Equations of a boost-invariant and cylindrically symmetric perfect hydrodynamics are solved numerically for initial conditions inspired by the wounded nucleon model. The energy-momentum and spin tensors are used in the form that describes a relativistic massive gas governed by Boltzmann statistics. In contrast to one-dimensional boost-invariant expansion, we find a coupling between the azimuthal and longitudinal components of the electric and magnetic components of the spin polarization tensor. This feature is similar to that found earlier for the Gubser symmetry; however, our treatment allows for a more general form of initial conditions and expansion geometry. Defining the freeze-out hypersurface by the constant temperature condition, we evaluate the Pauli-Lubański four-vector and find that for the assumed geometry the only nonzero total polarization may be induced by the longitudinal component of the magnetic part of the spin polarization tensor, coupled with the azimuthal electric component. The obtained results may serve as a reference point for more realistic models of hydrodynamic expansion.

Poland

Zbigniew Drogosz

Wojciech Florkowski

Jakub Witkowski

Boost-invariant and cylindrically symmetric perfect spin hydrodynamics

We study fluctuations and correlations of conserved charges in QCD using a string-based description of the hadronic mass spectrum. Mesons and baryons are modeled as open relativistic strings with quark-antiquark and quark-diquark endpoints, respectively, leading to an exponential Hagedorn growth of states with a limiting temperature fixed by the string tension. We find that continuous Hagedorn spectra constrained by experimentally established hadrons underestimate net-baryon number fluctuations obtained in lattice QCD (LQCD) calculations. By extracting the Hagedorn string spectrum directly from lattice QCD through a fit to the second-order net-baryon number susceptibility, we obtain a consistent description of a broad set of fluctuations of conserved charges from LQCD with the Hagedorn temperature <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>T</mi><mi>H</mi></msub><mo>≃</mo><mn>323</mn><mspace width="0.16em"></mspace><mi>MeV</mi></mrow></math>, without introducing additional free parameters. Our results provide thermodynamic evidence in support of a string quark-diquark picture of baryons in the confined phase of QCD.

Japan

Michał Marczenko

Krzysztof Redlich

Evidence for quark-diquark structure of baryons from fluctuations of conserved charges

We investigate the elliptic-flow splitting between prompt <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>D</mi><mn>0</mn></msup></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mi>D</mi><mi>s</mi><mo>+</mo></msubsup></math> mesons within a heavy-quark transport framework with sequential hadronization, in which <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mi>D</mi><mi>s</mi><mo>+</mo></msubsup></math> forms at <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>1.2</mn><mspace width="0.16em"></mspace><msub><mi>T</mi><mi>c</mi></msub></mrow></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>D</mi><mn>0</mn></msup></math> at <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>T</mi><mi>c</mi></msub></math>. We present model calculations for the <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi>T</mi></msub></math>-differential <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>v</mi><mn>2</mn></msub></math> and predictions for the <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msubsup><mi>D</mi><mi>s</mi><mo>+</mo></msubsup><mo>/</mo><msup><mi>D</mi><mn>0</mn></msup></mrow></math> yield ratio in O-O <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>0</mn><mo>–</mo><mspace width="0.16em"></mspace><mn>20</mn><mo>%</mo></mrow></math> collisions at <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>5.36</mn></mrow></math> TeV, where preliminary ALICE data are available. The sequential scenario reproduces the observed <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>v</mi><mn>2</mn></msub><mrow><mo>(</mo><msup><mi>D</mi><mn>0</mn></msup><mo>)</mo></mrow><mo>&gt;</mo><msub><mi>v</mi><mn>2</mn></msub><mrow><mo>(</mo><msubsup><mi>D</mi><mi>s</mi><mo>+</mo></msubsup><mo>)</mo></mrow></mrow></math> ordering and predicts an enhanced <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msubsup><mi>D</mi><mi>s</mi><mo>+</mo></msubsup><mo>/</mo><msup><mi>D</mi><mn>0</mn></msup></mrow></math> ratio at low <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi>T</mi></msub></math>, whereas a simultaneous baseline yields the opposite ordering. Decomposing the hadronic splitting into its partonic components, we show that the <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>v</mi><mn>2</mn></msub></math> ordering is driven by the late-stage flow accumulated by <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>D</mi><mn>0</mn></msup></math>-parent charm quarks during the <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mn>1.2</mn><mspace width="0.16em"></mspace><msub><mi>T</mi><mi>c</mi></msub><mo>→</mo><msub><mi>T</mi><mi>c</mi></msub></mrow></math> interval, with hadronic rescattering essential to preserve the signal in small systems. A systematic scan across nine collision configurations spanning O-O and Pb-Pb centralities reveals a universal linear scaling between the hadronic splitting and the partonic flow increment accumulated during this window. This establishes the <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi>D</mi><mn>0</mn></msup><mo>–</mo><msubsup><mi>D</mi><mi>s</mi><mo>+</mo></msubsup></mrow></math> flow splitting as a hadronization chronometer of the QGP at <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>5.36</mn></mrow></math> TeV.

China

Hui Du

Xiao-Wei Hao

Wei Dai

Germany

Jiaxing Zhao

Ben-Wei Zhang

Enke Wang

System-size dependence of the <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msup><mi>D</mi><mn>0</mn></msup><mtext>−</mtext><msubsup><mi>D</mi><mi>s</mi><mo>+</mo></msubsup></mrow></math> flow splitting from early <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mi>D</mi><mi>s</mi><mo>+</mo></msubsup></math> formation at <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>5.36</mn></mrow></math> TeV

Jet substructure studies at the CERN Large Hadron Collider have been used to constrain parton distribution functions, test perturbative quantum chromodynamics, measure the strong-coupling constant, and probe the properties of the quark-gluon plasma. We extend these studies to lower collision energies at the BNL Relativistic Heavy Ion Collider (RHIC) and present a feasibility and strategy study for the sPHENIX experiment. In this work, we perform a <math xmlns="http://www.w3.org/1998/Math/MathML"><mi mathvariant="normal" mathsize="small">PYTHIA8</mi></math> based Monte Carlo study of jet substructure in <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mo>+</mo><mi>p</mi></mrow></math> collisions at <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>200</mn></mrow></math> GeV with selection criteria adapted for a realistic measurement at sPHENIX. We consider jet substructure observables such as jet angularities and the primary Lund plane projection. In particular, we quantify the discriminating power of jet angularities for quark/gluon and light versus heavy flavor jets, and demonstrate how these observables can serve as experimentally accessible proxies for flavor separation at RHIC. We further show that the primary Lund plane exhibits a characteristic suppression pattern consistent with the dead cone effect for heavy quarks, and outline a concrete measurement strategy at sPHENIX based on a combination of heavy-flavor jet tagging and differential substructure observables.

United States

Zhuoheng Yang

Finland

Oleh Fedkevych

Roli Esha

Simulation-based study of jet substructure of light- and heavy-flavor jets in <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>p</mi><mo>+</mo><mi>p</mi></mrow></math> collisions at 200 GeV

In this study, we calculate the <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>γ</mi><mi>Z</mi></mrow></math>-exchange contribution to elastic <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>e</mi><mi>p</mi></mrow></math> scattering at large momentum transfer within perturbative quantum chromodynamics (pQCD). We present analytical expressions for the <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>γ</mi><mi>Z</mi></mrow></math>-exchange contributions to the amplitudes. We also estimate the asymptotic behaviors of the amplitude contributions and of the physical quantity <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>A</mi><mtext>PV</mtext></msub></math> at high momentum transfer. These asymptotic behaviors determine the subtraction order in the dispersion relations (DRs) satisfied by the amplitudes. At high <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>Q</mi><mn>2</mn></msup></math> beyond the forward limit, our pQCD analysis shows that this subtraction order is fixed by the high-<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>ν</mi></math> behavior of the amplitude coefficients <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mi>c</mi><mi>i</mi><mrow><mi>γ</mi><mi>Z</mi><mo>,</mo><mtext>PV</mtext></mrow></msubsup></math>, and that <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mi>c</mi><mrow><mn>3</mn><mo>,</mo><mi>A</mi></mrow><mrow><mi>γ</mi><mi>Z</mi><mo>,</mo><mtext>PV</mtext></mrow></msubsup></math> requires a once-subtracted DR. Within the present pQCD framework and the adopted proton distribution amplitudes, these high-energy results also provide nontrivial constraints on low-energy DR assumptions.

Qian-Qian Guo

Hui-Yun Cao

Hai-Qing Zhou

<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>γ</mi><mi>Z</mi></mrow></math>-exchange contribution in elastic <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>e</mi><mi>p</mi></mrow></math> scattering by perturbative QCD

Heavy-ion collisions produce final states with thousands to tens of thousands of particles, making their simulation among the most computationally intensive tasks in high-energy nuclear physics. We present a fast, high-fidelity generative model for heavy-ion events based on a score-driven diffusion process and the point-edge transformer architecture within the OmniLearn framework. A two-stage training strategy is performed: Stage 1 trains on lower-multiplicity O-O collisions, allowing the model to learn a stable event-and-particles representation; this is followed in Stage 2 by fine tuning on challenging high-multiplicity Pb-Pb collisions. We benchmark the generator with a broad set of closure checks, including agreement of event- and particle-level observables in one and two dimensions, flow consistency reconstructed from the generated particles, end-to-end jet finding with FastJet including key jet and substructure observables, and a classifier-based application to quantify the sample fidelity. The results are promising, showing that a compact generative model can produce realistic, high-multiplicity heavy-ion events, at a level that makes local-scale generation for heavy-ion collisions at high energies a practical goal.

Rita Sadek

Vinicius Mikuni

Mateusz Płoskoń

Diffusion-based point-cloud generation of heavy-ion events

We develop the theoretical framework for semi-inclusive deep-inelastic scattering on a polarized spin-1 target and apply it to scattering on the polarized deuteron with spectator nucleon tagging. In Part I (this article) we present the general form of the semi-inclusive cross section and polarization observables for the spin-1 target. A relativistically covariant formulation in terms of four-vectors and invariant polarization parameters is employed. The target polarization is described by a spin density matrix with vector and tensor polarization. The spin and azimuthal angle dependence of the semi-inclusive cross section is derived and parametrized in terms of invariant structure functions. To validate the result, the structure functions are expressed as photon-target helicity amplitudes with known symmetry properties. The expressions presented here are kinematic (no assumptions about particle production dynamics) and valid in all regions of the deep-inelastic final state (current and target fragmentation regions). In Part II (following article), we consider deep-inelastic scattering on the polarized deuteron with spectator nucleon tagging as a special case of target fragmentation. The semi-inclusive structure functions are computed by separating nuclear and hadronic structure, and the polarization observables are explored as functions of the tagged nucleon momentum.

W. Cosyn

C. Weiss

Semi-inclusive deep-inelastic scattering on a polarized spin-1 target. I. Cross section and spin observables

We systematically investigate the scaling properties of the transverse momentum spectra for pions, kaons, and protons in <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>Au</mi><mo>+</mo><mi>Au</mi></mrow></math> collisions at <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>7.7</mn></mrow></math>, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV, as well as in <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi mathvariant="normal">U</mi><mo>+</mo><mi mathvariant="normal">U</mi></mrow></math> collisions at <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msqrt><msub><mi>s</mi><mrow><mi>N</mi><mi>N</mi></mrow></msub></msqrt><mo>=</mo><mn>193</mn><mspace width="0.16em"></mspace><mi>GeV</mi></mrow></math>, across different centrality classes, using experimental data from the collaborations at the Relativistic Heavy Ion Collider (RHIC). Universal scaling emerges when the particle transverse momentum spectra are scaled by global physical quantities, i.e., the average total particle multiplicity and mean transverse momentum, confirming recent scaling findings from the data at the Large Hadron Collider (LHC) by the ExTrEMe Collaboration. The scaling behavior breaks down in the high <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi mathvariant="normal">T</mi></msub></math> region and in peripheral collisions. We provide a natural explanation for these observations in the framework of a blast-wave-type model built upon the Cooper-Frye formula. Furthermore, we demonstrate the equivalence between the scaling found by the ExTrEMe Collaboration and the Hwa-Yang scaling, which was proposed two decades ago.

Xi-Yao Guo

Hua Zheng

Wen-Chao Zhang

Li-Lin Zhu

Xing-Quan Liu

Zhi-Guang Tan

Dai-Mei Zhou

Ben-Hao Sa

Deciphering the universal scaling of particle transverse momentum spectra in heavy-ion collisions

We develop the theoretical framework for semi-inclusive deep-inelastic scattering on a polarized spin-1 target and apply it to scattering on the polarized deuteron with spectator nucleon tagging. In Part I (previous article), we present the general form of the semi-inclusive cross section and polarization observables for the spin-1 target. In Part II (this article), we consider deep-inelastic scattering on the polarized deuteron with spectator nucleon tagging as a special case of target fragmentation. Methods of light-front quantization are employed to separate nuclear and hadronic structure in the high-energy process and achieve a composite description. The light-front wave function of the polarized deuteron is obtained from a rotationally covariant three-dimensional wave function in the center-of-mass frame of the proton-neutron system. The tagged structure functions are computed in the impulse approximation. The momentum and spin distribution of the active nucleon are controlled by the deuteron polarization and the detected spectator momentum (<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>D</mi><mo>/</mo><mi>S</mi></mrow></math> wave ratio). The cross section and spin asymmetries are evaluated for general deuteron polarization (vector and tensor, longitudinal and transverse) as functions of the spectator momentum. Tensor-polarized spin asymmetries of order unity are achieved for spectator momenta of approximately 300 MeV, which select configurations with a large <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>D</mi></math> wave. Sum rules for the tagged spin structure functions are derived. The results can be used for simulations of spectator tagging in future polarized fixed-target experiments (Jefferson Lab) or at the Electron-Ion Collider.

Semi-inclusive deep-inelastic scattering on a polarized spin-1 target. II. Deuteron and spectator nucleon tagging

The thermal modification of the exclusive decay <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msubsup><mi>K</mi><mn>1</mn><mo>+</mo></msubsup><mrow><mo>(</mo><mn>1270</mn><mo>)</mo></mrow><mo>→</mo><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup><msup><mi>K</mi><mo>+</mo></msup></mrow></math> in a hot hadronic medium is studied. The decay amplitude is constructed from effective hadronic interactions dominated by the <math xmlns="http://www.w3.org/1998/Math/MathML"><mi>ρ</mi></math>- and <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>K</mi><mo>*</mo></msup></math>-pole contributions, thereby enabling a Dalitz-level analysis of the three-body decay in the medium. Thermal effects associated with partial chiral-symmetry restoration are incorporated through a phenomenological interpolation toward vector–axial-vector degeneracy near the chiral crossover. As the temperature increases, the reduction of the parent <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>K</mi><mn>1</mn></msub></math> mass strongly compresses the available three-body phase space, leading to substantial deformation of the Dalitz distribution and invariant-mass spectra, as well as to a pronounced suppression of <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi mathvariant="normal">Γ</mi><mrow><msub><mi>K</mi><mn>1</mn></msub><mo>→</mo><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup><msup><mi>K</mi><mo>+</mo></msup></mrow></msub><mrow><mo>(</mo><mi>T</mi><mo>)</mo></mrow></mrow></math>. As a further step toward future transport-based phenomenological comparisons, Normalized shape observables are introduced to quantify the thermal evolution of the <math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>K</mi><mo>*</mo></msup></math>-dominated <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>π</mi><mi>K</mi></mrow></math> region, the upper-edge weight of the <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><mi>π</mi><mi>π</mi></mrow></math> spectrum, and the compactification of the Dalitz population. The dominant effect identified in the present framework is therefore kinematic: thermal phase-space reduction in the strange axial-vector channel. These results suggest that the exclusive <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>K</mi><mn>1</mn></msub><mrow><mo>(</mo><mn>1270</mn><mo>)</mo></mrow></mrow></math> channel may provide a useful qualitative probe of in-medium strange axial-vector dynamics near the pseudocritical region.

South Korea

Seung-il Nam

Thermal modification of <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>K</mi><mn>1</mn></msub><mrow><mo>(</mo><mn>1270</mn><mo>)</mo></mrow><mo>→</mo><msup><mi>π</mi><mo>+</mo></msup><msup><mi>π</mi><mo>−</mo></msup><msup><mi>K</mi><mo>+</mo></msup></mrow></math> in a hot hadronic medium