Superheavy Particle Production in High Energy Heavy Ion Collisions

The existence of several TeV superheavy particles (SHPs) is predicted by theories beyond the Standard Model. Particles with a mass exceeding the energy in the center of mass in the collision of protons with protons can be produced in subthreshold heavy ion collisions at the LHC. The purpose of the performed research was to estimate the rate of a rare process of SHPs production. It was shown that the data on the subthreshold production of antiprotons can be explained by the phenomenological parton model. The obtained parton distribution function was used to determine the number of SHPs produced in subthreshold heavy ion collisions at the LHC. In one month of collision of lead with lead, the yield of 16 TeVparticles is about 70 per year. To study the kinematically forbidden phenomena in proton-proton interactions in collisions of heavy nuclei at the LHC, an experiment on the production of antiprotons is proposed in the ALICE fixed target project.

Entropy Production and Fractal Dimensions in Heavy Ion Nuclear Reaction at Intermediate Energies

The characteristics of the nonlinear dynamics in the Heavy Ion Collision (HIC) at intermediate energies have been studied by evaluating the productions of the Generalized Entropy (GE) and the Multifragmentation Entropy (ME) as well as the features of the information and fractal dimensions within the Isospin Quantum Molecular Dynamical Model compensated by the lattice methods. Results demonstrate from various views that the existence of deterministic chaos in the dynamical process of reaction.

Different coalescence sources of light nucleus production in Au-Au collisions at √sNN=3 GeV

We study the production of light nuclei in the coalescence mechanism of Au-Au collisions at midrapidity at √sNN=3GeV.We derive analytic formulas of the momentum distributions of two bodies,three bodies,and four nucleons coalescing into light nuclei and naturally explain the transverse momentum spectra of the deuteron(d),triton(t),helium-3(3He),and helium-4(4He).We reproduce data on the yield rapidity densities,yield ratios,and averaged transverse momenta of d,t,3He,and 4He and provide the proportions of contributions from different coalescence sources for t,3He,and 4He in their production.We find that besides nucleon coalescence,nucleon+nucleus coalescence and nucleus+nucleus coalescence may play requisite roles in light nucleus production in Au-Au collisions at √sNN=3 GeV.

Effects of a phase transition on two-pion interferometry in heavy ion collisions at √SNN=2.4-7.7 GeV

Hanbury-Brown-Twiss(HBT) correlations for charged pions in central Au+Au collisions at √SNN=2.4-7.7 GeV(corresponding to beam kinetic energies in the fixed target frame from Elab=1.23 to 30 GeV/nucleon) are calculated using the ultra-relativistic quantum molecular dynamics model with different equations of state(EoSs).The effects of a phase transition at high baryon densities are clearly observed in the explored HBT parameters.The results show that the available data on the HBT radii,RO/RSand R^(2)_(O)-R^(2)_(S),in the investigated energy region favor a relatively stiff EoS at low beam energies,which then turns into a soft EoS at high collision energies consistent with astrophysical constraints on the high-density EoS of quantum chromodynamics(QCD).The specific effects of two different phase transition scenarios on RO/RSand R^(2)_(O)-R^(2)_(S)are investigated.A phase transition with a significant softening of the EoS below four times the nuclear saturation density can be excluded using HBT data.Our results highlight that the pion’s RO/RSand R^(2)_(O)-R^(2)_(S)are sensitive to the stiffness of the EoS and can be used to constrain and understand the QCD EoS in a high baryon density region.

Production of X_(eses)in heavy ion collisions

The yields of X_(cscs)with its two possible configurations,i.e.,the hadronic molecular state and tetraquark state,for Pb-Pb collisions at√sNN=5.02 TeV is studied.A volume effect is found from the centrality distribution of X_(cscs),which could help to distinguish the inner structure of X_(cscs).We also show the rapidity and the transverse momentum distributions of X_(cscs) production as well as its elliptic flow coefficient as a function of the transverse momentum.

Interactions between heavy quarks and tilted QGP fireballs in 200 A GeV Au+Au collisions

Heavy quark observables are applied to probe the initial energy density distribution with violation of longitudinal boost invariance produced in relativistic heavy-ion collisions.Using an improved Langevin model coupled to a(3+1)-dimensional viscous hydrodynamic model,we study the nuclear modification factor(R_(AA)) and directed flow(v_(1)) and elliptic flow(v_(2)) coefficients of heavy mesons and their decayed electrons at an RHIC energy.We find that the counter-clockwise tilt of nuclear matter in the reaction plane results in a positive(negative) heavy flavor vi in the backward(forward) rapidity region,whose magnitude increases with the heavy quark transverse momentum.The difference in the heavy flavor R_(AA) between different angular regions is also proposed as a complementary tool to characterize the asymmetry of the medium profile.Our model results are consistent with currently available data at the RHIC and provide predictions that can be tested by future measurements.

Antiproton Production with a Fixed Target and Search for Superheavy Particles at the LHC

A proposal for an experiment to measure the cross section of antiproton production in a proton-nuclear collision in a kinematically forbidden region for nucleon-nucleon interaction on a fixed LHC target is considered. It is shown that this process can be separated from the kinematically allowed production process using the existing detectors of the ALICE facility at a proton energy of 7 TeV with a fixed nuclear target. Assuming the scale dependence of the cross section, the data obtained can be used to estimate the subthreshold cross section for the production of superheavy particles with a mass of several tens of TeV in the LHC lead nucleus beam.

Interaction of 28Si with Emulsion Nuclei at 4.5 AGeV/c in View of Thermo-Statistical Approach

In this work, we study some changes of nuclear matter in the interactions of 28Si with emulsion nuclei at 4.5 AGeV/c. From the experimental quantities investigated using Tsallis’ statistics, we deduced the temperature, entropy density and non-equilibrium factor of the nuclear medium. These obtained parameters were used to reveal variations in the nuclear matter at the stated interaction energy. The results that came up from this study were compared with their corresponding results obtained from other heavy ion collision experiments at wide energy range.

Progress of quantum molecular dynamics model and its applications in heavy ion collisions

In this review article,we first briefty introduce the transport theory and quantum molecular dynamics model applied in the study of the heavy ion collisions from low to intermediate energies.The developments of improved quantum molecular dynamics model(ImQMD)and ultra-relativistic quantum molecular dynamics model(UrQMD),are reviewed.The reaction mechanism and phenomena related to the fusion,multinucleon transrer,fragmentation,collective flow and particle production are reviewed and discussed within the framework of the two models.The constraints on the isospin asymmetric muclear equation of state and in-medium nucleon nucleon cross sections by comparing the heavy ion collision data with transport models calculations in last decades are also discussed,and the uncertainties of these constraints are analyzed as well.Finally,we discuss the future direction of the development of the transport models for improving the understanding of the reaction mechanism,the descriptions of various observables,the constraint on the nuclear equation of state,as well as for the constraint on in-medium nucleon-nucleon cross sections.

Application of microscopic transport model in the study of nuclear equation of state from heavy ion collisions at intermediate energies

The equation of state(EOS)of nuclear matter,i.e.,the thermodynamic relationship between the binding energy per nucleon,temperature,density,as well as the isospin asymmetry,has been a hot topic in nuclear physics and astrophysics for a long time.The knowledge of the nuclear EOS is essential for studying the propertics of muclei,the structure of neutron stars,the dynamics of heavy ion collision(HIC),as well as neutron star mergers.HIC offers a unique way to create muclear matter with high density and isospin asymmetry in terrestrial laboratory,but the formed dense nuclear matter exists only for a very short period,one cannot measure the nuclear EOS directly in experiments.Practically,transport models which often incorporate phenomenological potentials as an input are utilized to deduce the EOS from the comparison with the observables mcasured in laboratory.The ultrarelativistic quantum molecular dynamics(UrQMD)model has been widely employed for investigating HIC from the Fermi energy(40 MeV per nucleon)up to the CERN Large Hadron Collider energies(TeV).With further improvement in the nmuclear mean-field potential term,the collision term,and the cluster recognition term of the UrQMD model,the newly measured collective flow and nuclear stopping data of light charged particles by the FOPI Collaboration can be reproduced.In this article we highlight our recent results on the studies of the muclear EOS and the muclear symmetry energy with the UrQMD model.New opportunities and challenges in the extraction of the nuclear EOS from transport models and HIC experiments are discussed.

Signals of α clusters in ^(16)O+^(16)O collisions at the LHC from relativistic hydrodynamic simulations

In relativistic heavy ion collisions,the fluctuations of initial entropy density convert to the correlations of final state hadrons in momentum space through the collective expansion of strongly interacting QCD matter.Using a(3+1)D viscous hydrodynamic program,CL Visc,we consider whether the nuclear structure,which provides initial state fluctuations as well as correlations,can affect the final state of heavy ion collisions,and whether one can find signals of α cluster structures in oxygen using final state observables in ^(16)O+ ^(16)O collisions at the CERN Large Hadron Collider.For the initial nucleon distributions in oxygen nuclei,we compare three different configurations,a tetrahedral structure with four-α clusters,the deformed Woods-Saxon distribution,and a spherical symmetric Woods-Saxon distribution.Our results show that the charged multiplicity as a function of centrality and the elliptic flow at the most central collisions using the four-α structure differs from those with the Woods-Saxon and deformed Woods-Saxon distributions,which may help to identify α clustering structures in oxygen nuclei.

Hydrodynamic simulations of directed flow for light hadrons in Au+Au and isobarcollisions at √SNN=200 GeV

Using a(3+1)-D hydrodynamic model,CLVisc,we study the directed flow(vi)of light hadrons pro-duced in Au+Au,Ru+Ru,and Zr+Zr collisions at √SNN=200 GeV.The evolution of tilted energy density,pres-sure gradient,and radial flow along the x-direction is systematically investigated.The counter-clockwise tilt of the initial fireball is shown to be a vital source of directed flow for final light hadrons.A good description of directed flow is provided for light hadrons in central and mid-central Au+Au and isobar collisions at the RHIC.Our numeric-al results show a clear system size dependence for light hadron Vi across different collision systems.We further study the effect of nuclear structure on the directed flow and find that v1 for light hadrons is insensitive to nuclei with quadrupole deformation.

Effect of non-uniform efficiency on higher-order cumulants in heavy-ion collisions

We perform a systematic study on the effect of non-uniform track efficiency correction in higher-order cumulant analysis in heavy-ion collisions.Through analytical derivation,we find that the true values of cumulants can be successfully reproduced by the efficiency correction with an average of the realistic detector efficiency for particles with the same charges within each single phase space.The theoretical conclusions are supported by a toy model simulation by tuning the non-uniformity of the efficiency employed in the track-by-track efficiency correction method.The valid averaged efficiency is found to suppress the statistical uncertainties of the reproduced cumulants dramatically.Thus,usage of the averaged efficiency requires a careful study of phase space dependence.This study is important for carrying out precision measurements of higher-order cumulants in heavy-ion collision experiments at present and in future.

Flavor-dependent photoproduction in heavyion collisions

Strong electromagnetic fields produced in the non-central heavy-ion collisions can induce vector meson photoproduction.In this paper,we study the photoproduction J/ψandφmesons in the relativistic heavy-ion collision from ultra-peripheral nuclear collisions to peripheral hadronic heavy ion collisions.And then include both initial hadronic production and thermal production in quark-gluon plasma(QGP).We find,for the charm anti-charm bound state J/ψ,the photoproduced J/ψs are mainly in the very low momentum region and clearly exceed the hadronic production.However,considering the thermal production of strange quark anti-quark pairs in QGP produced in relativistic heavy-ion collisions,the photoproducedφis usually smaller than the thermal production and only evident at very peripheral collisions as even their photoproduction is much larger than J/ψ.

Hydrodynamic description of D meson production in high-energy heavy-ion collisions

The large values and constituent-quark-number scaling of the elliptic flow of low-pT D mesons imply that charm quarks,initially produced through hard processes,might be partially thermalized through strong interactions with quark-gluon plasma(QGP)in high-energy heavy-ion collisions.To quantify the degree of thermalization of low-pT charm quarks,we compare the D^(0)meson spectra and elliptic flow from a hydrodynamic model to experimental data as well as transport model simulations.We use an effective charm chemical potential at the freeze-out temperature to account for the initial charm quark production from hard processes and assume that they are thermalized in the local comoving frame of the medium before freeze-out.D^(0)mesons are sampled statistically from the freeze-out hyper-surface of the expanding QGP as described by the event-by-event(3+1)D viscous hydrodynamic model CLVisc.Both the hydrodynamic and transport models can describe the elliptic flow of D^(0)mesons at pT~3 GeV/c as measured in Au+Au collisions at√SNN=200 GeV.Though the experimental data on D^(0)spectra are consistent with the hydrodynamic result at small pT~1 GeV/c,they deviate from the hydrodynamic model at high transverse momentum,pT~2 GeV/c.The diffusion and parton energy loss mechanisms in the transport model can describe the measured spectra reasonably well within the theoretical uncertainty.Our comparative study indicates that charm quarks only approach local thermal equilibrium at small pT,even though they acquire sizable elliptic flow comparable to light-quark hadrons at both small and intermediate pT.

Determination of the impact parameter in high-energy heavy-ion collisions via deep learning

In this study,Au+Au collisions with an impact parameter of 0≤b≤12.5 fm at√(s_(NN))=200 GeV are simulated using the AMPT model to provide preliminary final-state information.After transforming this information into appropriate input data(the energy spectra of final-state charged hadrons),we construct a multi-layer perceptron(MLP)and convolutional neural network(CNN)to connect final-state observables with the impact parameters.The results show that both the MLP and CNN can reconstruct the impact parameters with a mean absolute error approximately 0.4 fm,although the CNN behaves slightly better.Subsequently,we test the neural networks at different beam energies and pseudorapidity ranges in this task.These two models work well at both low and high energies.However,when conducting a test for a larger pseudorapidity window,the CNN exhibits a higher prediction accuracy than the MLP.Using the Grad-CAM method,we shed light on the'attention'mechanism of the CNN model.

Nucleon effective mass splitting and density-dependent symmetry energy effects on elliptic flow in heavy ion collisions at Elab= 0.09 ~ 1.5 GeV/nucleon

By incorporating an iso spin-dependent form of the momentum-dependent potential in the ultra-relativistic quantum molecular dynamics(UrQMD)model,we systematically investigate effects of the neutron-proton effective mass splitting m*n-p=m*n-m*p/m and the density-dependent nuclear symmetry energy Esym(ρ)on the elliptic flow v2 in197Au+197 Au collisions at beam energies from 0.09 to 1.5 GeV/nucleon.It is found that at higher beam energies(≥0.25 GeV/nucleon)with the approximately 75 MeV difference in slopes of the two different Esym(ρ),and the variation of m*n-p ranging from-0.03 to 0.03 at saturation density with isospin asymmetryδ=(ρn-ρp)/ρ-0.2,the Esym(ρ)has a stronger influence on the difference in v2 between neutrons and protons,i.e.,v2n-v2p,than m*n-p has.Meanwhile,at lower beam energies(≤0.25 GeV/nucleon),v2n-v2p is sensitive to both the Esym(ρ)and the m*n-p.Moreover,the influence of m*n-p on v2n-v2p is more evident with the parameters of this study when using the soft,rather than stiff,symmetry energy.

Measurements of dihadron correlations relative to the event plane in Au+Au collisions at√^(S)NN=200 GeV

Dihadron azimuthal correlations containing a high transverse momentum(pr)trigger particle are sensit-ive to the properties of the nuclear medium created at RHIC through the strong interactions occurring between the traversing parton and the medium,ie.jet-quenching.Previous measurements revealed a strong modification to di-hadron azimuthal correlations in Au+Au collisions with respect to ptp and d+Au collisions.The modification in-creases with the collision centrality,suggesting a path-length or energy density dependence to the je-quenching ef-fect.This paper reports STAR measurements of dihadron azimuthal correlations in mid-central(20%-60%)Au+Au collisions at√^(S)NN=200 GeV as a function of the trigger particle's azimuthal angle relative to the event plane,Ф_(s)=|Ф_(t)-ψ_(Ep)|.The azimuthal correlation is studied as a function of both the trigger and associated particle pr.The subtractions of the combinatorial background and anisotropic flow,assuming Zero Yield At Minimum(ZYAM),are described.The correlation results are first discussed with subtraction of the even harmonic(elliptic and quadrangu-lar)flow backgrounds.The away-side correlation is strongly modifed,and the modification varies withФ_(s),with a double-peak structure for out-of-plane trigger particles.The near-side ridge(long range pseudo-rapidity△_(η)correla-tion)appears to drop with increasingФ_(s)while the jet-like component remains approximately constant.The correla-tion functions are further studied with the subtraction of odd harmonic triangular flow background arising from fluc-tuations.It is found that the triangular flow,while responsible for the majority of the amplitudes,is not sufficient to explain theφs-dependence of the ridge or the away-side double-peak structure.The dropping ridge withФ_(s)could be attributed to aФ_(s)-dependent lliptie anisotropy;however,the physics mechanism of the ridge remains an open ques-tion.Even with aФ_(s)-dependent elliptic flow,the away-side correlation structure is robust.These results,with extens-ive systematic studies of the dihadron correlations as a function ofФ_(s),trigger and associated particle pT,and the pseudo-rapidity range△_(η),should provide stringent inputs to help understand the underlying physics mechanisms of jet-medium interactions in high energy nuclear collisions.

Correlations of baryon and charge stopping in heavy ion collisions

Baryon numbers are theorized to be carried by valence quarks in the standard QCD picture of the baryon structure.Another theory proposed an alternative baryon number carrier,a non-perturbative Y-shaped configuration of the gluon field,called the baryon junction in the 1970s.However,neither of these theories has been verified experimentally.Recently,searching for the baryon junction by investigating the correlation of net-charge and net-baryon yields at midrapidity in heavy-ion collisions has been suggested.This paper presents studies of such correlations in collisions of various heavy ions from oxygen to uranium with the UrQMD Monte Carlo model.The UrQMD model implements valence quark transport as the primary means of charge and baryon stopping at midrapidity.Detailed studies are also conducted for isobaric ^(96)_(40)Zr+^(96)_(40)Zr and ^(96)_(44)Ru+^(96)_(44)Ru collisions.We found a universal trend of charge stopping with respect to baryon stopping and discovered that the charge stopping is always greater than the baryon stopping.This study provides a model baseline in valence quark transport for what is expected in net-charge and net-baryon yields at the midrapidity of relativistic heavy-ion collisions.

Revisiting the spin effects induced by thermal vorticity

We revisit the spin effects induced by thermal vorticity by calculating them directly from the spin-dependent distribution functions.For spin-1/2 particles,we provide the polarization up to the first order of thermal vorticity and compare it with the usual results calculated from the spin vector.For spin-1 particles,we show that all the non-diagonal elements vanish and there is no spin alignment up to the first order of thermal vortcity.We present the spin alignment at second-order contribution from thermal vorticity.We also show that the spin effects for both Dirac and vector particles receive an extra contribution when the spin direction is associated with the momentum of the particle.