State In Relativistic Heavy Ion Collisions Research Articles

Separating the Impact of Nuclear Skin and Nuclear Deformation in High-Energy Isobar Collisions.

Bulk nuclear structure properties, such as radii and deformations, leave distinct signatures in the final state of relativistic heavy-ion collisions. Isobaric collisions offer an easy route to establish explicit connections between the colliding nuclei's structure and the observable outcomes. Here, we investigate the effects of nuclear skin thickness and nuclear deformations on the elliptic flow (v_{2}) and its fluctuations in high-energy ^{96}Ru+^{96}Ru and ^{96}Zr+^{96}Zr collisions. Our findings reveal that the difference in skin thickness between these isobars only influences the inherent ellipticity of the collision systems, v_{2}^{rp}. In contrast, differences in nuclear deformations solely impact the fluctuations of v_{2} around v_{2}^{rp}. Hence, we have identified a data-driven method to disentangle the effects of nuclear skin and nuclear deformations, marking a significant step toward assessing the consistency of nuclear phenomena across energy scales.

Search for jet quenching effects on the plain jet mass in Pb+Pb collisions at the LHC with a multiphase transport model

The plain jet mass distributions of reconstructed jets are investigated in p+p and 0–10 $$\%$$ most central Pb+Pb collisions at $$\sqrt{s_{\textrm{NN}}} = 2.76~\textrm{TeV}$$ using a dynamical multiphase transport model with a string melting mechanism. It is observed that the mean charged jet mass increases with increasing jet transverse momentum and jet radius in central Pb+Pb collisions. It is demonstrated that the plain jet mass of reconstructed partonic jet is shifted to a higher value after the evolution of partonic stage due to jet quenching in central Pb+Pb collisions. However, the jet mass shift effect is strongly weakened by non-perturbative effects from hadronization and hadronic rescatterings. This makes it difficult to observe significant hot medium modification effects on the plain jet mass distribution in the final state of relativistic heavy-ion collisions.

An improved method to access initial states in relativistic heavy-ion collisions

Observables in heavy-ion collisions are generally categorized into centralities, which reflect an average over events within a range of impact parameters including a wide variety of initial-state configurations. A multiple binning method using spectator neutrons within each centrality has been shown to provide access to events with rare initial-state conditions. This work suggests an improvement in quantifying the difference between standard centrality and spectator neutron binning towards accessing the initial-state properties. A selection of events with higher initial-state density at a fixed participating nucleon number was observed to result in larger final-state particle production and smaller elliptic flow. The relative difference between observables in centrality and spectator binning shows reduced sensitivity for the observables dominated by impact parameter fluctuations in the initial state, such as triangular flow. This property renders the spectator binning method a good candidate for separating geometric contributions from random fluctuations in the initial state towards final-state observables.

Eccentricities, fluctuations and A-dependence of elliptic and triangular flows in heavy-ion collisions

A simple geometrical model with event-by-event fluctuations is suggested to study elliptical and triangular eccentricities in the initial state of relativistic heavy-ion collisions. This model describes rather well the ALICE and ATLAS data for Pb + Pb collisions at center-of-mass energy TeV per nucleon pair, assuming that the second, v 2, and third, v 3, harmonics of the anisotropic flow are simply linearly proportional to the eccentricities ɛ 2 and ɛ 3, respectively. We show that the eccentricity ɛ 3 has a pure fluctuation origin and is substantially dependent on the size of the overlap area only, while the eccentricity ɛ 2 is mainly related to the average collision geometry. Elliptic flow, therefore, is weakly dependent on the event-by-event fluctuations everywhere except of the very central collisions 0%–2%, whereas triangular flow is mostly determined by the fluctuations. The scaling dependence of the magnitude of the flow harmonics on atomic number, v n ∝ A −1/3, is predicted for this centrality interval.

Classification of equation of state in relativistic heavy-ion collisions using deep learning

Convolutional Neural Nets, which is a powerful method of Deep Learning, is applied to classify equation of state of heavy-ion collision event generated within the UrQMD model. Event-by-event transverse momentum and azimuthal angle distributions of protons are used to train a classifier. An overall accuracy of classification of 98% is reached for Au+Au events at sqrt{s_{NN}} = 11 GeV. Performance of classifiers, trained on events at different colliding energies, is investigated. Obtained results indicate extensive possibilities of application of Deep Learning methods to other problems in physics of heavy- ion collisions.

Multiplicity and entropy scaling of medium-energy protons emitted in relativistic heavy-ion collisions

The behavior and the properties of medium-energy protons with kinetic energies in the range 26 - 400 MeV is derived from measurements of the particle yields and spectra in the final state of relativistic heavy-ion collisions (16O-AgBr interactions at 60 A and 200 A GeV and 32S-AgBr interactions at 3.7 A and 200 A GeV) and their interpretation in terms of the higher order moments. The multiplicity distributions have been fitted well with the Gaussian distribution function. The data are also compared with the predictions of the modified FRITIOF model, showing that the FRITIOF model does not reproduce the trend and the magnitude of the data. Measurements of the ratio of the variance to the mean show that the production of target fragments at high energies cannot be considered as a statistically independent process. However, the deviation of each multiplicity distribution from a Poisson law provides evidence for correlations. The KNO scaling behavior of two types of scaling (Koba-Nielsen-Olesen (KNO) scaling and Hegyi scaling) functions in terms of the multiplicity distribution is investigated. A simplified universal function has been used in each scaling to display the experimental data. An examination of the relationship between the entropy, the average multiplicity, and the KNO function is performed. Entropy production and subsequent scaling in nucleus-nucleus collisions are carried out by analyzing the experimental data over a wide energy range (Dubna and SPS). Interestingly, the data points corresponding to various energies overlap and fall on a single curve, indicating the presence of a kind of entropy scaling.

Identity method to study chemical fluctuations in relativistic heavy-ion collisions

Event-by-event fluctuations of the chemical composition of the hadronic final state of relativistic heavy-ion collisions carry valuable information on the properties of strongly interacting matter produced in the collisions. However, in experiments incomplete particle identification distorts the observed fluctuation signals. The effect is quantitatively studied and a new technique for measuring chemical fluctuations, the identity method, is proposed. The method fully eliminates the effect of incomplete particle identification. The application of the identity method to experimental data is explained.

The realistic QCD equation of state in relativistic heavy-ion collisions and the early Universe

The realistic equation of state of strongly interacting matter, that has been successfully applied in the recent hydrodynamic studies of hadron production in relativistic heavy-ion collisions at RHIC, is used in the Friedmann equation to determine the precise time evolution of thermodynamic parameters in the early Universe. A comparison with the results obtained with simple ideal-gas equations of state is made. The realistic equation of state describes a crossover rather than the first-order phase transition between the quark–gluon plasma and hadronic matter. Our numerical calculations show that small inhomogeneities of strongly interacting matter in the early Universe are moderately damped during such crossover.

Thermal analysis of particle yields from RQMD

The thermal model is often used to characterize the hadronic final state of relativistic heavy-ion collisions. We investigate the bias of incomplete momentum space information on the extraction of thermal parameters. We use RQMD(v2.3) to generate full momentum space multiplicities. Then we apply cuts in rapidity and use measured subsets of stable particles for our investigation. We find that using the full set of stable particles decreases considerable the quality of the thermal fit compared to fits using subsets of particles typically measured in experiments. The particles responsible for making the fit worse are the multi-strange baryons and anti-baryons. Also rapidity cuts on particle multiplicities lead to worse fits.

X rays from radiative electron capture into continuum states in relativistic heavy-ion collisions

Continuum x rays originated from radiative electron capture into continuum states (RECC) of relativistic ${\mathrm{Ne}}^{10+}$ ions have been observed. It has been found that the intensities of x rays decrease as their energy increases and the observed spectra show a clear edge which corresponds to the maximum energy transferred to free electrons under energetic projectile-ion collisions. The edge becomes dramatically sharp as the incident projectile energy decreases from 290 to 75 MeV/amu, indicating that the cross sections for RECC sharply increase at low projectile energies, which is in agreement with the theoretical expectation.

Pi -N correlations probe the nuclear equation of state in relativistic heavy-ion collisions.

pi -N correlations probe the nuclear equation of state in relativistic heavy-ion collisions.

Excitation of multiphonon giant resonance states in relativistic heavy-ion collisions

We investigate the excitation of multiphonon states in relativistic heavy-ion collisions. A semiclassical formalism is used to determine the one-step and two-step Coulomb excitation of the double-phonon GDR × GDR excitation. The excitation cross sections of the L = 0 and L = 2 states are determined separately with the use of sum rules for estimating corresponding matrix elements. The nuclear contributions to the excitation process are also calculated and shown to be small. The widths of final and intermediate states are taken into account; physical problems related to the spreading width of the double-phonon states are discussed in detail. The recent experimental data are discussed and compared to our calculations.

Excitation of multiphonon states in relativistic heavy-ion collisions.

One- and two-step Coulomb excitation cross sections of the double-phonon giant dipole resonance are calculated in a semiclassical formalism. The contribution of nuclear interaction is shown to be small. The multipole sum rules are used to evaluate corresponding nuclear matrix elements; the widths of final and intermediate states are taken into account. The recent experimental data re discussed and compared to our calculations.

Lattice calculation of muon-pair production with capture in relativistic heavy-ion collisions.

We present a schematic study of muon-pair production with capture of the negative muon into the $1{s}_{\frac{1}{2}}$ ground state in relativistic heavy-ion collisions. The muon pairs are generated by a time-dependent, screened electromagnetic field. Our numerical calculations are carried out on a three-dimensional lattice with 20-30 mesh points in each direction, utilizing the $B$-spline collocation method. Numerical convergence has been demonstrated using a Lorentz-boosted screened Coulomb potential. For the system $^{197}\mathrm{Au}$+$^{197}\mathrm{Au}$ at collider energies of 0.2, 1.0 and 2.0 GeV/nucleon, we obtain probabilities for dimuon production with capture between 1.0 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ and 7.0 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}2}$ at grazing impact parameters. This may indicate that the electromagnetic lepton-pair production is strongly nonperturbative.

Angular distribution of electrons emitted from 1s1/2 and 2s1/2 states in relativistic heavy-ion collisions

The energy and angular distributions of electrons emitted from the 1s1/2 and 2s1/2 states in relativistic heavy-ion collisions are evaluated in the semiclassical approximation employing the first-order perturbation theory. The angular anisotropy of the electron-emission becomes increasingly peaked in forward direction with growing kinetic energy of the electrons.

Charmonium disintegration by field ionization

Charmonium bound states immersed in a coherent chromo-electric field are easily ripped apart for field strengths comparable to the QCD string tension. Estimates based on flux tube models suggest that field strengths of such magnitude may be achieved in heavy-ion collisions at ultrarelativistic energies. Our results suggest that charmonium suppression would not discriminate between a coherent and a thermalized post collision state in relativistic heavy-ion collisions.