High Energy Nuclear Collisions Research Articles

Measurements of jet quenching using semi-inclusive hadron+jet distributions in pp and central Pb-Pb collisions at sNN=5.02 TeV

The ALICE Collaboration reports measurements of the semi-inclusive distribution of charged-particle jets recoiling from a high transverse momentum (high pT) charged hadron, in pp and central Pb-Pb collisions at center-of-mass energy per nucleon–nucleon collision sNN=5.02 TeV. The large uncorrelated background in central Pb-Pb collisions is corrected using a data-driven statistical approach which enables precise measurement of recoil jet distributions over a broad range in pT,chjet and jet resolution parameter R. Recoil jet yields are reported for R=0.2, 0.4, and 0.5 in the range 7<pT,chjet<140 GeV/c and π/2<Δφ<π, where Δφ is the azimuthal angular separation between hadron trigger and recoil jet. The low-pT,chjet reach of the measurement explores unique phase space for studying jet quenching, the interaction of jets with the quark–gluon plasma generated in high-energy nuclear collisions. Comparison of pT,chjet distributions from pp and central Pb-Pb collisions probes medium-induced jet energy loss and intra-jet broadening, while comparison of their acoplanarity distributions explores in-medium jet scattering and medium response. The measurements are compared to theoretical calculations incorporating jet quenching. ©2024 CERN, for the ALICE Collaboration 2024 CERN

Unveiling the dynamics of little-bang nucleosynthesis

High-energy nuclear collisions provide a unique site for the synthesis of both nuclei and antinuclei at temperatures of kT ≈ 100 − 150 MeV. In these little bangs of transient collisions, a quark-gluon plasma (QGP) of nearly vanishing viscosity is created, which is believed to have existed in the early universe within the first few microseconds after the Big Bang. Analyses of identified particles produced in these little bangs based on the statistical hadronization model for the QGP have suggested that light (anti)nuclei are produced from the QGP as other hadrons and their abundances are little affected by later hadronic dynamics. Here, we find a strong reduction of the triton yield by about a factor of 1.8 in high-energy heavy-ion collisions based on a kinetic approach that includes the effects of hadronic re-scatterings, particularly that due to pion-catalyzed multi-body reactions. This finding is supported by the latest experimental measurements and thus unveils the important role of hadronic dynamics in the little-bang nucleosynthesis.

Hyperon polarization and its relation with directed flow in high-energy nuclear collisions

Hyperon polarization and its relation with directed flow in high-energy nuclear collisions

Parameterizing smooth viscous fluid dynamics with a viscous blast wave

Blast wave fits are widely used in high energy nuclear collisions to capture essential features of global properties of systems near kinetic equilibrium. They usually provide temperature fields and collective velocity fields on a given hypersurface. We systematically compare blast wave fits of fluid dynamic simulations for Au+Au collisions at and Pb+Pb collisions at TeV with the original simulations. In particular, we investigate how faithful the viscous blast wave introduced in Yang and Fries (2022 Phys. Rev. C 105 014910) can reproduce the given temperature and specific shear viscosity fixed at freeze-out of a viscous fluid dynamic calculation, if the final spectrum and elliptic flow of several particle species are fitted. We find that viscous blast wave fits describe fluid dynamic pseudodata rather well and reproduce the specific shear viscosities to good accuracy. However, extracted temperatures tend to be underpredicted, especially for peripheral collisions. We investigate possible reasons for these deviations. We establish maps from true to fitted values. These maps can be used to improve raw fit results from viscous blast wave fits. Although our work is limited to two specific, albeit important, parameters and two collision systems, the same procedure can be easily generalized to other parameters and collision systems.

A review on anisotropic flow in heavy-ion collisions

We present a review of published experimental measurements on anisotropic flow in heavy-ion collisions. Flow coefficients [Formula: see text] are presented for charged and identified hadrons at various beam energies ranging from [Formula: see text][Formula: see text]GeV to 200[Formula: see text]GeV at the Relativistic Heavy Ion Collider (RHIC) and [Formula: see text][Formula: see text]GeV at the Large Hadron Collider (LHC). We present [Formula: see text] and its slope [Formula: see text] and discuss possible first-order Quantum chromodynamics (QCD) phase transition. [Formula: see text] of particles containing heavy quarks (Charm quarks) is also discussed to illustrate the effect of the magnetic field produced in relativistic heavy-ion collisions. We also present flow harmonics ([Formula: see text], [Formula: see text] and [Formula: see text]) for inclusive charged and identified hadrons. We compare the charged particle [Formula: see text] coefficients with hydrodynamics to study and constrain initial conditions in the transport models. The results of identified particle [Formula: see text] from ideal and viscous hydrodynamics are discussed to understand the dynamics and mechanism of particle production in high-energy nuclear collisions.

The photon production and collective flows from magnetic induced gluon fusion and splitting in early stage of high energy nuclear collision

We present an event-by-event study of photon production in early stage of high energy nuclear collisions, where the system is dominant by highly occupied of gluons and initialized by McLerran-Venugopalan model. The photons are produced through the gluon fusion and splitting processes when strong magnetic field is included. We study the spectra and collective flows of the photons and show their dependence on transverse momentum qT. It is found that in our approach the photons from boost invariant evolving glasma provide visible enhancement on spectrum and obvious contribution on v2 of the total direct photons. The results, by weighting on top of parton-hadron-string dynamics (PHSD) model, agree even better with experiment measurements in Au-Au 20%-40% centrality collisions at sNN=200 GeV.

Space–Time Structure of Particle Emission and Femtoscopy Scales in Ultrarelativistic Heavy-Ion Collisions

The analysis of the spatiotemporal picture of particle radiation in relativistic heavy-ion collisions in terms of correlation femtoscopy scales, emission, and source functions allows one to probe the character of the evolution of the system created in the collision. Realistic models, such as the integrated hydrokinetic model (iHKM), used in the present work, are able to simulate the entire evolution process of strongly interacting matter produced in high-energy nuclear collisions. The mentioned model describes all the stages of the system’s evolution, including thermalisation and hydrodynamisation, which can help researchers figure out the specific details of the process and better understand the formation mechanisms of certain observables. In the current paper, we investigated the behaviour of the pion and kaon interferometry radii and their connection with emission functions in ultrarelativistic heavy-ion collisions at the Large Hadron Collider within iHKM. We focused on the study of the emission time scales at different energies for both particle species (pions and kaons) aiming to gain deeper insight into relation of these scales and the peculiarities of the mentioned system’s collective expansion and decay with the experimentally observed femtoscopy radii. One of our main interests was the problem of the total system’s lifetime estimation based on the femtoscopy analysis.

Thermalization and isotropization of heavy quarks in a non-Markovian medium in high-energy nuclear collisions

Thermalization and isotropization of heavy quarks in a non-Markovian medium in high-energy nuclear collisions

Poincaré covariant cascade method for high-energy nuclear collisions

Poincaré covariant cascade method for high-energy nuclear collisions

Decomposition of multi-particle azimuthal correlations in Q-cumulant analysis* *Supported by the Ministry of Education Science and Technological Development, Republic of Serbia, the National Natural Science Foundation of China (12035006, 12075085, 12147219), and the U.S. Department of Energy (de-sc0012910).

The method of Q-cumulants is a powerful tool for studying the fine details of azimuthal anisotropies in high energy nuclear collisions. This paper presents a new method, based on mathematical induction, to evaluate the analytical form of high-order Q-cumulants. The capability of this method is demonstrated via a toy model that uses the elliptic power distribution to simulate the anisotropic emission of particles, quantified in terms of Fourier flow harmonics . The method can help in studying the large amount of event statistics that can be collected in the future and allow measurements of the very high central moments of the distribution. This can, in turn, facilitate progress in understanding the initial geometry, the input to the hydrodynamic calculations of medium expansion in high energy nuclear collisions, and the constraints on it.

Longitudinal momentum fraction of heavy-flavor mesons in jets in high-energy nuclear collisions

Longitudinal momentum fraction of heavy-flavor mesons in jets in high-energy nuclear collisions

Nuclear Modification Factor of Neutral Pions in the Forward and Backward Regions in p-Pb Collisions.

The nuclear modification factor of neutral pions is measured in proton-lead collisions collected at a center-of-mass energy per nucleon of 8.16TeV with the LHCb detector. The π^{0} production cross section is measured differentially in transverse momentum (p_{T}) for 1.5<p_{T}<10.0 GeV and in center-of-mass pseudorapidity (η_{c.m.}) regions 2.5<η_{c.m.}<3.5 (forward) and -4.0<η_{c.m.}<-3.0 (backward) defined relative to the proton beam direction. The forward measurement shows a sizable suppression of π^{0} production, while the backward measurement shows the first evidence of π^{0} enhancement in proton-lead collisions at the LHC. Together, these measurements provide precise constraints on models of nuclear structure and particle production in high-energy nuclear collisions.

Elliptic flow of heavy-flavor decay electrons in Au+Au collisions at [formula omitted] = 27 and 54.4 GeV at RHIC

We report on new measurements of elliptic flow (v2) of electrons from heavy-flavor hadron decays at mid-rapidity (|y|<0.8) in Au+Au collisions at sNN = 27 and 54.4 GeV from the STAR experiment. Heavy-flavor decay electrons (eHF) in Au+Au collisions at sNN = 54.4 GeV exhibit a non-zero v2 in the transverse momentum (pT) region of pT< 2 GeV/c with the magnitude comparable to that at sNN=200 GeV. The measured eHFv2 at 54.4 GeV is also consistent with the expectation of their parent charm hadron v2 following number-of-constituent-quark scaling as other light and strange flavor hadrons at this energy. These suggest that charm quarks gain significant collectivity through the evolution of the QCD medium and may reach local thermal equilibrium in Au+Au collisions at sNN=54.4 GeV. The measured eHFv2 in Au+Au collisions at sNN= 27 GeV is consistent with zero within large uncertainties. The energy dependence of v2 for different flavor particles (π,ϕ,D0/eHF) shows an indication of quark mass hierarchy in reaching thermalization in high-energy nuclear collisions.

DDFRG2: Double-Differential FRaGmentation model for pion production in high energy nuclear collisions

A set of Double-Differential FRaGmentation (DDFRG) models for production of fragments from nucleon–nucleus and nucleus–nucleus collisions, relevant to space radiation, is currently being developed. DDFRG1 was a previous model developed for production of protons and light ions. A new model for pion production is developed in the present work and is called DDFRG2, which is based upon thermal production of pions from a central fireball source. The Lorentz-invariant double-differential cross section is calculated in the central fireball frame, and is Lorentz transformed to the laboratory frame, which results in a closed-form analytic formula involving no numerical integration, and which can be run very efficiently in radiation transport codes. The Lorentz-invariant double-differential cross section is then integrated over angle to give the single-differential energy spectral distribution. The calculated Lorentz-invariant double-differential cross sections are compared to an extensive data set containing over 1000 data points. Agreement between model calculations and data is found to be excellent.

Spin polarization and correlation of quarks from the glasma

We investigate the interaction of strong color fields in the glasma stage of high-energy nuclear collisions with the spins of quarks and antiquarks. We employ the perturbative solution of the quantum kinetic theory for the spin transport of (massive) quarks in a background color field governed by the linearized Yang-Mills equation and derive expressions for the quark-spin polarization and quark-antiquark spin correlation at small momentum in terms of field correlators. For the Golec-Biernat W\"usthoff dipole distribution the quark-spin polarization vanishes, but the out-of-plane spin correlation of quarks and antiquarks is nonzero. Our order-of-magnitude estimate of the correlation far exceeds that caused by vorticity effects, but does not fully explain the data for vector meson alignment. We identify possible mechanisms that could further increase the predicted spin correlation.

Imprints of clustering in multiplicity fluctuations

In this paper, we investigate the multiplicity fluctuations of charged particles observed in high-energy nuclear collisions and relate them to the size of hadronizing systems which happen during such processes. We use the average multiplicities langle Nrangle and variances Varleft( Nright) of multiplicity distributions of charged particles produced in centrality selected collisions of relativistic heavy-ion nuclei to evaluate the dynamic variance varOmega and study its dependence on the size of colliding systems. We connect the observed system-size dependence of multiplicity fluctuations with the clustering phenomena and the finiteness of the hadronizing sources and the thermal bath.

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.

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.

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.