arXiv subjects

Tetsufumi Hirano

Publications and source records attributed to Tetsufumi Hirano.

At least 19 recordsLinked to original sources

arXiv2026

Necessary and sufficient conditions of nonlinear causality in viscous anisotropic hydrodynamics

We derive the necessary and sufficient conditions for nonlinear causality in viscous anisotropic hydrodynamics (VAH) within the approximation that neglects small correction terms. Relativistic hydrodynamics provides a successful description of the space-time evolution of the matter produced in relativistic heavy-ion collisions, yet the earliest stage at which a hydrodynamic description becomes valid remains an open question. VAH has been proposed as an extension of conventional viscous hydrodynamics (VH) that can accommodate the large pressure anisotropies of the early-time dynamics. However, in such far-from-equilibrium regimes, the nonlinear causality of the theory is not guaranteed. By analyzing the characteristic velocities of the VAH equations of motion, we derive a set of inequalities that ensures causal signal propagation in all directions. The resulting conditions take a remarkably simple form and admit a clear physical interpretation in terms of the characteristic modes of the anisotropic medium. These results establish the regime of validity of VAH and provide a foundation for its application to the early-time dynamics of relativistic heavy-ion collisions.

nucl-th↗

arXiv2026

Diffusion and shear viscosity coefficients of hot isospin asymmetric strange hadronic matter using a chiral SU(3) model

We study the diffusion and shear viscosity coefficients of hot isospin asymmetric strange hadronic matter. The effects due to the baryon density, isospin, strangeness, and temperature on the nucleons and hyperons are studied within a chiral SU(3) model. The medium modifications of the baryons arise due to interactions with the mean scalar and vector fields within the model. The thermodynamic and transport properties are studied in the hot strange hadronic matter. The diffusion matrix associated with the multiple charges (baryon number, isospin, and strangeness), as well as the coefficient of shear viscosity, are computed from the Boltzmann equation using first-order Chapman--Enskog expansion within the relaxation time approximation. There are observed to be significant effects from the isospin asymmetry as well as the strangeness of the medium on the diffusion coefficients. The coefficient of shear viscosity, $η$ is observed to have a large enhancement in the presence of finite strangeness in the medium due to additional contributions from the hyperons. The effects due to isospin asymmetry on the shear viscosity coefficient is however observed to be marginal both in nuclear and hyperonic matter. The present study can be relevant for the experimental observables of asymmetric relativistic heavy-ion collisions, e.g., in the compressed baryonic matter (CBM) experiment at the FAIR facility at GSI as well as in the future J-PARC-HI program.

nucl-th↗

arXiv2026

Forward hadron production in pp collisions at LHC energies from an event generator based on the color glass condensate framework

We investigate inclusive forward single-hadron production in high-energy proton--proton collisions using a CGC-inspired Monte Carlo event generator, MC-CGC. We carried out a systematic study of the sensitivity of the running-coupling Balitsky-Kovchegov (rcBK) evolution equation to its initial conditions by comparing three parameterizations: the McLerran-Venugopalan (MV) model and its two HERA DIS-constrained variants, MV$^γ$ and MV$^e$. Our results indicate that the current LHCb data favor the MV$^γ$ and MV$^e$ models, while the differences from the original MV model become more pronounced at higher transverse momentum and at mid-rapidity. As a complementary analysis, we also compared the dilute-dense (DHJ factorization) and dense-dense ($k_T$ factorization) frameworks. We found that the $k_T$ factorization framework provides a better description of the particle production spectra at mid-rapidity than the DHJ framework, where both the projectile and target are in the dense regime at LHC energies. Predictions for the FoCal measurements at ALICE, including the production of identified neutral mesons and jets, are also presented.

hep-ph↗

arXiv2026

Equilibrated fraction of QCD matter in high-energy oxygen--oxygen collisions

We quantify to what degree the QCD matter created in high-energy oxygen--oxygen ($\mathrm{O}+\mathrm{O}$) collisions at $\sqrt{s_{NN}} = 5.36$ TeV reaches a locally equilibrated state. For this purpose, we employ a novel framework based on the core--corona picture that describes the dynamics of both locally equilibrated fluids (the core) and nonequilibrium particles (the corona). Contributions from the core become larger than those from the corona above charged-particle multiplicity at midrapidity, $\langle dN_{\mathrm{ch}}/dη\rangle_{|η|<0.5} \approx 20$. We also find that nonnegligible contributions from the corona still remain even in central $\mathrm{O}+\mathrm{O}$ collisions. The yield ratios of strange baryons to charged pions exhibit an increasing behavior with increasing multiplicity at midrapidity. However, these ratios are smaller than those obtained when assuming that QCD matter has reached complete chemical equilibrium. These results demonstrate that a purely hydrodynamic approach is insufficient and that the inclusion of a corona component is essential for describing the dynamics of intermediate-size systems such as $\mathrm{O}+\mathrm{O}$ collisions.

nucl-th↗

arXiv2025

Rapidity scan with DCCI at LHC energy

We extend the dynamical core-corona initialization (DCCI2) model to include the baryon number evolution in the entire system created in high-energy heavy-ion collisions. Introducing the source term for the baryon number, we describe the early-stage equilibration and later-stage hydrodynamic evolution of the baryon number throughout the system from midrapidity to forward rapidity. Through numerical simulations with this extended model, we show that extremely large baryon chemical potentials are realized in forward rapidity regions at the LHC energy and are comparable to those of the BES energies. Moreover, we show that fluctuations of baryon chemical potentials are large and, consequently, negative baryon chemical potential regions appear at midrapidity.

nucl-th↗

arXiv2025

Dynamical evolution of critical fluctuations with second-order baryon diffusion coupled to chiral condensate

We develop a dynamical model to describe critical fluctuations in heavy-ion collisions, incorporating the baryon diffusion current and chiral condensate as dynamical degrees of freedom, to address their nontrivial scale separation. The model couples fluctuations of the chiral condensate $σ$ with baryon density fluctuations $n$ and the diffusion current $ν$ based on a second-order diffusion equation with a finite relaxation time of the baryon diffusion $τ_\mathrm{R}$. We analyze the spacetime evolution and these correlation functions of the fluctuations in one-dimensionally expanding background. We confirm that an appropriate relaxation time $τ_\mathrm{R}$ ensures causality. We show that propagating waves with finite $τ_\mathrm{R}$ split into two modes at the critical temperature due to a rapid change of kinetic coefficients. In the correlation functions, we find that dynamical $σ$ blurs the structure and peak around the critical temperature. With finite $τ_\mathrm{R}$, the effect of the critical fluctuations persists longer into the later stages of the evolution. These findings suggest importance of dynamical effects of the chiral condensate and baryon diffusion current in identifying critical-point signals in heavy-ion collisions, where the scale separation is nontrivial.

nucl-th↗

arXiv2025

Constraint on initial conditions of one-dimensional expanding fluids from nonlinear causality

The initial conditions of one-dimensional expanding viscous fluids in relativistic heavy-ion collisions are scrutinized in terms of nonlinear causality of the relativistic hydrodynamic equations. Conventionally, it is believed that the matter generated in relativistic heavy-ion collisions starts to behave as a fluid all at once at some initial time. However, it is by no means trivial how soon after the first contact of two high-energy nuclei the fluid picture can be applied. It is demonstrated that one-dimensional expanding viscous fluids violate the necessary and the sufficient conditions of nonlinear causality at large departures from local equilibrium. We therefore quantify the inverse Reynolds number to justify the hydrodynamic description to be valid. The initial conditions are strictly constrained not to violate the causality conditions during the time evolution. With the help of the transverse energies per rapidity measured at RHIC and LHC, we obtain the minimum initial proper time and the maximum energy density allowed by nonlinear causality. This analysis strongly suggests that the initial stage of relativistic heavy-ion collisions needs to be described by a non-equilibrium description other than the framework of relativistic dissipative hydrodynamics.

nucl-th↗

arXiv2024

p--$ϕ$ femtoscopic correlation analysis using a dynamical model

We analyse the p--$ϕ$ correlation functions in high-multiplicity proton+proton collisions at the LHC using a dynamical model, DCCI2, and discuss the effects of collision dynamics on the p--$ϕ$ femtoscopic study. Collision dynamics, such as collective expansion and hadronic rescatterings, leads to the relative momentum-dependent non-Gaussian source functions. This results in deviations in the correlation functions compared to those using the Gaussian source function adopted in existing studies. Our analysis shows the importance of using the source functions that reflect more realistic collision dynamics for future precision analysis of hadron interactions via femtoscopy.

hep-ph↗

arXiv2024

Causal hydrodynamic fluctuations in a one-dimensional expanding system

We derive equations of motion of hydrodynamic fluctuations performing perturbative expansion of the energy-momentum conservation equations around the boost invariant solution in one-dimensional expanding system. In the course of derivation, we do not assume any specific forms of constitutive equations for shear stress tensor $π^{μν}$ and bulk pressure $Π$. Therefore, the framework enables us to employ any constitutive equations beyond the Navier-Stokes theory which satisfy the causality. Employing Israel-Stewart equations as examples of the constitutive equations, we demonstrate the dynamics of causal hydrodynamic fluctuations in (1+1)-dimensional Milne coordinates. We observe that structure of energy density fluctuations is almost frozen in the early stage of the expansion. Two-point correlations of energy density fluctuations turn out to be closely related with the properties of the medium such as sound velocity, viscosity, and relaxation time. Furthermore, we show that two-particle correlation functions of final hadrons after freezeout inherit correlations of thermodynamic variables and flow rapidity. This opens a new door for an analysis of transport properties of the medium produced in relativistic heavy ion collisions.

nucl-th↗

arXiv2024

Hadronisation in event generators from small to large systems

The results of the dynamical core-corona initialisation framework in p+p and Pb+Pb collisions at the LHC energies are presented. We extract the fractions of final hadron yields originating from equilibrated and non-equilibrated matter as functions of multiplicity. We show that the contribution from non-equilibrated matter is non-negligible even in intermediate and central Pb+Pb collisions. The particle production from non-equilibrated matter behaves as a correction on $c_2\{4\}$ that is purely obtained from the equilibrated matter. The result poses a warning on Bayesian parameter estimation with conventional hydrodynamic models. The observed flow coefficients might need a reinterpretation with new dynamical models which incorporate the particle production from non-equilibrated matter.

nucl-th↗

arXiv2023

Dynamics of causal hydrodynamic fluctuations in an expanding system

We develop a framework of causal hydrodynamic fluctuations in one-dimensional expanding system performing linearisation of the hydrodynamic equations around the boost invariant solution. Through the description of space-time evolution of thermodynamic variables and flow velocity, we find a novel phenomenon that the structure of thermodynamic variables is almost frozen. We also show that two-particle correlation functions of final hadrons after freezeout are closely related with the mass of hadrons and properties of the medium such as viscosity, relaxation time and equation of state.

nucl-th↗

arXiv2023

Hydrodynamic fluctuations and ultra-central flow puzzle in heavy-ion collisions

One of the long-standing problems in the field of high-energy heavy-ion collisions is that the dynamical models based on viscous hydrodynamics fail to describe the experimental elliptic flow $v_2$ and the triangular flow $v_3$ simultaneously in ultra-central collisions. The problem, known as the "ultra-central flow puzzle", is specifically that hydrodynamics-based models predict the flow ratio of the two-particle cumulant method $v_2\{2\}/v_3\{2\} > 1$ while $v_2\{2\}/v_3\{2\} \sim 1$ in the experimental data. In this Letter, we focus on the effects of hydrodynamic fluctuations during the space-time evolution of the QGP fluid on the flow observables in the ultra-central collisions. Using the (3+1)-dimensional integrated dynamical model which includes relativistic fluctuating hydrodynamics, we analyze the anisotropic flow coefficients $v_n\{2\}$ in 0-0.2% central Pb+Pb collisions at $\sqrt{s_\text{NN}}=2.76~\text{TeV}$. We find that the hydrodynamic fluctuations decrease the model overestimate of $v_2\{2\}/v_3\{2\}$ from the experimental data by about 19% within the present setup of $η/s = 1/2π$. This means that the hydrodynamic fluctuations qualitatively have an effect to improve the situation for the puzzle, but the effect of the hydrodynamic fluctuations alone is quantitatively insufficient to resolve the puzzle. The decrease of the ratio largely depends on the shear viscosity $η/s$, which calls for future comprehensive analyses with, for example, a realistic temperature-dependent viscosity.

nucl-th↗

arXiv2022

Interplay between core and corona from small to large systems

We present new results in $p$+$p$ and Pb+Pb collisions at the LHC energies from the updated dynamical core--corona initialization framework (DCCI2). The fractions of final hadron yields originating from equilibrated and non-equilibrated components are extracted as functions of multiplicity. We find that the contributions from non-equilibrated components are non-negligible even in Pb+Pb collisions and affect $p_T$-integrated multi-particle correlations. These suggest the importance of non-equilibrated components for the sophisticated extraction of properties of the quark gluon plasma from comparisons between dynamical frameworks and experimental data.

nucl-th↗

arXiv2022

Non-equilibrium components in very low transverse momentum region in high-energy nuclear collisions

We analyze Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV with a novel framework based on the dynamical core--corona picture that describes particle productions from both equilibrium and non-equilibrium components. We remark the possibility of the contribution from non-equilibrium components at very low transverse momentum ($p_{T}$) region and show that such contributions significantly affect $p_{T}$-integrated four-particle cumulants. These results strongly suggest the necessity of non-equilibrium components when one extracts properties of the quark-gluon plasma from experimental data using sophisticated dynamical models based on relativistic hydrodynamics.

nucl-th↗

arXiv2021

Effects of hydrodynamic and initial longitudinal fluctuations on rapidity decorrelation of collective flow

We investigate the interplay between hydrodynamic fluctuations and initial longitudinal fluctuations for their effects on the rapidity decorrelation of collective flow in high-energy nuclear collisions. We use a (3+1)-dimensional integrated dynamical model in which we combine initial conditions with longitudinal fluctuations, fluctuating hydrodynamics and hadronic cascades. We analyse the factorisation ratio in the longitudinal direction to study the effect of these fluctuations on the rapidity decorrelation. We find an essential difference between the effects of the hydrodynamic fluctuations and the initial longitudinal fluctuations in the centrality dependence of the factorisation ratios. A combination of the hydrodynamic fluctuations and the initial longitudinal fluctuations leads to reproduction of the centrality dependence of the second-order factorisation ratio, $r_2(η_\mathrm{p}^\mathrm{a},η_\mathrm{p}^\mathrm{b})$, measured by the CMS Collaboration. Our model also qualitatively describes the centrality dependence of the third-order factorisation ratio, $r_3(η_\mathrm{p}^\mathrm{a},η_\mathrm{p}^\mathrm{b})$. These results demonstrate the importance of the hydrodynamic fluctuations, as well as the initial longitudinal fluctuations, in understanding the longitudinal dynamics of high-energy nuclear collision reactions.

nucl-th↗

arXiv2021

Interplay between core and corona components in high-energy nuclear collisions

We establish the updated version of dynamical core--corona initialization framework (DCCI2) as a unified description from small to large colliding systems and from low to high transverse momentum ($p_T$) regions. Using DCCI2, we investigate effects of interplay between locally equilibrated and non-equilibrated systems, in other words, core and corona components in high-energy nuclear collisions. Given experimental multiplicity distributions and yield ratios of $Ω$ baryons to charged pions as inputs, we extract the fraction of core and corona components in p+p collisions at $\sqrt{s}=7$ TeV and Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV. We find core contribution overtakes corona contribution as increasing multiplicity above $\langle dN_{\mathrm{ch}}/dη\rangle_{|η|<0.5} \sim 18$ regardless of the collision system or energy. We also see that the core contribution exceeds the corona contribution only in 0.0-0.95\% multiplicity class in p+p collisions. Notably, there is a small enhancement of corona contribution with $\sim20$\% below $p_T\sim 1$ GeV even in minimum bias Pb+Pb collisions. We find that the corona contribution at low $p_T$ gives $\sim 15$-$38$ $\%$ correction on $v_2\{2\}$ at $N_{\mathrm{ch}}\lesssim 370$. This raises a problem in conventional hydrodynamic analyses in which low $p_T$ soft hadrons originate solely from core components. We finally scrutinize the roles of string fragmentation and the longitudinal expansion in the transverse energy per unit rapidity, which is crucial in initial conditions for hydrodynamics from event generators based on string models.

nucl-th↗

arXiv2020

Rapidity decorrelation of anisotropic flow caused by hydrodynamic fluctuations

We investigate the effect of hydrodynamic fluctuations on the rapidity decorrelations of anisotropic flow in high-energy nuclear collisions using a (3+1)-dimensional integrated dynamical model. The integrated dynamical model consists of twisted initial conditions, fluctuating hydrodynamics, and hadronic cascades on an event-by-event basis. To understand the rapidity decorrelation, we analyze the factorization ratio in the longitudinal direction. Comparing the factorization ratios between fluctuating hydrodynamics and ordinary viscous hydrodynamics, we find a sizable effect of hydrodynamic fluctuations on rapidity decorrelations. We also propose to calculate the Legendre coefficients of the flow magnitude and the event-plane angle to understand the decorrelation of anisotropic flow in the longitudinal direction.

nucl-th↗

arXiv2020

Unified description of hadron yield ratios from dynamical core-corona initialization

We develop the dynamical core-corona initialization framework as a phenomenological description of the formation of quark gluon plasma (QGP) fluids in high-energy nuclear collisions. Using this framework, we investigate the fraction of the fluidized energy to the total energy and strange hadron yield ratios as functions of multiplicity and scrutinize the multiplicity scaling of hadron yield ratios recently reported by the ALICE Collaboration. Our results strongly indicate that the QGP fluids are partly formed even at the averaged multiplicity for nonsingle diffractive p+p events.

nucl-th↗