We propose a new mechanism of generating spin polarization in heavy-ion collisions dominated by nucleon degree of freedom. By incorporating the spin change in nucleon-nucleon scatterings based on the phase shift data together with the constraint of rigorous angular momentum conservation and Pauli blocking, we illustrate through a Boltzmann-Uehling-Uhlenbeck transport model that appreciable spin polarization (about <math altimg='si1.svg'><mn>1</mn><mo>&#8764;</mo><mn>2</mn><mtext>%</mtext></math>) can be generated in intermediate-energy heavy-ion collisions. This mechanism, together with the nuclear spin-orbit potential, may help to understand the spin polarization in few-GeV heavy-ion collisions dominated by nucleon degree of freedom.

China

Rong-Jun Liu

Jun Xu

Yu-Gang Ma

Spin polarization from nucleon-nucleon scatterings in intermediate-energy heavy-ion collisions

We study the transverse momentum ( $p_T$ ) differential anisotropic flow and flow fluctuation in Pb+Pb collisions at $\sqrt{s_{_{NN}}}$ =5.02 TeV at the LHC. A (3+1)-dimensional CLVisc hydrodynamics framework with fluctuating TRENTO (or AMPT) initial conditions is utilized to simulate the space-time evolution of the quark-gluon plasma (QGP) medium. The effects of shear viscosity and the sub-nucleon structure on anisotropic flow and flow fluctuation are analyzed. Our result shows that shear viscosity tends to suppress both flow coefficients ( ${v_2\{2}\}$ , ${v_2\{4\}}$ , and ${\langle v_2\rangle}$ ) and flow fluctuation ( ${\sigma_{v_2}}$ ) owing to its smearing effect on local density fluctuation. The flow coefficients appear to be insensitive to the sub-nucleon structure, whereas the flow fluctuation ${\sigma_{v_2}}$ tends to be suppressed by the sub-nucleon structure in central collisions but enhanced in peripheral collisions. After taking into account the sub-nucleon structure effect, our numerical result can qualitatively describe the relative flow fluctuations ( ${v_2\{4\}/v_2\{2\}}$ , $F({v_2})$ ) measured by the ALICE Collaboration at the LHC. We further investigate the effects of shear viscosity, sub-nucleon structure, and initial condition model on the flow angle and flow magnitude decorrelations ( ${A_2^f}$ , ${M_2^f}$ ) using the four-particle correlation method. We find that the flow decorrelation effect is typically stronger in central collisions than in peripheral collisions. The flow angle decorrelation is found to be insensitive to the shear viscosity and sub-nucleon structure, whereas the flow magnitude decorrelation shows quite different behaviors when using the TRENTO and AMPT initial condition models. Our study sheds light on the anisotropic flow, transport properties, and initial structure of the QGP created in high-energy nuclear collisions.

Germany

Jie Zhu

Canada

Xiang-Yu Wu

Guang-You Qin

Anisotropic flow, flow fluctuation, and flow decorrelation in relativistic heavy-ion collisions: the roles of sub-nucleon structure and shear viscosity * Supported in part by the Natural Science Foundation of China (12225503, 11890710, 11890711, 11935007). Zhu Jie is supported in part by the China Scholarship Council (202306770009). Wu Xiang-Yu is supported in part by the National Science Foundation within the framework of the JETSCAPE collaboration (OAC-2004571) and in part by the Natural Sciences and Engineering Research Council of Canada. Some of the calculations were performed in the Nuclear Science Computing Center at Central China Normal University (NSC$^{3}$), Wuhan, Hubei, China.

Investigating nucleon-nucleon correlations inherent to the strong nuclear force is one of the core goals in nuclear physics research. We showcase the unique opportunities offered by collisions of 16O nuclei at high-energy facilities to reveal detailed many-body properties of the nuclear ground state. We interface existing knowledge about the geometry of 16O coming from <italic>ab-initio</italic> calculations of nuclear structure with transport simulations of high-energy 16O+ 16O collisions. Bulk observables in these processes, such as the elliptic flow or the fluctuations of the mean transverse momentum, are found to depend significantly on the input nuclear model and to be sensitive to realistic clustering and short-range repulsive correlations, effectively opening a new avenue to probe these features experimentally. This finding demonstrates collisions of oxygen nuclei as a tool to elucidate initial conditions of small collision systems while fostering connections with effective field theories of nuclei rooted in quantum chromodynamics (QCD).

United States

Chunjian Zhang

Jinhui Chen

Giuliano Giacalone

Shengli Huang

Jiangyong Jia

<italic>Ab-initio</italic> nucleon-nucleon correlations and their impact on high energy 16O+ 16O collisions

The study of physics at the Planck scale has garnered significant attention due to its implications for understanding the fundamental nature of the universe. At the Planck scale, quantum fluctuations challenge the classical notion of spacetime as a smooth continuum, revealing a complex microstructure that defies traditional models. This study introduces a novel scaling-based framework to investigate the properties of spacetime microstructures. By deriving a scaling-characterized metric tensor and reformulating fundamental equations—including the geodesic, Einstein field, Klein-Gordon, and Dirac equations—into scaling forms, the research reveals new properties of local spacetime dynamics. Remarkably, the golden ratio emerges naturally in linear scale measurements, offering a potential explanation for the role of the Planck length in resolving ultraviolet (UV) divergence. Furthermore, the study demonstrates how scale invariance in spacetime can restore classical geometric stability through the renormalization group equations. These findings significantly revise classical geometric intuitions, providing a fresh lens for understanding quantum fluctuations and offering promising insights for advancing quantum gravity theories.

Weihu Ma

A novel framework for characterizing spacetime microstructure with scaling

Jet-associated deuteron production in pp collisions at <math><msqrt><mrow><mi>s</mi></mrow></msqrt><mo>=</mo><mn>13</mn></math> TeV and p-Pb collisions at <math><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>N</mi><mi>N</mi></mrow></msub></mrow></msqrt><mo>=</mo><mn>5.02</mn></math> TeV is studied in the coalescence model by using the phase-space information of proton and neutron pairs from a multiphase transport (AMPT) model at the kinetic freezeout. In the low transverse momentum (<math><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub></math>) region <math><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub><mo>/</mo><mi>A</mi><mo>&lt;</mo><mn>1.5</mn></math> GeV/c, where A is the mass number of a nucleus, the in-jet coalescence factor <math><msubsup><mrow><mi>B</mi></mrow><mrow><mn>2</mn></mrow><mrow><mtext>In-jet</mtext></mrow></msubsup></math> for deuteron production, given by the ratio of the in-jet deuteron number to the square of the in-jet proton number, is found to be larger than the coalescence factor <math><msub><mrow><mi>B</mi></mrow><mrow><mn>2</mn></mrow></msub></math> in the medium perpendicular to the jet by a factor of about 10 in pp collisions and of 25 in p−Pb collisions, which are consistent with the ALICE measurements at the LHC. Such large low-momentum enhancements mainly come from coalescence of nucleons inside the jet with the medium nucleons. Coalescence of nucleons inside the jet dominates deuteron production only at the higher <math><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub></math> region of <math><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub><mo>/</mo><mi>A</mi><mo>≳</mo><mn>4</mn></math> GeV/c, where both the yield ratio <math><mi>d</mi><mo>/</mo><mi>p</mi></math> of deuteron to proton numbers and the <math><msub><mrow><mi>B</mi></mrow><mrow><mn>2</mn></mrow></msub></math> are also significantly larger in the jet direction than in the direction perpendicular to the jet due to the strong collinear correlation among particles produced from jet fragmentation. Studying jet-associated deuteron production in relativistic nuclear collisions thus opens up a new window to probe the phase-space structure of nucleons inside jets.

Yi-Heng Feng

Che Ming Ko

Kai-Jia Sun

Xin-Nian Wang

Zhong Yang

Song Zhang

Jet-induced enhancement of deuteron production in And -Pb collisions at the LHC

The bubble nuclei are important components of exotic nuclear structures characterized by special depletions of central densities. Focusing on bubble structures of $^{36}$Ar, the characterizations of bubble nuclei were explored with the framework of the extended quantum molecular dynamics model. Three density distribution modes were uncovered for the first time, i.e. micro-bubble, bubble, and cluster resonances, which show unique spectral signature compared to the monopole resonance spectrum as the excitation intensity was increased in bubbles. Of pivotal importance is the revelation that the bubble mode's oscillation frequency closely resembles macroscopic bubble dynamics, building a connection between classical macroscopic phenomena and the quantum complexity of the nuclear structure. The discovery marks a crucial step forward in deciphering the relationship between classical and quantum domains within the enigmatic world of atomic nuclei.

Ge Ren

Chun-Wang Ma

Xi-Guang Cao

Bubble Ar and its new breathing modes

Understanding the properties of hypernuclei helps to constrain the interaction between hyperon and nucleon, which is known to play an essential role in determining the properties of neutron stars. Experimental measurements have suggested that the hypertriton (<math><mmultiscripts><mrow><mtext>H</mtext></mrow><mprescripts></mprescripts><mrow><mi>Λ</mi></mrow><mrow><mn>3</mn></mrow></mmultiscripts></math>), the lightest hypernucleus, exhibits a halo structure with a deuteron core encircled by a Λ hyperon at a distance of about 10 fm. This large <math><mi>Λ</mi><mo>−</mo><mi>d</mi></math> distance in <math><mmultiscripts><mrow><mtext>H</mtext></mrow><mprescripts></mprescripts><mrow><mi>Λ</mi></mrow><mrow><mn>3</mn></mrow></mmultiscripts></math> wave function is found to cause a suppressed <math><mmultiscripts><mrow><mtext>H</mtext></mrow><mprescripts></mprescripts><mrow><mi>Λ</mi></mrow><mrow><mn>3</mn></mrow></mmultiscripts></math> yield and a softening of its transverse momentum (<math><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub></math>) spectrum in relativistic heavy-ion collisions. Within the coalescence model based on nucleons and Λ hyperons from a microscopic hybrid hydro model with a hadronic afterburner for nuclear cluster production in Pb-Pb collisions at <math><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>N</mi><mi>N</mi></mrow></msub></mrow></msqrt></math>= 5.02 TeV, we show how this softening of the hypertriton <math><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub></math> spectrum appears and leads to a smaller mean <math><msub><mrow><mi>p</mi></mrow><mrow><mi>T</mi></mrow></msub></math> for <math><mmultiscripts><mrow><mtext>H</mtext></mrow><mprescripts></mprescripts><mrow><mi>Λ</mi></mrow><mrow><mn>3</mn></mrow></mmultiscripts></math> than for helium-3 ($^{3}$He). The latter is opposite to the predictions from the blast-wave model which assumes that <math><mmultiscripts><mrow><mtext>H</mtext></mrow><mprescripts></mprescripts><mrow><mi>Λ</mi></mrow><mrow><mn>3</mn></mrow></mmultiscripts></math> and $^{3}$He are thermally produced at the kinetic freeze-out of heavy-ion collisions. The discovered quantum mechanical softening of the (anti-)hypertriton spectrum can be experimentally tested in relativistic heavy-ion collisions at different collision energies and centralities and used to obtain valuable insights to the mechanisms for light (hyper-)nuclei production in these collisions.

Dai-Neng Liu

Italy

Francesco Mazzaschi

CERN

Maximiliano Puccio

Qi-Ye Shou

Yuan-Zhe Wang

Softening of the hypertriton transverse momentum spectrum in heavy-ion collisions

Within the Ultra-relativistic Quantum Molecular Dynamics model (UrQMD) coupled with nucleon coalescence model and Mini-Spanning-Tree model, the yields of light nuclei have been stimulated in Au + Au collisions over an energy range of <math><msqrt><mrow><msub><mrow><mi>s</mi></mrow><mrow><mi>N</mi><mi>N</mi></mrow></msub></mrow></msqrt><mo>=</mo><mn>2.0</mn><mo>∼</mo><mn>7.7</mn><mspace width="0.25em"></mspace><mrow><mi>GeV</mi></mrow></math> and the momentum correlation functions of light nuclei pairs have been calculated by both the Lednický-Lyuboshitz and the Correlation After Burner methods. We compare the yields of light nuclei and their momentum correlation functions at midrapidity in this energy region with experimental data. It is found that there are differences between the results of the two models, and the coalescence method seems less valid at low collision energy. Furthermore, both the peak values of proton-proton correlation functions and the transition point of elliptic flows from out-of-plane to in-plane emission show a turning point around 3-4 GeV, which suggests that there is a relation between momentum correlation function and collective flow of particles.

Feng-Hua Qiao

Xian-Gai Deng

Momentum correlation of light nuclei in Au + Au collisions at GeV

Momentum correlation functions between <math><mi>π</mi><mo>,</mo><mo> </mo><mi>K</mi></math>, and <math><mi>p</mi></math> are calculated for several heavy-ion collision systems, namely <math><mrow><mmultiscripts><mi>B</mi><mprescripts></mprescripts><mn>5</mn><mn>10</mn></mmultiscripts><mo>+</mo><mmultiscripts><mi>B</mi><mprescripts></mprescripts><mn>5</mn><mn>10</mn></mmultiscripts></mrow><mo>,</mo><mo> </mo><mrow><mmultiscripts><mi>O</mi><mprescripts></mprescripts><mn>8</mn><mn>16</mn></mmultiscripts><mo>+</mo><mmultiscripts><mi>O</mi><mprescripts></mprescripts><mn>8</mn><mn>16</mn></mmultiscripts></mrow><mo>,</mo><mo> </mo><mrow><mmultiscripts><mi>Ca</mi><mprescripts></mprescripts><mn>20</mn><mn>40</mn></mmultiscripts><mo>+</mo><mmultiscripts><mi>Ca</mi><mprescripts></mprescripts><mn>20</mn><mn>40</mn></mmultiscripts></mrow></math>, and <math><mrow><mmultiscripts><mi>Au</mi><mprescripts></mprescripts><mn>79</mn><mn>197</mn></mmultiscripts><mo>+</mo><mmultiscripts><mi>Au</mi><mprescripts></mprescripts><mn>79</mn><mn>197</mn></mmultiscripts></mrow></math> in central collisions as well as <math><mrow><mmultiscripts><mi>Au</mi><mprescripts></mprescripts><mn>79</mn><mn>197</mn></mmultiscripts><mo>+</mo><mmultiscripts><mi>Au</mi><mprescripts></mprescripts><mn>79</mn><mn>197</mn></mmultiscripts></mrow></math> collision in different centralities at center-of-mass energy <math><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>39</mn></math> GeV within the framework of a multiphase transport model complemented by the Lednický and Lyuboshitz analytical method. The results present the centrality and system-size dependence of the momentum correlation functions among pairs of <math><mi>π</mi><mo>,</mo><mo> </mo><mi>K</mi></math>, and <math><mi>p</mi></math>, from which the emission source-size can be deduced. It is found that the deduced source sizes increase with the decreasing of centrality for Au <math><mo>+</mo></math> Au system or with the increasing of system size in central collisions with different nuclear size. In addition, through the momentum correlation functions of nonidentical particle pairs gated on velocity, the average emission sequence of nonidentical particles can be indicated. The results illustrate that in the small relative momentum region, protons are emitted in average earlier than <math><msup><mi>π</mi><mo>+</mo></msup></math> and <math><msup><mi>K</mi><mo>+</mo></msup></math>, and <math><msup><mi>K</mi><mo>+</mo></msup></math> are emitted averagely earlier than <math><msup><mi>π</mi><mo>+</mo></msup></math>. Furthermore, it seems that larger interval of the average emission order among them is exhibited for smaller collision systems. The present study sheds light on the dynamics of light particle emission at RHIC energy.

Ting-Ting Wang

Calculation of momentum correlation functions between <math><mi>π</mi><mo>,</mo><mo> </mo><mi>K</mi></math>, and <math><mi>p</mi></math> for several heavy-ion collision systems at <math><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>39</mn></mrow></math> GeV

The longitudinal asymmetry in relativistic heavy ion collisions arises from the fluctuation in the number of nucleons involved. This asymmetry causes a rapidity shift in the center of mass of the participating zone. Both the rapidity shift and the longitudinal asymmetry have been found to be significant at the top CERN Large Hadron Collider (LHC) energy for collisions of identical nuclei, and the longitudinal asymmetry is important for reconstructing the colliding vertex and correcting the rapidity shift. However, much discussion of the longitudinal asymmetry has treated the initial condition as a nonzero momentum contributed only by the number of participants, i.e., the asymmetry depends only on the number of participating nucleons. So we naturally raise a physical problem, can other initial conditions, such as two typical initial conditions for nuclei, geometric configuration, and momentum distribution, provide effects on the longitudinal asymmetry? Therefore, in this work we consider other effects on the longitudinal asymmetry other than the fluctuation in the number of participants, e.g., the <math><mi>α</mi></math> clustering structure as well as the intrinsic momentum distribution in the target and projectile nuclei for the collisions in the framework of a multiphase transport (AMPT) model. By introducing systems with different <math><mi>α</mi></math>-clustering structure and intrinsic momentum distribution, we calculated the ratio of the rapidity distributions of different systems and extracted expansion coefficients to analyze the difference contributed by these factors. We also investigated the possible effect of the non-Gaussian distribution on the rapidity distribution. These results can help us to constrain the initial conditions and reconstruct the colliding vertex in ultrarelativistic heavy ion collisions, and suggest a quantitative correction on the final state measurement and a possible correlation between the initial condition and the final state observable in LHC and BNL Relativistic Heavy Ion Collider energy.

Ru-Xin Cao

Effects of the <math><mi>α</mi></math>-cluster structure and the intrinsic momentum component of nuclei on the longitudinal asymmetry in relativistic heavy-ion collisions