HIJING Model Research Articles

Event-by-event fluctuations of mean transverse momentum in Pb–Pb and Xe–Xe collisions with ALICE

Event-by-event fluctuations of the mean transverse momentum of charged particles produced in Pb–Pb and Xe–Xe collisions at TeV and TeV, respectively, are studied as a function of the charged-particle multiplicity using the ALICE detector at the LHC. Dynamical fluctuations are observed in both collision systems which indicate correlated particle emission. The central Pb–Pb and Xe–Xe collisions show a significant reduction of the fluctuation in comparison to peripheral collisions and are in qualitative agreement with previous measurements in Pb–Pb collisions at TeV. The results are compared with the HIJING model. A clear deviation from simple superposition of independent nucleon-nucleon collisions scenario, where the final state particles are produced from superposition of particle emitting sources, is observed.

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.

Higher-order transverse momentum fluctuations in heavy-ion collisions

In relativistic heavy-ion collisions, the event-by-event mean transverse momentum fluctuations are sensitive to overlap area and energy density fluctuations in initial state. We present a framework to calculate $p_{\mathrm{T}}$ fluctuations up to $4^{\mathrm{th}}$-order using standard and subevent methods, which is validated using the HIJING model. We observe a power-law dependence for cumulants of all orders as a function of charged particle multiplicity $N_{\mathrm{ch}}$, consistent with a simple independent source picture. The fluctuation in $pp$ collisions is observed to be larger than for $p+$Pb, Pb+Pb and Xe+Xe collisions at the same $N_{\mathrm{ch}}$ due to bias in number of contributing sources. The short-range correlations are greatly suppressed in subevent method in comparison to calculations based on the standard method. This study provides a baseline for transverse momentum fluctuations without the presence of final state effects.

Event-by-Event Particle Ratio Fluctuations at LHC Energies

A Monte Carlo study of identified particle ratio fluctuations at LHC energies is carried out in the framework of HIJING model using the fluctuation variable ν dyn . The simulated events for Pb-Pb collisions at s N N = 2.76 and 5.02 TeV and Xe-Xe collisions at s N N = 5.44 TeV are analyzed. From this study, it is observed that the values of π , K , p , K , and π , p follow the similar trends of energy dependence as observed in the most central collision data by NA49, STAR, and ALICE experiments. It is also observed that ν dyn for all the three combinations of particles for semicentral and central collisions, the model predicted values of ν dyn A , B for Pb-Pb collisions at s N N = 2.76 TeV agree fairly well with those observed in the ALICE experiment. For peripheral collisions, however, the model predicted values of ν dyn π , K are somewhat smaller, whereas for p , K and π , p it predicts larger values as compared to the corresponding experimental values. The possible reasons for the observed differences are discussed. The ν dyn values scaled with charged particle density when plotted against N part exhibit a flat behaviour, as expected from the independent particle emission sources. For p , K and π , p combinations, a departure from the flat trend is, however, observed in central collisions in the case of low p T window when the effect of jet quenching or resonances is considered. Furthermore, the study of ν dyn A , B dependence on particle density for various collision systems (including proton-proton collisions) suggests that at LHC energies ν dyn values for a given particle pair are simply a function of charged particle density, irrespective of system size, beam energy, and collision centrality.

Cumulants of Net-charge Distribution from Particle–Antiparticle Sources

It is shown how high-order cumulants of net-charge distribution in hadronic collisions at LHC energies can be expressed via lower-order terms under the assumption that particle-antiparticle pairs are produced in independent local processes. It is argued and tested with HIJING model that this assumption is typically valid for net-proton fluctuations in the case when no critical behaviour is present in the system. Values estimated in such a way can be considered as baselines for direct measurements of high-order net-charge fluctuations in real data.

Using the HIJING Model in Modeling Nucleus–Nucleus Interaction at Energies of Nucleon–Nucleon Collisions 5–15 GeV

A new set of HIJING model parameters is found. Changes in the model are proposed that allow NA49 and NA61/SHINE experimental data on pp interactions to be described at projectile proton momenta $${{p}_{{{\text{lab}}}}}$$ = 20, 31, 40, 80 and 158 GeV c−1 in the rest frame of target protons. The modified model is used to analyze data on nucleus–nucleus interaction. It is shown that the model allows the main properties of these interactions to be described.

Improvement of heavy flavor production in a multiphase transport model updated with modern nuclear parton distribution functions

Recently we have updated a multi-phase transport (AMPT) model with modern parton distribution functions of nuclei (nPDFs). Here we study open charm production in the updated AMPT model and compare to the experimental data from $pp$ and $AA$ collisions over a wide range of collision energies. Besides the update of nPDFs, we have removed the transverse momentum cutoff on initial heavy quark productions and also included the resultant heavy flavor cross section into the total minijet cross section in the initial condition as described by the HIJING model. We show that the AMPT model with these updates provides a much better description of the yields and transverse momentum spectra of various open charm hadrons in comparison with the experimental data. This lays the foundation for further heavy flavor studies within the transport model approach.

Forward-backward multiplicity fluctuation and longitudinal harmonics in high-energy nuclear collisions

Forward-backward (FB) multiplicity fluctuation in high-energy nuclear collisions can be quantified by two-particle pseudo-rapidity correlation function and its expansion into Legendre polynomials. The corresponding coefficients represent different fluctuation modes in longitudinal direction. The leading term 〈a12〉 corresponds to the asymmetry of numbers of the participants from the two colliding nuclei. This method is tested in events generated from AMPT and HIJING model. The an signal are found to be strongly dampened in AMPT than in HIJIGN, due to weaker short-range correlaitons and final-state effects in AMPT. Two-particle correlation also reveals an intresting shallow minimum around Δη≈0 in AMPT events, which is absent in HIJING results. The method opens a new avenue to elucidate the particle production mechanism and early time dynamics in heavy-ion collisions.

Vorticity in heavy-ion collisions

We study the event-by-event generation of flow vorticity in RHIC Au + Au collisions and LHC Pb + Pb collisions by using the HIJING model. Different definitions of the vorticity field and velocity field are considered. A variety of properties of the vorticity are explored, including the impact parameter dependence, the collision energy dependence, the spatial distribution, the event-by-event fluctuation of the magnitude and azimuthal direction, and the time evolution. In addition, the spatial distribution of the flow helicity is also studied.

Electric fields and chiral magnetic effect in Cu + Au collisions

The non-central Cu+Au collisions can create strong out-of-plane magnetic fields and in-plane electric fields. By using the HIJING model, we study the general properties of the electromagnetic fields in Cu+Au collisions at 200 GeV and their impacts on the charge-dependent two-particle correlator γq1q2=〈cos⁡(ϕ1+ϕ2−2ψRP)〉 (see main text for definition) which was used for the detection of the chiral magnetic effect (CME). Compared with Au+Au collisions, we find that the in-plane electric fields in Cu+Au collisions can strongly suppress the two-particle correlator or even reverse its sign if the lifetime of the electric fields is long. Combining with the expectation that if γq1q2 is induced by elliptic-flow driven effects we would not see such strong suppression or reversion, our results suggest to use Cu+Au collisions to test CME and understand the mechanisms that underlie γq1q2.

Event structure investigation of AuAu interactions from HIJING model using fractal dimensions

This work presents an implementation of fractal geometry methods in the study of event structure for AuAu interactions at collision energies √sNN = 9.2, 62 and 200 GeV for different interaction dynamics. The events are generated by using the HIJING model. It is shown, that the fractal dimension of events in phase space projections rapidity-transverse momentum (y - pt) and azimuthal angle-transverse momentum (φ - pt) are sensitive to the interaction dynamics.

Event-by-event generation of electromagnetic fields in heavy-ion collisions

We compute the electromagnetic fields generated in heavy-ion collisions by using the HIJING model. Although after averaging over many events only the magnetic field perpendicular to the reaction plane is sizable, we find very strong magnetic and electric fields both parallel and perpendicular to the reaction plane on the event-by-event basis. We study the time evolution and the spatial distribution of these fields. Especially, the electromagnetic response of the quark-gluon plasma can give non-trivial evolution of the electromagnetic fields. The implications of the strong electromagnetic fields on the hadronic observables are also discussed.

Pb-Pb collisions atsNN=2.76TeV in a multiphase transport model

The multiplicity and elliptic flow of charged particles produced in Pb-Pb collisions at center of mass energy $\sqrt{{s}_{\mathit{NN}}}=2.76$ TeV from the Large Hadron Collider are studied in a multiphase transport (AMPT) model. With the standard parameters in the HIJING model, which is used as initial conditions for subsequent partonic and hadronic scatterings in the AMPT model, the resulting multiplicity of final charged particles at mid-pseudorapidity is consistent with the experimental data measured by the ALICE Collaboration. This value is, however, increased by about $25%$ if the final-state partonic and hadronic scatterings are turned off. Because of final-state scatterings, particularly those among partons, the final elliptic flow of charged hadrons is also consistent with the ALICE data if a smaller but more isotropic parton scattering cross section than previously used in the AMPT model for describing the charged hadron elliptic flow in heavy ion collisions at the Relativistic Heavy Ion Collider is used. The resulting transverse momentum spectra of charged particles as well as the centrality dependence of their multiplicity density and the elliptic flow are also in reasonable agreement with the ALICE data. Furthermore, the multiplicities, transverse momentum spectra, and elliptic flows of identified hadrons such as protons, kaons, and pions are predicted.

Study on `soft' and `hard' interactions in bar p p (pp) collisions using HIJING and PYTHIA

This paper presents a study on `hard' and `soft' interactions in p (pp) collisions using a phenomenological model of HIJING, the jet-cone reconstruction method is employed to select the `hard' and `soft' event sub-samples from minimum bias events. It is found that the HIJING model can reproduce the energy scaling behaviour of mean transverse momentum ( ) distributions of charged hadrons versus multiplicity (Nch) in `soft' events. From the PYTHIA simulation comparing with the HIJING model, the enhancement of the kaon and proton yields from `hard' interactions comparing with `soft' interactions is observed to be due to the mini-jets effect. These mechanisms responsible for the increase of charged hadron's are different in `soft' and `hard' interactions.

The background for Cherenkov gluons at RHIC and LHC energies

The pseudorapidity distribution of centers of dense isolated groups of particles in HIJING model is determined. It can be considered as the background for Cherenkov gluons. If peaks over this background were found in experiment, they would indicate the onset of new collective effects.

High energy Cherenkov gluons at RHIC and LHC

The collective effect of emission by the forward moving partons of high energy Cherenkov gluons in nucleus-nucleus collisions at RHIC and LHC energies is considered. It can reveal itself as peaks in the pseudorapidity distribution of jets at midrapidities or as a ring-like structure of individual events in event-by-event analysis. The pseudorapidity distribution of centers of dense isolated groups of particles in HIJING model is determined. It can be considered as the background for Cherenkov gluons. If peaks above this background were found in experiment, they would indicate new collective effects.

Effect of parton shadowing on the distributions of global observables

The effect of parton shadowing on the distributions of global observables (total transverse energy ET and charged-particle multiplicity nch) is investigated on the basis of the HIJING model of nucleus-nucleus interactions. It is shown that, for the interaction of lead nuclei at \(\sqrt s = 5\) TeV per nucleon, the shadowing effect results in a considerable reduction of the cross section for (mini)jet production (approximately by a factor of 4); this in turn reduces the total transverse energy and the charged-particle multiplicity by a factor of 2.7. Upon taking into account the QCD evolution of nuclear structure functions in Q2, the shadowing effect becomes less pronounced (by a factor of 1.9 for PbPb interactions). It is shown that global observables can be used to test models of parton shadowing.

Energy and centrality dependence of rapidity densities at RHIC energies.

The energy and centrality dependence of the charged multiplicity per participant nucleon is shown to be able to differentiate between final state saturation and fixed scale perturbative QCD models of initial entropy production in high-energy heavy-ion collisions. The energy dependence is shown to test the nuclear enhancement of the minijet component of the initial conditions, while the centrality dependence provides a key test of whether gluon saturation is reached at RHIC energies. The HIJING model predicts that the rapidity density per participant increases with centrality, while the saturation model prediction is essentially independent of centrality.

Bremsstrahlung from an equilibrating quark-gluon plasma

The photon production rate from a chemically equilibrating quark-gluon plasma likely to be produced at RHIC (BNL) and LHC (CERN) energies is estimated taking into account bremsstrahlung. The plasma is assumed to be in local thermal equilibrium, but with a phase space distribution that deviates from the Fermi or Bose distribution by space-time dependent factors (fugacities). The photon spectrum is obtained by integrating the photon rate over the space-time history of the plasma, adopting a boost invariant cylindrically symmetric transverse expansion of the system with different nuclear profile functions. Initial conditions obtained from a self-screened parton cascade calculation and, for comparison, from the HIJING model are used. Compared to an equilibrated plasma at the same initial energy density, taken from the self-screened parton cascade, a moderate suppression of the photon yield by a factor of 1 to 5 depending on the collision energy and the photon momentum is observed. The individual contributions to the photon production, however, are completely different in the both scenarios.

Specific interactions at cosmic ray energies for extensive air shower experiments

The HIJING and VENUS models of relativistic hadron-nucleus and nucleus-nucleus collisions are used to study interactions of hadron-hadron, hadron-nitrogen and nucleus-nitrogen collisions, specific for the extensive air shower developments initiated by cosmic rays in the atmosphere. The transverse energy, transverse momenta and spectra of secondary particles as well as their energy and mass dependence have been investigated in detail. Results are presented with particular emphasis on the contributions of mini-jets in the HIJING model and the validity of superposition models in this energy range.