arXiv subjects

Azumi Sakai

Publications and source records attributed to Azumi Sakai.

9 recordsLinked to original sources

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↗

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 $\sigma$ with baryon density fluctuations $n$ and the diffusion current $\nu$ based on a second-order diffusion equation with a finite relaxation time of the baryon diffusion $\tau_\mathrm{R}$. We analyze the spacetime evolution and these correlation functions of the fluctuations in one-dimensionally expanding background. We confirm that an appropriate relaxation time $\tau_\mathrm{R}$ ensures causality. We show that propagating waves with finite $\tau_\mathrm{R}$ split into two modes at the critical temperature due to a rapid change of kinetic coefficients. In the correlation functions, we find that dynamical $\sigma$ blurs the structure and peak around the critical temperature. With finite $\tau_\mathrm{R}$, 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↗

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

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.

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↗

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↗

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↗

arXiv2019

Rapidity Decorrelation from Hydrodynamic Fluctuations and Initial Fluctuations

Rapidity decorrelation in high energy heavy-ion collisions is one of the hot topics in understanding longitudinal dynamics of the quark gluon plasma (QGP). In this study we employ an integrated dynamical model with full three dimensional relativistic hydrodynamics and perform event-by-event numerical simulations of Pb+Pb collisions at the LHC energy. We analyze factorization ratios to understand rapidity decorrelation from hydrodynamic fluctuations and initial longitudinal fluctuations. We show that factorization breaking happens due to both hydrodynamic fluctuations and initial longitudinal fluctuations. We conclude hydrodynamic fluctuations and initial longitudinal fluctuations are both important in understanding rapidity decorrelation.

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arXiv2018

Rapidity decorrelation from hydrodynamic fluctuations

We discuss rapidity decorrelation caused by hydrodynamic fluctuations in high-energy nuclear collisions at the LHC energy. We employ an integrated dynamical model which is a combination of the Monte Carlo version of Glauber model with extension to longitudinal direction for initial conditions, full three-dimensional relativistic fluctuating hydrodynamics for the space-time evolution of created matter in the intermediate stage and a hadronic cascade model in the late stage. We switch on and off the hydrodynamic fluctuations in the hydrodynamic stage to understand the effects of hydrodynamic fluctuations on factorisation ratios $r_{n}(η^a_p, η^b_p)$. To understand the rapidity gap dependence of the factorisation ratio comprehensively, we analyse Legendre coefficients $A^k_2$ and $B^k_2$.

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