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Chiho Nonaka

Publications and source records attributed to Chiho Nonaka.

At least 19 recordsLinked to original sources
arXiv2026

Non-perturbative quark production and transport in the evolving glasma: the WAGASHI event generator

We study non-perturbative quark-antiquark pair production and the subsequent quark dynamics in the earliest glasma stage of relativistic heavy-ion collisions. For that, we develop a new model, WAGASHI (Wong-precessing Anisotropic Glasma And ScHwinger-produced Initial-conditions), which combines classical Yang-Mills glasma evolution, Wong-equation transport, and Schwinger pair production on an event-by-event basis. We find that a sizeable number of quarks, comparable to the final hadron yields, are produced already during the glasma stage and subsequently undergo substantial momentum broadening and spin randomization, suggesting a significant contribution toward the early equilibration of the quark-gluon plasma. We also determine the event-by-event distributions of baryon number, electric charge, strangeness, and spin polarization in Pb-Pb and O-O collisions at LHC energies, finding particularly large fluctuations in the smaller O-O system. These results provide dynamical initial conditions for the subsequent hydrodynamic evolution of the quark-gluon plasma.

hep-ph↗
arXiv2026

Thermal dileptons to probe the baryon-rich QCD matter in the forward region of LHC energy heavy-ion collisions

We investigate thermal dilepton production from a quark-gluon plasma (QGP) with finite baryon chemical potential ($\mu_{\text{B}}$) in central Pb-Pb collisions at $\sqrt{s_{\text{NN}}}=5.02~\text{TeV}$. Recent studies suggest that sizable baryon densities can be achieved at forward rapidity even at LHC energies. We incorporate finite $\mu_{\text{B}}$ into a (3+1)-dimensional hydrodynamic framework and find that $\mu_{\text{B}}$ exceeds 500 MeV around $\eta_\text{s} = 6$ during the medium evolution. Using this framework, we calculate thermal dilepton spectra over a wide rapidity range and evaluate the impact of finite $\mu_{\text{B}}$ on dilepton production. A suppression of 3-4% is observed in the forward-rapidity region $5.2 < y < 7.2$ due to the reduced quark-antiquark abundance at finite baryon density. We further examine the effective temperature extracted from dilepton mass spectra in the intermediate-mass region $1.2 < M_{\ell \ell} < 2.6~\text{GeV}$ . The effective temperature remains strongly correlated with the underlying hydrodynamic temperature and retains sensitivity to the early high-temperature stage of the QGP evolution. These results demonstrate that forward-rapidity dileptons remain effective thermometers while providing sensitivity to finite baryon density at the LHC.

hep-ph↗
arXiv2026

Chemical Equilibration and Thermalization of Quark-Gluon Plasma in a Parton Cascade Model with 2-to-3 Quark Interactions

We investigate the thermalization, chemical equilibration, and hydrodynamization behavior of the far-from-equilibrium, gluon-dominated quark gluon plasma (QGP) produced in Au+Au collisions at $\sqrt{s_{\text{NN}}} = 200$ GeV using the hadronic transport model SMASH extended to simulate partonic interactions. The initial conditions are prepared using the mini-jet model with nuclear parton distribution functions. We first validate the model in a box simulation with the periodic boundary condition to establish indicators for thermalization, chemical equilibration, and hydrodynamization by analyzing energy spectrum and momentum anisotropy. We observe that the additional inelastic channels accelerate thermalization and chemical equilibration compared to the gluon-only scheme. Applying the same framework to the expanding medium, we find that the energy spectrum converges toward the Boltzmann distribution at $t \sim 0.2$ fm while momentum isotropization is achieved at $t \sim 2$ fm, but chemical equilibration is not clearly established even after 5 fm. The Knudsen number rises above unity after $\sim 4$ fm, indicating a breakdown of the hydrodynamic regime at later times consistent with other kinetic theory approaches.

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arXiv2025

Investigating effects of the electrical conductivity of QCD matter on charge-dependent directed flow

Charge dependent directed flow is an important observable of electromagnetic fields in relativistic heavy-ion collisions. We demonstrate how the difference in charge dependent directed flows between protons and antiprotons is sensitive to the resistivity, inverse of quark-gluon plasma's electric conductivity, over different collision centralities. Our model numerically solves the 3+1D relativistic resistive magneto-hydrodynamic (RRMHD) equations, assuming the electric conductivity to be a scalar. For this work, we focus on symmetric Au + Au collisions at the top RHIC energy of $\sqrt{s}=200$ GeV. We illustrate the time evolution of the electromagnetic fields in our model and connect that to the charge dependent directed flow results. Our results highlight the importance of modeling quark-gluon plasma's electric conductivity for charge dependent observables in relativistic heavy-ion collisions.

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arXiv2024

Estimate of virtual photon polarization due to the intense magnetic field in Pb-Pb collisions at the LHC energies

We present the first numerical calculation of the virtual photon polarization and assess the feasibility of measuring the polarization via the anisotropic decay $\gamma^{*} \rightarrow \mu\mu$ using the LHC-ALICE detector. In presence of intense magnetic fields generated in high-energy non-central heavy-ion collisions that exceed the critical magnetic field intensity of quantum electrodynamics (QED), prompt virtual photons are predicted to decay anisotropically into lepton pairs, which we call virtual photon polarization. Using a relativistic resistive magnetohydrodynamics model, we computed the time evolution of the magnetic field and used these results to estimate the averaged polarization by calculating the vacuum polarization under the influence of the magnetic field at specific times. The estimated polarization deviates from zero with a statistical significance of $0.07\sigma$ with the data statistics collected from 2010 to 2011 and $0.15\sigma$ with the one from 2015 to 2018. It is understandable that the magnetic field could not be detected through polarization due to low statistical significance. With the data collecting the ongoing ALICE run from 2023 to 2026, the statistics dramatically increase by the upgraded LHC and the new data processing system. Thereby we expect that the statistical significance could reach $\sim 1.7\sigma$, resulting in a promising probe for detecting the intense magnetic fields.

hep-ph↗
arXiv2023

Fate of the $\rho$-$a_1$ 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 $\rho$ 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

Probing the QCD phase transition with chiral mixing in dilepton production

We perform a systematic study of dilepton emission in a hot QCD medium based on three different scenarios of chiral mixing, each of which yields a characteristic structure in the vector spectral function. The in-medium spectral functions are accommodated into the state-of-the-art hydrodynamic simulations for a relativistic viscous fluid to calculate the dilepton production rate, fully accounting for the space-time evolution of a created fireball in relativistic heavy-ion collisions. We demonstrate that the low-temperature theorem of chiral mixing extrapolated toward a chiral crossover, often used in the literature, leads to critical shortcomings: the inadequacy of width broadening, and a substantial overestimate of the dilepton yield maximized around the invariant mass of $M = 1.2$ GeV. The proper prescription offers a milder yet sizable increase in the window of $1.1 < M < 1.4$ GeV as the direct signature of chiral symmetry restoration.

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arXiv2022

Charge-dependent anisotropic flow in high-energy heavy-ion collisions from relativistic resistive magneto-hydrodynamic expansion

We have investigated the charge-dependent anisotropic flow in high-energy heavy-ion collisions, using relativistic resistive magneto-hydrodynamics (RRMHD). We consider the optical Glauber model as an initial model of the quark-gluon plasma (QGP) and the solution of the Maxwell equations with source term of the charged particles in two colliding nuclei as initial electromagnetic fields. The RRMHD simulation is performed with these initial conditions in Au-Au and Cu-Au collisions at $\sqrt{s_{\mathrm{NN}}} = 200$ GeV. We have calculated the charge-odd contribution to the directed flow $\Delta v_1$ and elliptic flow $\Delta v_2$ in both collisions based on electric charge distributions as a consequence of RRMHD. Our results show that the $\Delta v_1$ and $\Delta v_2$ are approximately proportional to the electrical conductivity ($\sigma$) of the medium. In the $\sigma=0.023~\mathrm{fm}^{-1}$ case, our result of $\Delta v_1$ is consistent with STAR data in Au-Au collisions. Furthermore, in Cu-Au collisions, $\Delta v_1$ has a non-zero value at $\eta = 0$. We conclude that the charge-dependent anisotropic flow is a good probe to extract the electrical conductivity of the QGP medium in high-energy heavy-ion experiments.

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arXiv2022

Relativistic resistive magneto-hydrodynamics code for high-energy heavy-ion collisions

We construct a relativistic resistive magneto-hydrodynamic (RRMHD) numerical simulation code for high-energy heavy-ion collisions. We split the system of differential equations into two parts, a non-stiff and a stiff part. For the non-stiff part, we evaluate the numerical flux using HLL approximated Riemann solver and execute the time integration by the second-order of Runge-Kutta algorithm. For the stiff part, which appears in Ampere's law, we integrate the equations using semi-analytic solutions of the electric field. We employ the generalized Lagrange multiplier method to ensure the divergence-free constraint for the magnetic field and Gauss's law. We confirm that our code reproduces well the results of standard RRMHD tests in the Cartesian coordinates. In the Milne coordinates, the code with high conductivity is validated against relativistic ideal MHD tests. We also verify the semi-analytic solutions of the accelerating longitudinal expansion of relativistic resistive magneto-hydrodynamics in high-energy heavy-ion collisions in a comparison with our numerical result. Our numerical code reproduces these solutions.

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arXiv2022

Directed flow in relativistic resistive magneto-hydrodynamic expansion for symmetric and asymmetric collision systems

We construct a dynamical model for high-energy heavy-ion collision based on the relativistic resistive magneto-hydrodynamic framework. Using our newly developed (3+1)-dimensional relativistic resistive magneto-hydrodynamics code, we investigate magneto-hydrodynamic expansion in symmetric and asymmetric collision systems as a first application to high-energy heavy-ion collisions. As a realistic initial condition for electromagnetic fields, we consider the solutions of the Maxwell equations with the source term of point charged particles moving in the direction of the beam axis, including finite constant electrical conductivity of the medium. We evaluate the directed flow in the symmetric and asymmetric collisions at RHIC energy. We find a significant effect of finite electrical conductivity on the directed flow in the asymmetric collision system. We confirm that a certain amount of energy transfer by dissipation associated with Ohmic conduction occurs in the asymmetric collision system because of asymmetry of the electric field produced by two different colliding nuclei. Because this energy transfer makes the pressure gradient of the medium flatter, the growth of directed flow decreases.

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arXiv2022

Radiative hadronization: Photon emission at hadronization from quark-gluon plasma

We investigate photon emission at the hadronization stage from a quark-gluon plasma created in relativistic heavy-ion collisions. A recombination-model picture suggests that a quark and an antiquark bind into a meson state in hadronization, which would apparently violate the energy conservation if there is nothing else involved. We consider here a hadronization process where the recombination accompanies a photon emission. This is an analog of the "{\it radiative recombination}" known in plasma physics, such as $e^- + p^+ \to {\rm H}^0 +\gamma$, which occurs when an electromagnetic plasma goes back to a neutral atomic gas. The "radiative hadronization" picture will bring about (i) an enhancement of the photon yield, (ii) significant flow of photons similar to that of hadrons, and (iii) the photon transverse momentum ($p_T$) distribution with a thermal profile whose effective temperature is given by blue-shifted temperature of quarks. Here as a simplest and phenomenological realization of the radiative hadronization, we modify the recombination model to involve a photon emission and evaluate the photon yield with this modified model. Adding this contribution to the direct photon yield along with thermal photon contribution calculated with a hydrodynamic model and a parametrized contribution of prompt photons, we study the $p_T$ spectrum and elliptic flow of the photons produced in heavy-ion collisions at RHIC and LHC energies.

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arXiv2021

Inhomogeneous Phases in the Chiral Gross-Neveu Model on the Lattice

We discuss possible existence of inhomogeneous phase in low temperature and high density region in the 1+1 dimensional chiral Gross-Neveu ( $χ$GN$_2$ ) model on the lattice. First we investigate the phase structure of the $χ$GN$_2$ model at vanishing chemical potential, changing temperature. From behavior of $Δ^2 = Σ^2 + Π^2$ as a function of Monte Carlo time, a candidate of an order parameter for chiral symmetry in the $χ$GN$_2$ model is $Δ$. At vanishing chemical potential, we observe restoration of chiral symmetry at high $T$. In low temperature and high chemical potential region, we find existence of inhomogeneous phase in spatial correlation functions of $σ$ and $π$. The signal of inhomogeneous phase in correlation function of $Δ$ is more clearly than that of $σ$ or $π$.

hep-lat↗
arXiv2019

Dynamically Integrated Transport Approach for High-Energy Nuclear Collisions at High Baryon Density

To explore the structure of the QCD phase diagram in high baryon density domain, several high-energy nuclear collision experiments in a wide range of beam energies are currently performed or planned using many accelerator facilities. In these experiments search for a first-order phase transition and the QCD critical point is one of the most important topics. To find the signature of the phase transition, experimental data should be compared to appropriate dynamical models which quantitatively describe the process of the collisions. In this study we develop a new dynamical model on the basis of the non-equilibrium hadronic transport model JAM and 3+1D hydrodynamics. We show that the new model reproduce well the experimental beam-energy dependence of hadron yields and particle ratio by the partial thermalization of the system in our core-corona approach.

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arXiv2019

First 4D lattice calculation of transport coefficient $\hat{q}$ for pure gluon plasma

The transport coefficient $\hat{q}$ plays a pivotal role in describing the phenomenon of jet quenching in the quark-gluon plasma (QGP) produced in ultra-relativistic nucleus-nucleus collisions. It is challenging to compute this coefficient from first principles due to its non-perturbative nature. In this article, we present an $ab$-$initio$ formulation of $\hat{q}$ based on the standard techniques of perturbative quantum chromodynamics (pQCD) and lattice gauge theory. We construct $\hat{q}$ by considering a leading order (LO) process where a hard parton produced from the hard scattering undergoes transverse broadening due to scatterings with the thermal medium. We do an analytic continuation to the Euclidean region and use the dispersion relation to express $\hat{q}$ in terms of series of local Field-Strength-Field-Strength (FF) operators. Each term in the series is suppressed by the hard scale $q^{-}$. Finally, we compute the local operators on the quenched SU(3) lattice and present our estimates for $\hat{q}$.

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arXiv2019

Temperature dependence of transport coefficients of QCD in high-energy heavy-ion collisions

Using our developed new relativistic viscous hydrodynamics code, we investigate the temperature dependence of shear and bulk viscosities from comparison with the ALICE data: single particle spectra and collective flows of Pb+Pb $\sqrt{s_{\rm NN}}=2.76$ TeV collisions at the Large Hadron Collider. We find that from the comprehensive analyses of centrality dependence of single particle spectra and collective flows we can extract detailed information on the quark-gluon plasma bulk property, without the information being smeared by the final state interactions.

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arXiv2018

Nature of the $a_1$ meson in lattice quantum chromodynamics studied with chiral fermions

We study the $a_1$ meson using a quenched lattice quantum chromodynamics simulation with the truncated overlap fermions formalism based on the domain wall fermions. The obtained lightest mass of the $a_1$ meson, 1272(45) MeV, is consistent with the experimental value for $a_1$(1260). Thus, $a_1$(1260) can be identified to have a simple two-body constituent-quark structure. Our quenched simulation result of $a_1$(1420) can not explain the experimental mass value, which suggests $a_1$(1420) is not a simple $q\bar{q}$ two quark state.

hep-lat↗
arXiv2018

First calculation of $\hat{q}$ on a quenched SU(3) plasma

The jet transport coefficient $\hat{q}$ is the leading transport coefficient that controls the modification of hard jets produced in heavy-ion collisions. This coefficient is inherently non-perturbative, and hence, is challenging to compute from first principles. In this report, we present a perturbative quantum chromodynamics (pQCD) and lattice gauge theory based formulation to study $\hat{q}$. We formulate $\hat{q}$ within a 4-dimensional (4D) quenched SU(3) lattice. We consider a leading order diagram for a hard parton passing through the quark-gluon plasma. The non-perturbative part is expressed in terms of a non-local (two-point) Field-Strength-Field-Strength (FF) operator product which can be Taylor expanded after analytic continuation to the Euclidean region. Such an expansion allows us to write $\hat{q}$ in terms of the expectation of local operators. Finally, we present our results for $\hat{q}$ in a pure gluon plasma.

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