Nuclear Collisions Research Articles

A model-free procedure to correct for volume fluctuations in E-by-E analyses of particle multiplicities

We develop an innovative and unbiased procedure, based on event mixing, to account for unavoidable contributions from volume (or system size) fluctuations to experimentally measured moments of particle multiplicity distributions produced in relativistic nuclear collisions. Within the wounded-nucleon model they are characterized by fluctuations of the number of wounded nucleons, the latter usually referred to as participants. For the first time we extract participant fluctuations directly from the data used for the fluctuation analysis, i.e., without involving model calculations. To achieve this we constructed a dedicated event-mixing algorithm that eliminates all possible correlations between produced particles while preserving the volume fluctuations. The procedure provides direct access to the cumulants of wounded-nucleon distributions, which can be used to account for non-critical contributions to the experimentally measured cumulants of multiplicity distributions.

Heavy-Flavour Jets in High-Energy Nuclear Collisions

Reconstructed jets initiated from heavy quarks provide a powerful tool to probe the properties of the quark–gluon plasma (QGP) and to explore the mass hierarchy of jet quenching. In this article, we review the recent theoretical progresses on heavy-flavour jets in high-energy nuclear collisions at the RHIC and LHC. We focus on the yields and substructures of charm and bottom quark jets with jet-quenching effects, such as the nuclear modification factors, transverse momentum imbalance, angular correlation, radial profiles, fragmentation functions, the “dead-cone” effect, etc.

Semianalytical calculation of the trajectory of relativistic nuclear collisions in the QCD phase diagram

We extend a semianalytical model that includes the finite nuclear thickness to calculate the energy density $\ensuremath{\epsilon}(t)$ and conserved-charge densities including the net-baryon density ${n}_{\mathrm{B}}(t)$ produced at mid-spacetime-rapidity in central Au + Au collisions. Assuming the formation of a quark-gluon plasma with an ideal gas equation of state of either quantum or Boltzmann statistics or with a lattice QCD-based equation of state, we extract the temperature $T$ and chemical potentials ${\ensuremath{\mu}}_{\mathrm{B}}, {\ensuremath{\mu}}_{\mathrm{Q}}$, and ${\ensuremath{\mu}}_{\mathrm{S}}$ as functions of time. This then allows us to semianalytically calculate the $T\text{\ensuremath{-}}{\ensuremath{\mu}}_{\mathrm{B}}$ trajectory of relativistic nuclear collisions in the QCD phase diagram, which should benefit the studies of high density physics including the search for the critical endpoint. This model is also useful for exploring the trajectories in the more general $T\text{\ensuremath{-}}{\ensuremath{\mu}}_{\mathrm{B}}\text{\ensuremath{-}}{\ensuremath{\mu}}_{\mathrm{Q}}\text{\ensuremath{-}}{\ensuremath{\mu}}_{\mathrm{S}}$ QCD phase space.

Statistical analysis of initial-state and final-state response in heavy-ion collisions

High-energy nuclear collisions are analyzed theoretically using multistage models assembled to simulate the different stages of the reaction. This paper presents a new method for systematically characterizing the early-time density of a heavy-ion collision system. It is then used to analyze events from different models as well as the system response to these initial conditions. This method will help distinguish between different approaches to describing the dynamics of relativistic heavy-ion collisions at very early times.

Z 0 boson associated b-jet production in high-energy nuclear collisions* *Supported by the Guangdong Major Project of Basic and Applied Basic Research (2020B0301030008), the Science and Technology Program of Guangzhou (2019050001), and the NSFC of China (11935007, 12035007, 12247127). S. Wang is supported by China Postdoctoral Science Foundation (2021M701279)

The production of vector boson tagged heavy quark jets potentially provides new tools to probe the jet quenching effect. In this paper, we present the first theoretical study on the angular correlations (), transverse momentum imbalance (), and nuclear modification factor () of boson tagged b-jets in heavy-ion collisions, which was performed using a Monte Carlo transport model. We find that the medium modification of the for + b-jet has a weaker dependence on than that for + jet, and the modification patterns are sensitive to the initial jet distribution. Additionally, with the high purity of the quark jet in + (b-) jet production, we calculate the momentum imbalance and the nuclear modification factor of + b-jet in Pb+Pb collisions. We observe a smaller and larger of + b-jet in Pb+Pb collisions relative to those of + jet, which may be an indication of the mass effect of jet quenching and can be tested in future measurements.

Ultraperipheral nuclear collisions as a source of heavy quarkonia

Heavy quarkonium production in ultraperipheral nuclear collisions (UPC) is described within the QCD dipole formalism. Realistic quarkonium wave functions in the $Q\overline{Q}$ rest frame are calculated by solving the Schr\"odinger equation with a subsequent Lorentz boost to high energy. We rely on several realistic $Q\overline{Q}$ potentials, which allow us to describe well the quarkonium masses and decay widths, as well as data on diffractive electroproduction of quarkonia on protons. Nuclear effects are calculated with the phenomenological dipole cross sections fitted to deep-inelastic scattering (DIS) data. The higher twist quark shadowing related to the lowest $Q\overline{Q}$ Fock component of the photon, as well as the leading twist gluon shadowing, related to higher components containing gluons, are included. The results for coherent and incoherent photoproduction of charmonia and bottomonia in UPC of heavy nuclei are in good accord with available data from the LHC. They can also be verified in future experiments at electron-ion colliders.

Phase transition amplification of proton number fluctuations in nuclear collisions from a transport model approach

The time evolution of particle number fluctuations in nuclear collisions at intermediate energies ($E_{\rm lab} = 1.23-10A$ GeV) is studied by means of the UrQMD-3.5 transport model. The transport description incorporates baryonic interactions through a density-dependent potential. This allows for an implementation of a first order phase transition including a mechanically unstable region at large baryon density. The scaled variance of the baryon and proton number distributions is calculated in the central cubic spatial volume of the collisions at different times. A significant enhancement of fluctuations associated with the unstable region is observed. This enhancement persists to late times reflecting a memory effect for the fluctuations. The presence of the phase transition has a much smaller influence on the observable event-by-event fluctuations of protons in momentum space.

Diagram of High-Energy Nuclear Collisions

Many new particles, mostly hadrons, are produced in high-energy collisions between atomic nuclei. The most popular models describing the hadron-production process are based on the creation, evolution and decay of resonances, strings or quark–gluon plasma. The validity of these models is under vivid discussion, and it seems that a common framework for this discussion is missing. Here, for the first time, we explicitly introduce the diagram of high-energy nuclear collisions, where domains of the dominance of different hadron-production processes in the space of laboratory-controlled parameters, the collision energy and nuclear-mass number of colliding nuclei are indicated. We argue that the recent experimental results suggest the location of boundaries between the domains, allowing for the first time to sketch an example diagram. Finally, we discuss the immediate implications for experimental measurements and model development following the proposed sketch of the diagram.

Collective Excitation in High-Energy Nuclear Collisions—In Memory of Professor Lianshou Liu

We celebrate the legacies of our friend and mentor Professor Lianshou Liu who was one of the pioneers for the phenomenology of multi-particle interactions and initiated the physics of relativistic heavy-ion collisions in China. In this article, we discuss some of the recent exciting experimental observations on the collective phenomena including collectivity, chirality, criticality, strangeness production, and thermal equilibrium in high-energy nuclear collisions. Future directions, especially the physics at high baryon density, will be discussed with a focus on the first-order phase boundary and hyperon–nucleon interactions.

Characterizing initial- and final-state effects of relativistic nuclear collisions

The Multi-Phase Transport model (AMPT) is used to study the final state effects on the Symmetric Correlations (SC), Asymmetric Correlations (ASC), Normalized Symmetric Correlations (NSC), and Normalized Asymmetric Correlations (NASC) in Au+Au collisions at 200~GeV. The correlators' sensitivity to non-flow effects associated with long- and short-range non-flow correlations are also discussed using the HIJING model. The results indicate that SC, ASC, NSC, and NASC can give accompanying constraints for initial and final state effects. In addition, conducting further detailed experimental measurements spanning a broad range of collision systems and beam energies will serve as an additional constraint for the theoretical models' calculations.

Experimental investigation into impacts of neutron irradiation on pMOS dosimeter behaviors

The p-channel metal–oxide–semiconductor (pMOS) transistor is a frequently used dosimeter for ionizing radiation, such as γ-rays, since its threshold voltage changes (ΔVth) in proportion to the ionization dose. When a pMOS is subjected to γ-ray and neutron radiation fields, the neutrons can influence the ΔVth response to γ-rays. Individual cobalt-60 γ-ray irradiation and sequential irradiation by fission neutrons and subsequent cobalt-60 γ-rays are used to explore the neutron-induced impacts. Experiments show that preceding neutron irradiation can reduce γ-ray sensitivity of ΔVth while accelerating the recovery of ΔVth during the post-γ-irradiation annealing. Additionally, the component of ΔVth induced by oxide charges (ΔVox) is separated from that induced by interface traps as determined by mid-gap tests. This proves that the neutron sensitivity of ΔVox accounts for that of ΔVth. According to nuclear collision theory, this work suggests that the electronic loss of neutrons rather than the nuclear loss dominates the neutron-induced influences on ΔVth.

Evolution of global polarization in relativistic heavy-ion collisions within a perturbative approach

Extremely large angular orbital momentum can be produced in non-central heavy-ion collisions, leading to a strong transverse polarization of partons that scatter through the quark-gluon plasma (QGP) due to spin-orbital coupling. We develop a perturbative approach to describe the formation and spacetime evolution of quark polarization inside the QGP. Polarization from both the initial hard scatterings and interactions with the QGP have been consistently described using the quark-potential scattering approach, which has been coupled to realistic initial condition calculation and the subsequent (3+1)-dimensional viscous hydrodynamic simulation of the QGP for the first time. Within this improved approach, we have found that different spacetime-rapidity-dependent initial energy density distributions generate different time evolution profiles of the longitudinal flow velocity gradient of the QGP, which further lead to an approximately 15% difference in the final polarization of quarks collected on the hadronization hypersurface of the QGP. Therefore, in addition to the collective flow coefficients, the hyperon polarization may serve as a novel tool to help constrain the initial condition of the hot nuclear matter created in high-energy nuclear collisions.

Complementary two-particle correlation observables for relativistic nuclear collisions

Two-particle correlations are a widely used tool for studying relativistic nuclear collisions. Multiplicity fluctuations comparing charge and particle species have been studied as a possible signal for quark-gluon plasma (QGP) and the QCD critical point. These fluctuation studies all make use of particle variances which can be shown to originate with a two-particle correlation function. Momentum correlations and covariances of momentum fluctuations, which arise from the same correlation function, have also been used to extract properties of the nuclear collision medium such as the shear viscosity to entropy density ratio, the shear relaxation time, and temperature fluctuations. Searches for critical fluctuations are also done with these correlation observables. We derive a mathematical relationship between several number and momentum density correlation observables and outline the different physics mechanisms often ascribed to each. This set of observables also contains a new multiplicity-momentum correlation. Our mathematical relation can be used as a validation tool for measurements, as a method for interpreting the relative contributions of different physics mechanisms on correlation observables, and as a test for theoretical and phenomenological models to simultaneously explain all observables. We compare an independent source model to simulated events from pythia for all observables in the set.

3D structure of anisotropic flow in small collision systems at energies available at the BNL Relativistic Heavy Ion Collider

We present (3+1)D dynamical simulations of asymmetric nuclear collisions at the Relativistic Heavy Ion Collider (RHIC). Employing a dynamical initial state model coupled to (3+1)D viscous relativistic hydrodynamics, we explore the rapidity dependence of anisotropic flow in the RHIC small system scan at 200 GeV center of mass energy. We calibrate parameters to describe central $^3$He+Au collisions and make extrapolations to d+Au and p+Au collisions. Our calculations demonstrate that approximately 50% of the $v_3(p_T)$ difference between the measurements by the STAR and PHENIX Collaborations can be explained by the use of reference flow vectors from different rapidity regions. This emphasizes the importance of longitudinal flow decorrelation for anisotropic flow measurements in asymmetric nuclear collisions, and the need for (3+1)D simulations. We also present results for the beam energy dependence of particle spectra and anisotropic flow in d+Au collisions.

On the effect of oxygen on the creation of colour centres in swift heavy ion-irradiated AlN

We readdress the nature and creation mechanism of the defects absorbing at 4.7 eV in Swift Heavy Ion (SHI)-irradiated AlN semiconductor. Irradiations are performed in different controlled atmospheres (vacuum, oxygen, helium or argon partial pressure) to investigate the coupled effects of (i) the synergy between electronic excitations (Se) and nuclear collisions (Sn) for point defect creation and (ii) SHI-induced impurity introduction/diffusion into the AlN layer. We found that the synergy, previously reported, between Se and Sn for defect formation must also be related to the presence of oxygen during irradiation. At the same Se, the synergy parameter is indeed increased by up to two orders of magnitude during irradiation under O2 compared to irradiation under vacuum. By SHI irradiation and by a fine-tuning of the irradiation atmosphere, we then control the activation or inhibition of these light-emitted point defects (oxygen-vacancy complexes) which can be used as qubits for applications in quantum computing.

Manifestation of Pairing Modes in Nuclear Collisions

We discuss the possible manifestation of pairing dynamics in nuclear collisions beyond the standard quasi-static treatment of pairing correlations. These involve solitonic excitations induced by pairing phase difference of colliding nuclei and pairing dynamic enhancement in the di-nuclear system formed by merging nuclei.

Causality violations in realistic simulations of relativistic nuclear collisions

We show that relativistic causality is violated in the early stages of state-of-the-art heavy-ion hydrodynamic simulations of nuclear collisions. Up to 75% of the initial fluid cells violate nonlinear causality constraints, while superluminal propagation is observed by up to 15% the speed of light. Only after 2-3 fm/c of evolution, do 50% of the fluid cells become definitely causal. Inclusion of pre-equilibrium evolution significantly reduces the number of acausal cells, but it does not eliminate them. Relativistic causality thus imposes constraints on the model parameter space of heavy-ion collision simulations.

Physics programme of the ALICE 3 experiment for the LHC Runs 5 and 6

Ultrarelativistic heavy-ion collisions are used to study the physics of strongly interacting matter under extreme conditions, similar to those of the early universe. In such collisions a deconfined state of quarks and gluons, the quark-gluon plasma (QGP), is formed. Nuclear collisions at the LHC provide access to the highest-temperature, longest-lived experimentally accessible QGP. After three years of Long Shutdown and intensive installation of detector and accelerator upgrades, ALICE is about to take data at a peak Pb–Pb collision rate of 50 kHz to further characterize the properties of the QGP. Even after the ambitious scientific programme for the upcoming Runs 3 and 4, open questions will remain. Therefore, a next-generation LHC heavy-ion experiment ALICE 3 is proposed for the 2030s. It should give access to next-level measurements of electromagnetic probes and heavy-flavour hadrons, including multi-charm states and exotic hadrons, inaccessible in the LHC Runs 3 and 4.

Fragmentation-induced fission reactions of 236U in inverse kinematics to investigate the pre-fragment angular momentum parameterizations

In the last decades, measurements of spallation, fragmentation and Coulex induced fission reactions in inverse kinematics have provided valuable data to accurately investigate the fission dynamics and nuclear structure at large deformations of a large variety of stable and non-stable heavy nuclei. The collected data were used to constrain dynamic and nuclear structure parameters of different de-excitation models, such as ABLA and GEF, but the data can also be used to constrain the parameterizations describing the pre-fragment properties after the nuclear collision, such as the angular momentum gained by the pre-fragment. In this work, the fissioning system yields are compared to calculations assuming different parameterizations for modeling the angular momentum gained by the compound nuclei. Our findings indicate that the parameterizations utilized by abrasion models clearly underestimate the angular momentum, resulting in the underestimation of the production of lighter fissioning systems.

Exploring the hadronic phase of relativistic heavy-ion collisions with resonances in ALICE

Hadronic resonances, having short lifetimes, are useful for studying the hadrongas phase that characterizes the late-stage evolution of high-energy nuclear collisions. Indeed, regeneration and rescattering processes occurring in the hadron gas modify the measured yields of hadronic resonances and can be studied by measuring resonance yields as a function of system size and comparing them to model predictions with and without hadronic interactions. Measurements of the differential yields of resonances with different lifetime, mass, quark content, and quantum numbers help in understanding particle production mechanisms, the lifetime of the hadronic phase, strangeness production, parton energy loss, rapidity yield asymmetry, and collective effects. With its excellent tracking and particle identification capabilities, the ALICE experiment measured a comprehensive set of mesonic and baryonic resonances. We present recent results on resonance production in pp, p–Pb, Xe–Xe and Pb–Pb collisions at various center-of-mass energies, highlighting new results of K*±, Λ(1520), and Σ*±(1385). The results are also compared to lower energy measurements and model calculations.