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Chihiro Sasaki

Publications and source records attributed to Chihiro Sasaki.

At least 19 recordsLinked to original sources

Chiral Doubling, Renormalization Group Fixed Points, and Dense Matter Equations of State

We review a top-down fixed-point-extrapolation paradigm as a unified effective field theory framework for hadronic spectroscopy, dense baryonic matter, and compact star physics. By formulating our effective Lagrangian directly at renormalization group (RG) fixed points and introducing minimal symmetry breaking, we extrapolate the theory back to physical environments. In the vacuum, this framework naturally reproduces the heavy-light meson parity-doubling spectrum via light vector meson loops. In dense matter, the interplay between the chiral-invariant nucleon mass m0 and walking vector couplings reconciles gravitational-wave constraints with $2M_\odot$ massive neutron stars. Generalization to a quark-hadron hybrid approach further illuminates sequential deconfinement, core stability, and baryon number fluctuations, demonstrating how underlying RG fixed points dictate hadronic dynamics from vacuum to neutron star interiors.

hep-ph

Finite-size effects and scaling properties of chiral and baryon-number fluctuations

An effective chiral model is introduced to illustrate finite-size effects, incorporating the standard zero-mode treatment, momentum-space discretization, and gradient effects modeled via a prescribed finite-volume profile. The fluctuations of the chiral order parameter and the net-baryon number, as well as their scaling properties, are investigated near a critical point and a first-order transition in finite-size systems. The finite-volume effects on the Binder cumulant and the kurtosis are shown along the phase boundary and the approximate freeze-out line, respectively. Phenomenological implications on fluctuation observables are pointed out and explained.

hep-ph

Scaling properties of net-baryon number fluctuations at the deconfinement critical point

We investigate the critical behavior of the first four cumulants of the net-baryon number near the deconfinement critical point in QCD in the limit of heavy quarks. By connecting baryon-number fluctuations to Polyakov loop susceptibilities, we analyze their mean-field scaling properties at zero and non-zero baryon chemical potentials. In the mean-field approximation we derive critical exponents via the Landau theory and validate them through explicit numerical calculations in an effective Polyakov loop model. Using a Ginzburg-Landau criterion as a diagnostic of beyond-mean-field effects, we estimate the size of the critical region and find that it shrinks with increasing baryon density. By utilizing the exact scaling function in the 3D Ising model, we estimate critical exponents beyond the mean-field approximation.

hep-ph

Selected Topics in Quark-Hadron Physics: From Scalar Nonets to Topological Glueballs

This contribution reviews recent progress in the low-lying scalar mesons and glueballs. We propose a new classification for the scalar nonet that includes $f_0(980)$ and $a_0(980)$ as the lowest states, while we identify $f_0(1500)$ as a primary glueball candidate. We demonstrate that the production yields of these states in heavy-ion collisions are mutually consistent across statistical, coalescence, and S-matrix frameworks. To investigate their internal structure, we move beyond standard phenomenology by describing glueballs as topological solitons. This approach yields an energy spectrum in excellent agreement with lattice QCD and experimental data, while interpreting $f_0(2470)$ as a tightly bound glueballonium to explain its anomalously long lifetime. This non-perturbative framework provides a predictive basis for the future experimental verification of exotic scalar states.

hep-ph

Dilepton Production as a Probe of Pion Condensation in Hot and Dense QCD Matter

We investigate dilepton production from an isospin-asymmetric hot and dense medium in order to explore the role of isospin imbalance in electromagnetic spectral properties. We focus in particular on modifications of the dilepton production rate associated with the onset of pion condensation, which can occur in the presence of a finite isospin chemical potential. We employ the Nambu--Jona-Lasinio model with isoscalar--vector interaction. We examine the phase structure in the $T-μ_I$ plane and estimate the vector current correlator--resummed dilepton rate for an effective quark chemical potential. We find that the interplay between isospin asymmetry, pion condensation, and vector interactions leads to nontrivial modifications of the dilepton yield. In particular, we observe two key features of the pion condensed phase: an enhancement at lower invariant mass and a prominent plateau-like structure which also help clearly identify the pion condensed phase from a chirally broken/restored phase. These results highlight the potential sensitivity of dilepton observables to pion-condensed phase of QCD matter, with possible implications for future low-energy heavy-ion collision experiments as well as isospin-rich environments such as neutron star matter.

hep-ph

New nonet scalar mesons and glueballs: the mass spectra and the production yields in relativistic heavy ion collisions

We propose a new nonet scheme for scalar mesons consisting of $f_{0}(980)$, $a_{0}(980)$, $K_{0}^{\ast}(1430)$, and $f_{0}(1770)$, regarding them as quark-antiquark $P$-wave states classified by $\mathrm{SU}(3)$ light flavor symmetry. We investigate their production in relativistic heavy ion collisions, and estimate their yields by applying the statistical model and the quark coalescence model. In contrast to these scalar mesons, we regard $f_{0}(1500)$ as a glueball that is not included in the proposed nonet. We quantify the production yields of $f_{0}(1500)$ by accounting for various internal structures of this state, and compare their yields with those of the new nonet scalar mesons. Given the production yields of these hadrons, our results strongly suggests that $f_{0}(1500)$ is a glueball.

hep-ph

Charm Quark Kinetics in Heavy Ion Collisions

We study the evolution of charm $(c)$ quarks in hot QCD matter with $N_f=2+1(+1)$ quark flavors by analyzing the charm production rate and the time dependence of their abundance. Microscopically, the system is described within a quasiparticle model, in which interactions among dynamical quarks and gluons are encoded in their effective masses with the running coupling constrained by lattice QCD data. We investigate $c$-quark kinetics in a longitudinally propagating perfect fluid as well as in a viscous medium undergoing (2+1)D expansion, and find that the charm production rate decreases monotonically across all medium formulations. In the $N_f=2+1+1$ scenario, charm production is systematically suppressed due to the effective mass of heavy quasiparticles. Assuming an initial charm yield given by the Statistical Hadronization Model, we solve the rate equation and compute the total charm abundance in hot QCD medium. For all descriptions considered, the charm quark number remains approximately conserved, consistent with existing experimental evidence.

hep-ph

Thermodynamics of fermionic excitations in heavy-quark QCD

We investigate the thermodynamic properties of fermionic excitations in heavy-quark QCD on the lattice with Wilson fermions. The grand potential is calculated analytically in the hopping parameter expansion (HPE) on the basis of the cumulant expansion. Using the grand potential, we compute the quark number susceptibilities and their ratios up to next-to-leading order in the HPE. The ratio of fourth- to second-order susceptibilities is shown to be unity (nine) in the deconfined (confined) phase at the leading order. Excitation properties of baryonic and quark modes in each phase are also investigated utilizing the Boltzmann statistics. We obtain an analytic formula for the quark excitation energy in the deconfined phase, while that for baryonic excitations in the confined phase is decomposed into flavor multiplets.

hep-lat

Probing nuclear liquid-gas phase transition with isospin correlations

We investigate the fluctuations of the net-baryon number near the critical point of the liquid-gas phase transition. We use the parity doublet model in the mean-field approximation fixed to the zero-temperature properties of nuclear matter to account for critical behavior. We explicitly calculate the fluctuations of the net-proton and net-neutron numbers as well as their correlations in the isospin-symmetric matter. We focus on the qualitative properties and systematics of the first- to fourth-order susceptibilities and their ratios. We demonstrate that the fluctuations of net-proton number do not reflect the total net-baryon number fluctuations in the vicinity of the nuclear liquid-gas phase transition. We also study the behavior of the baryon and proton number factorial cumulants. We highlight the importance of the non-trivial correlations between protons and neutrons.

nucl-th

Emergent chirality and superfluidity of parity-doubled baryons in neutron stars

We propose novel superfluids induced by the parity-doubled baryons. The parity-doubled baryons, i.e., a nucleon $N(940)$ with spin-parity $J^{P}=1/2^{+}$ and an excited nucleon $N^{\ast}(1535)$ with $J^{P}=1/2^{-}$ in vacuum, become degenerate at sufficiently high density where the chiral symmetry is restored. In this study, we extend the conventional $\mathrm{U}(1)$ chiral symmetry to the higher dimensional symmetries, dubbed emergent chiral symmetries, including the naive and mirror assignments as their subgroups. Starting with the Lagrangian up to four-point interactions among the neutron $n$ and its chiral partner $n^{\ast}$, neutral components in $N$ and $N^{\ast}$, in pure neutron matter, we investigate the properties of the ground state with a pairing gap generated by the $n$ and $n^{\ast}$ in the mean-field approximation. We find vector-type condensates that induce the dynamical breaking of a new class of internal symmetries, emergent chiral symmetries, as well as the baryon number and the rotational symmetries of the real space, indicating the appearance of massless Nambu-Goldstone bosons consisting of six quarks: emergent pions, superfluid phonons, and magnons, respectively. We also study the fermionic excitation modes at low-energy scales, and show that they exhibit a spatial anisotropy of the propagation at the Dirac cone in momentum space. Some phenomenological implications are advocated, shedding new light on the properties of neutron stars.

nucl-th

Proton Spin Decomposition via QCD Sum Rules

We present a novel approach for investigating the spin structure of hadrons based on the two-point function in quantum field theory. In a rotating frame, we derive two independent expressions of the two-point function and identify their equivalence, which allows for a complete decomposition of the total spin of a composite system into the angular momenta of its constituent particles. Applying this approach to the proton, we analyze its spin structure in the massless quark limit. Our results indicate that the quark spin contribution accounts for approximately 27$\%$ of the total proton spin at a low-energy scale, significantly deviating from the prediction of the non-relativistic quark model.

hep-ph

Glueballonia as Hopfions

We work out the Hopfion description of glueballs by inclusively comparing the energy spectra obtained by quantizing Hopfions with experimental data and lattice QCD. Identifying a Hopfion carrying a unit topological charge as $f_0(1500)$, the Hopfions with the topological charge two are classified as glueballonia, i.e., two glueballs are bound together. We find a tightly and a loosely bound glueballonia complying with $f_0 (2470)$ and a novel scalar particle carrying the mass around 2814 MeV, respectively, and calculate their binding energies. By the rigid body quantization of Hopfions, we predict a characteristic multiplet structure of tensor glueball states. Some of them are missing in the current experimental data and can be verified in future measurements.

hep-ph

Magnetic effects in the Hadron Resonance Gas

We discuss the modeling of the hadronic phase of QCD at finite magnetic field in the framework of hadron resonance gas (HRG). We focus on the statistical description of particle yields that include contribution from resonance decays. We demonstrate that the swift increase in the number of protons with magnetic field predicted in the HRG is due to the ill-defined description of higher-spin states. We discuss fluctuations of conserved charges and show that at present the qualitative comparison of the model predictions with the Lattice QCD data should be treated with care. We also discuss the principle of detailed balance which allows to study the magnetic field dependence of neutral resonances.

hep-ph

Toward a Direct Measurement of Partial Restoration of Chiral Symmetry at J-PARC E16 via Density-induced Chiral Mixing

The degeneracy of chiral partners is an ideal signal for measuring the restoration of the spontaneously broken chiral symmetry in QCD. In this work, we investigate the observability of the $ϕ$ - $f_1(1420)$ degeneracy in the J-PARC E16 experiment, which measures di-electrons emitted from 30 GeV pA collisions. We for this purpose make use of an effective Lagrangian approach, which naturally incorporates the broken charge-conjugation symmetry in nuclear matter and the ensuing anomaly-induced mixing between vector and axial-vector mesons, to compute the spectral function relevant for the experimental measurement. The real-time dynamics of the pA collision is obtained from a transport simulation. Including experimental background and resolution effects on top of that, we find that a signal of the $ϕ$ - $f_1(1420)$ mixing can be observed around 2.5 $σ$ with the Run2 statistics planned for the J-PARC E16 experiment with an ideal mixing strength.

hep-ph

Fluctuations and correlations of baryonic chiral partners

Fluctuations and correlations of the net-baryon number play an important role in exploring critical phenomena in phase transitions of strongly interacting matter governed by Quantum chromodynamics (QCD). In this work, we use the parity doublet model to investigate the fluctuations of the net-baryon number density in hot and dense hadronic matter. The model accounts for chiral criticality within the mean-field approximation. We focus on the qualitative properties and systematics of the first- and second-order susceptibility of the net-baryon number density, and their ratios for nucleons of positive and negative parity, as well as their correlator. We show that the fluctuations of the positive-parity nucleon do not necessarily reflect the fluctuations of the total net-baryon number density at the phase boundary of the chiral phase transition. We also investigate the non-trivial structure of the correlator. Furthermore, we discuss and quantify the differences between the fluctuations of the net-baryon number density in the vicinity of the chiral and liquid-gas phase transition in nuclear matter. We indicate a possible relevance of our results with the interpretation of the experimental data on net-proton number fluctuations in heavy-ion collisions.

hep-ph

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.

nucl-th

Interplay of baryonic chiral partners in fluctuations of net-baryon number density

In this contribution, we use the parity doublet model to investigate the fluctuations of the net-baryon number density. We discuss the systematics of the susceptibilities and their ratios for nucleons of positive and negative parity, as well as their correlator. We demonstrate that the fluctuations of positive-parity nucleon do not reflect the fluctuations of the total net-baryon number at the chiral phase transition.

nucl-th