Heavy-Ion Collisions toward High-Density Nuclear Matter

Abstract

In the present paper, the current efforts in heavy-ion collisions toward high-density nuclear matter will be discussed. First, the essential points learned from RHIC and LHC will be reviewed. Then, the present data from the STAR Beam Energy Scan are discussed. Finally, the current efforts, NICA, FAIR, HIAF, and J-PARC-HI (heavy ion) are described. In particular, the efforts of the J-PARC-HI project are described in detail.

Keywords: FAIR; HIAF; J-PARC-HI; LHC; NICA; RHIC; high-density nuclear matter; high-temperature matter; relativistic heavy-ion collision.

Figure 1

A photo taken in 2005 for Professor ZIMÁNYI József (1931–2006) and Ms. ZIMÁNYI Magdolna.

Figure 2

Comparison of relativistic heavy-ion collisions at the AGS energy region (left) and the RHIC/LHC energy region (right). At the RHIC/LHC energy region, the Lorenz-contracted nuclei penetrate through each other to form a hot but baryon-free region in the middle, whereas at the AGS energy region, colliding nuclei stop each other to form a baryon-rich region in the middle. y_T and y_B indicate target rapidity and projectile rapidity, respectively.

Figure 3

The proposed phase diagram of nuclear matter in terms of temperature (T) and baryonic chemical potential (μ_B) where μ_B is a measure of density at above μ_B ~1 GeV.

Figure 4

Jet quenching (left) [2] and strong elliptic flow (right) [3] observed at RHIC.

Figure 5

Azimuthal asymmetry of transverse energies observed in central collisions at ATLAS [8]. A similar phenomenon is observed at both CMS at CERN [9] as well as STAR [10]. Pictorial explanations for both peripheral and central collisions are also shown in the figure.

Figure 6

Quark number scaling of the v₂ flow data: (a) measured and (b) normalized by n_q [11,12].

Figure 7

Analysis for the available data of v₂ to v₅ [13].

Figure 8

Comparison of η/s for various liquids. The figure is taken from [14].

Figure 9

Suppressions of J/ψ (c$\overline{c}$) and Υ(upsilon = b$\stackrel{⥪}{b}$) and their excited states observed by the CMS group at CERN-LHC [16,17]. The figure is taken from [18].

Figure 10

Chemical freezeout temperature obtained from data from SIS, AGS, SPS, RHIC, and LHC [19], as compared with the Lattice QCD result [20] indicated by the shadow area.

Figure 11

Net proton (proton–antiproton) distributions at AGS, SPS, and RHIC [22].

Figure 12

Schematic drawing when two colliding objects stop one another completely. Left figure is from Landau [23] and the right figure is from Goldhaber [25], though the illustrations are pictorial.

Figure 13

A hadron cascade calculation for heavy-ion collisions at 25 AGeV was created at the proposed JHF project in Japan (before J-PARC). At those energies, the collision reaches well above the expected value of 7ρ₀ [26].

Figure 14

RHIC Collider, STAR Beam Energy Scan (BES), and its extension named FXT superposed on the phase diagram. It is a common expectation that the first-order phase transition would start from the critical point toward high-density regions [31,34].

Figure 15

New data on fluctuations κσ² (right) at the critical point between the crossover transition and the first-order phase transition [34]. Statistics are still low, though. The figure on the left is a normalized baryon number.

Figure 16

In the left figure, (a) shows chemical freezeout temperature and kinetic freezeout temperature. (b) indicates an extracted blast flow [37]. The right figure shows the observed change in v₂ [38]. Note that at around √s_NN = 6–8 GeV, a sudden decrease occurs for T_ch, T_kin, <β>, and v₂.

Figure 17

The observed K/π ratios (left) [37] and expected formation of hypernuclei in relativistic heavy-ion collisions (right) [43]. The latter is based on the coalescence model.

Figure 18

Schematic explanation of the first-order phase transition and the crossover transition. Figures on the left-hand side were taken from [46]. The right-hand figure is a pictorial explanation.

Figure 19

A proposed phase diagram of high-density nuclear matter [33]. The crossover transition was proposed by Hatsuda et al. [45], and it is illustrated in the previous Figure 18.

Figure 20

Planned future accelerators toward high-density nuclear matter. A high flux up to 10 Mz interaction rates is planned [47]. Only STAR BES has been running. Other are the future plans.

Figure 21

NICA project, which was completed in 2020. Beams from the Nucleotron at 4.5 AGeV are injected into a collider called NICA. Figure is taken from [48]. In the inset, a proposed project from 1990, called the PS-Collider [49] at KEK, is shown, although this project was not approved.

Figure 22

The FAIR accelerator project at GSI. The SIS 18 will be used as an injector. The facility will be completed by 2025. The figure is taken from [50].

Figure 23

Two major experiments, CBM and NUSTAR, are planned at FAIR at CSI [51].

Figure 24

The HIAF facility for the study of nuclear matter (above) [51]. In parallel to this HIAF, extremely high-flux CiADS is being constructed. HIAF also uses neutron-rich isotopes by utilizing high-flux beams from CiADS as an ion source, as with ISOLDE at CERN, to significantly enrich the HIAF project [52].

Figure 25

A J-PARC heavy-ion project in Japan, that requires heavy-ion injector alone, as the 3 GeV RCS and the 30–40 GeV MR already exist [54].

Figure 26

The upper figure is the proposed linac from JAEA and the lower figure is a planned usage of the existing KEK-PS Booster ring (already disposed but working at KEK). This revised plan was proposed in 2020.

Figure 27

An initial plan for the J-PARC by using the existing E16 experiment [55].

Figure 27

An initial plan for the J-PARC by using the existing E16 experiment [55].