagen (spaghetti) picture of the QCD vacuum and calculate the ground-state baryon masses in a constituent quark model. We find that the calculated baryon mass does depend on a parameter that characterizes the Copenhagen picture of the QCD vacuum and satisfies the Gell-Mann-Okubo mass relation for the baryon octet. We also observe that the effective constituent quark mass defined in this study contains a contribution attributed to the Copenhagen vacuum, that is the gluon background field. We then estimate the value of the background gluon field as a function of the up (down) constituent quark mass by using the baryon masses as inputs.

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