State In Heavy Ion Collisions Research Articles

Elliptic and triangular flow of charmonium states in heavy ion collisions

Elliptic and triangular flow of charmonium states in heavy ion collisions

Examining the influence of hadronic interactions on the directed flow of identified particles in RHIC Beam Energy Scan energies using UrQMD model

The directed flow of identified particles can serve as a sensitive tool for investigating the interactions during initial and final states in heavy ion collisions. This study examines the rapidity distribution ([Formula: see text]), rapidity-odd directed flow ([Formula: see text]) and its slope ([Formula: see text]) for [Formula: see text], [Formula: see text], p, and [Formula: see text] in Au+Au collisions at different collision centralities and beam energies ([Formula: see text], 11.5, 14.5, 19.6, 27, and 39[Formula: see text]GeV) using the UrQMD model. We investigate the impact of late-stage hadronic interactions on charge-dependent [Formula: see text] and its slope by modifying the duration of the hadronic cascade lifetime ([Formula: see text]). The energy dependence of [Formula: see text] for p ([Formula: see text]) exhibits distinct pattern compared to [Formula: see text] and [Formula: see text]. Notably, we observe a change in the sign reversal position of proton [Formula: see text] at different beam energies with varying [Formula: see text] in central and mid-central collisions. Moreover, the difference in [Formula: see text] between positively and negatively charged hadrons ([Formula: see text]) demonstrates a stark centrality dependence for different particle species. The deuteron displays a significant increase in [Formula: see text] with increasing [Formula: see text] compared to p and n. This investigation underscores the importance of considering the temporal evolution and duration of the hadronic phase when interpreting the sign reversal, charge splitting of [Formula: see text] and light nuclei formation at lower RHIC energies.

Dissociation rates and recombination likelihood of bottomonium states in heavy-ion collisions at sNN=5.02 TeV

In the medium of relativistic heavy-ion collisions, dissociation of the quarkonium and its survival have been studied to understand the properties of Quark Gluon Plasma (QGP). The coupled rates of dissociation and recombination reactions in QGP are commonly solved with the Boltzmann transport equation in which the formation and dissociation reactions compete with each other. Since the dissociation of newly formed bound-states is not accounted in the Boltzmann equation, a framework of decoupled rates is developed to assess the combined effect of gluon-induced dissociation and recombination (though it is small for [Formula: see text]) together with color screening on bottomonium production in heavy-ion collisions at center of mass energy [Formula: see text][Formula: see text]TeV. To calculate the recombination rates, we have employed an effective method of Bateman solution which ensures the correlated effect between the recombination and the dissociation of the newly combined bottomonium in the QGP medium. The modifications of bottomonium have been estimated in an inflating QGP with the constraints matching with the dynamics of [Formula: see text] collision events at LHC.

Signals of α clusters in 16O+16O collisions at the LHC from relativistic hydrodynamic simulations* *Supported in part by the National Natural Science Foundation of China (12075098, 12147101, 11875066, 11861131009), and Computations Were Performed at the Nuclear Science Computer Center at the CCNU (NSC3).

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, CLVisc, 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 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.

The high-density equation of state in heavy-ion collisions: constraints from proton flow

A set of different equations of state is implemented in the molecular dynamics part of a non-equilibrium transport simulation (UrQMD) of heavy-ion collisions. It is shown how different flow observables are affected by the density dependence of the equation of state. In particular, the effects of a phase transition at high density are explored, including an expected reduction in mean m_T. We also show that an increase in v_2 is characteristic for a strong softening of the equation of state. The phase transitions with a low coexistence density, n_{text {CE}}<4 n_0, show a distinct minimum in the slope of the directed flow as a function of the beam energy, which would be a clear experimental signal. By comparing our results with experimental data, we can exclude any strong phase transition at densities below 4n_0.

Collective dynamics of heavy ion collisions in ATLAS

The latest measurements of collective behaviour in a variety of collision systems with the ATLAS detector at the LHC, including collisions at 13 TeV, collisions at 5.44 TeV, and \mbox{Pb+Pb}Pb+Pb collisions at 5.02 TeV, are presented. They include v_nvn-[p_{T}pT] correlations, which carry important information about the initial-state geometry of the quark-gluon plasma and can shed light on any quadrupole deformation in the Xe nucleus, and measurements of flow decorrelations differential in rapidity, which probe the longitudinal structure of the colliding system. These measurements furthermore provide stringent tests of the theoretical understanding of the initial state in heavy ion collisions.

Jet shape and redistribution of the lost energy from jets in Pb\xa0+\xa0Pb collisions at the LHC in a multiphase transport model

Jet-medium interaction involves two important effects: jet energy loss and medium response. The search for jet-induced medium excitations is one of the hot topics in jet quenching study in relativistic nuclear collisions. In this work, we perform a systematic study on how the lost energy from hard jets evolves with the bulk medium and redistributes in the final state of heavy-ion collisions via a multi-phase transport model. In particular, the (Delta eta , Delta phi ) distribution of charged particles with respect to the jet axis and jet shape function are studied for various Pb + Pb collision centralities and for different transverse momentum intervals of charged particles. Our numerical result shows a strong enhancement of soft particles at large angles for Pb + Pb collisions relative to p + p collisions at the LHC, qualitatively consistent with recent CMS data. This indicates that a significant fraction of the lost energy from hard jets is carried by soft particles at large angles away from the jet axis.

Ratio of photon anisotropic flow in relativistic heavy ion collisions

The elliptic and triangular flow parameters of direct photons are known to be dominated by thermal radiations. The nonthermal contributions dilute the photon anisotropic flow by adding extra weight factor in the ${v}_{n}$ calculation. The discrepancy between experimental photon anisotropic flow data and results from theoretical model calculations is not well understood even after significant developments in the model calculations as well as in the experimental analysis methods. We show that the ratio of photon ${v}_{n}$ can be a potential observable in this regard by minimizing the nonthermal contributions. We calculate the ${v}_{2}/{v}_{3}$ of photons as a function of ${p}_{T}$ from heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and compare the results with available experimental data. The ratio is found to be larger for peripheral collisions than for central collisions. However, the ${p}_{T}$ dependent behavior of the ratio is found to be different than the individual flow parameters. The ratio is found to be sensitive to the initial conditions of the model calculation at different ${p}_{T}$ regions compared to the individual anisotropic flow parameters. We show that the photon ${v}_{1}/{v}_{2}$ and ${v}_{1}/{v}_{3}$ along with the ${v}_{2}/{v}_{3}$ results may help us constrain the initial conditions in heavy ion collisions.

Thermal production of sexaquarks in heavy-ion collisions

We present new results on the thermal production yield of a hypothetical state made of six quarks [Formula: see text] assuming its production in heavy-ion collisions at the CERN Large Hadron Collider (LHC). A state with this quark content and mass low enough to be stable against decay in timescales of the order of the age of the Universe has been hypothesized by one of us (G. Farrar) and has been discussed as a possible dark matter candidate. In this work, we address for the first time the thermal production rate that can be expected for this state in heavy-ion collisions at colliders. For this estimate we use a thermal model which has been shown to describe accurately the production of hadrons and nuclei in heavy-ion collisions at LHC energy. This estimate is of great relevance for sexaquark searches at colliders as well as for its consideration as a dark matter candidate and for the composition of neutron stars.

Model investigations of the correlation between the mean transverse momentum and anisotropic flow in shape-engineered events

The correlation between the event mean-transverse momentum [pT], and the anisotropic flow magnitude vn, ρ(vn2,[pT]), has been argued to be sensitive to the initial conditions in heavy-ion collisions. We use simulated events generated with the AMPT and EPOS models for Au+Au at sNN = 200 GeV, to investigate the model dependence and the response and sensitivity of the ρ(v22,[pT]) correlator to collision-system size and shape, and the viscosity of the matter produced in the collisions. We find good qualitative agreement between the correlators for the string melting version of the AMPT model and the EPOS model. The model investigations for shape-engineered events as well as events with different viscosity (η/s), indicate that ρ(v22,[pT]) is sensitive to the initial-state geometry of the collision system but is insensitive to sizable changes in η/s for the medium produced in the collisions. These findings suggest that precise differential measurements of ρ(v22,[pT]) as a function of system size, shape, and beam-energy could provide more stringent constraints to discern between initial-state models and hence, more reliable extractions of η/s.

XJ(2900) states in a hot hadronic medium

In this work we investigate the hadronic effects on the $X_{J=0,1} (2900)$ states in heavy-ion collisions. We make use of Effective Lagrangians to estimate the cross sections and their thermal averages of the processes $X_J \pi \to \bar{D}^{*} K , K^{*} \bar{D} $, as well as those of the corresponding inverse processes, considering also the possibility of different isospin assignments ($I=0,1$). We complete the analysis by solving the rate equation to follow the time evolution of the $X_J (2900)$ multiplicities and determine how they are affected by the considered reactions during the expansion of the hadronic matter. We also perform a comparison of the $X_J (2900)$ abundances considering them as hadronic molecular states ($J=0$ as a $S$-wave and $J=1$ as a $P$-wave) and tetraquark states at kinetic freeze-out.

Gluonic hot spot initial conditions in heavy-ion collisions

The initial conditions in heavy-ion collisions are calculated in many different frameworks. The importance of nucleon position fluctuations within the nucleus and sub-nucleon structure has been established when modeling initial conditions for input to hydrodynamic calculations. However, there remain outstanding puzzles regarding these initial conditions, including the measurement of the near equivalence of the elliptical $v_{2}$ and triangular $v_{3}$ flow coefficients in ultra-central 0-1% Pb+Pb collisions at the LHC. Recently a calculation termed MAGMA incorporating gluonic hot spots via two-point correlators in the Color Glass Condensate framework, and no nucleons, provided a simultaneous match to these flow coefficients measured by the ATLAS experiment, including in ultra-central 0-1% collisions. Our calculations reveal that the MAGMA initial conditions do not describe the experimental data when run through full hydrodynamic SONIC simulations or when the hot spots from one nucleus resolve hot spots from the other nucleus, as predicted in the Color Glass Condensate framework. We also explore alternative initial condition calculations and discuss their implications.

Exploring Longitudinal Observables with 3+1D IP-Glasma

We present a formulation of the initial state of heavy ion collisions that generalizes the 2+1D boost invariant IP-Glasma [B. Schenke, P. Tribedy, R. Venugopalan, Fluctuating Glasma initial conditions and flow in heavy ion collisions, Phys. Rev. Lett. 108 (2012) 252301.] to 3+1D through JIMWLK rapidity evolution of the pre-collision Wilson lines. The rapidity dependence introduced by the JIMWLK evolution leads us to modify the initial condition for the gauge fields, and to solve Gauss' law iteratively in order to allow for temporal evolution on a 3-dimensional lattice.While the transverse physics of QGP has been studied nearly exhaustively, the effect of longitudinal fluctuations introduced by the JIMWLK evolution has yet to be studied in detail phenomenologically. Hence, we couple our 3+1D IP-Glasma model to MUSIC+UrQMD, for completely 3+1D simulations of heavy ion collisions. Specifically, we consider Pb-Pb collisions at s=2.76TeV and study the rapidity dependence of the charged hadron νn(η) via the η-dependent flow factorization ratios rn(ηa,ηb) as measured by CMS [V. Khachatryan, et al., Evidence for transverse momentum and pseudorapidity dependent event plane fluctuations in PbPb and pPb collisions, Phys. Rev. C 92 (3) (2015) 034911.], as well as the charged hadron multiplicity dNch/dη.

Azimuthal anisotropy and nuclear modification of Upsilon states in heavy-ion collisions with the CMS detector

Bottomonia are produced by hard scattering in the early times of a relativistic heavy-ion collision, so they serve as excellent probes of the quark-gluon plasma (QGP). Early CMS data showed that the yields of the ϒ(1S), ϒ(2S), and ϒ(3S) mesons are suppressed in PbPb relative to those in pp. In order to interpret the results in PbPb collision unambiguously, the cold nuclear matter effects need to be quantitatively estimated using pPb collisions data. Additionally, the measurement of the elliptic azimuthal anisotropy of bottomonium states have been suggested as a powerful tool to study the different in-medium effects such as dissociation and regeneration. In these proceedings, the new bottomonium results are reported for pPb and PbPb collisions data at 5.02 TeV with the CMS detector. First, the nuclear modification factors of the ϒ(1S), ϒ(2S), and ϒ(3S) mesons are presented in pPb collisions as functions of the transverse momentum and rapidity. Then, the new measurements of the azimuthal anisotropy (ν2) of the ϒ(1S) and ϒ(2S) mesons are reported using PbPb collisions data at 5.02 TeV taken in 2018.

Sea Quark Chemistry and Correlations in the Initial Conditions of Heavy Ion Collisions

QCD dynamics are directly studied in two different but intersecting contexts: high-temperature QCD in heavy-ion collisions and zero-temperature hadronic structure in electron-hadron collisions. The approaches to high-energy nuclear physics in these two contexts are very different, however, and bringing the two fields into dialogue can reveal new opportunities. In this paper I highlight one interesting intersection between the two: leveraging the sea quark chemistry of the proton to study charge transport in top-energy heavy-ion collisions.

The Nuclear Equation of State in Heavy Ion Collisions

We present several possibilities offered by the dynamics of intermediate energy heavy ion collisions to investigate the nuclear matter equation of state (EoS) beyond the ground state. In particular the relation between the reaction dynamics and the high density nuclear EoS is discussed by comparing theoretical results with experiments.

Correlations in the Initial Conditions of Heavy-Ion Collisions

Ultracentral collisions of heavy nuclei, in which the impact parameter is nearly zero, are especially sensitive to the details of the initial state model and the microscopic mechanism for collective flow. In a hydrodynamic “flow” picture, the final state momentum correlations are a direct response to the fluctuating initial geometry, although models of the initial geometry differ widely. Alternatively, dynamical mechanisms based in the color glass condensate (CGC) formalism can naturally lead to many-body correlations with very different systematics. Here we present a calculation of event-by-event elliptic flow in both the hydrodynamic and CGC paradigms and show that they can be qualitatively distinguished in ultracentral collisions of deformed nuclei. Specifically, the multiplicity dependence in such collisions is qualitatively opposite, with the CGC correlations increasing with multiplicity while the hydrodynamic correlations decrease. The consistency of the latter with experimental data on UU collisions appears to rule out a CGC-mediated explanation. We find that these qualitative features also persist in small deformed systems and can therefore be a valuable test of the microscopic physics in that regime. The authors acknowledge support from the US-DOE Nuclear Science Grant No. DE-SC0019175, and the Alfred P. Sloan Foundation, and the Zuckerman STEM Leadership Program.

Second to tenth order susceptibilities of conserved charges within a modified Nambu-Jona-Lasinio model * * Supported in part by the MoST of China 973-Project (2015CB856901), the National Natural Science Foundation of China (11475085, 11535005, 11690030, 11575069, 11221504, 11890711, 11861131009) and the Fundamental Research Funds for the Central Universities (020414380074, CCNU19QN054)

We discuss the sign and energy dependence of second to tenth order susceptibilities of the baryon number, charge number, and strangeness for the analysis of critical conditions in heavy ion collisions in the LHC and RHIC by applying a modified Nambu-Jona-Lasinio model. This model is fitted to the quark condensate of the lattice QCD result at finite temperature and zero baryon chemical potential. The presence of a critical point made these susceptibilities deviate considerably from a Hadron-Resonance-Gas model that shows no criticality. The sign, magnitude, and energy dependence of these higher order fluctuations hint towards the existence and location of a critical point that could be discovered in future heavy ion collision experiments.

Effective kinetic description of event-by-event pre-equilibrium dynamics in high-energy heavy-ion collisions

We develop a macroscopic description of the space-time evolution of the energy-momentum tensor during the pre-equilibrium stage of a high-energy heavy-ion collision. Based on a weak coupling effective kinetic description of the microscopic equilibration process (\`a la "bottom-up"), we calculate the non-equilibrium evolution of the local background energy-momentum tensor as well as the non-equilibrium linear response to transverse energy and momentum perturbations for realistic boost-invariant initial conditions for heavy ion collisions. We demonstrate how this framework can be used on an event-by-event basis to propagate the energy momentum tensor from far-from-equilibrium initial state models, e.g. IP-Glasma, to the time $\tau_\text{hydro}$ when the system is well described by relativistic viscous hydrodynamics. The subsequent hydrodynamic evolution becomes essentially independent of the hydrodynamic initialization time $\tau_\text{hydro}$ as long as $\tau_\text{hydro}$ is chosen in an appropriate range where both kinetic and hydrodynamic descriptions overlap. We find that for $\sqrt{s_{NN}}=2.76\,\text{TeV}$ central Pb-Pb collisions, the typical time scale when viscous hydrodynamics with shear viscosity over entropy ratio $\eta/s=0.16$ becomes applicable is $\tau_\text{hydro}\sim 1\,\text{fm/c}$ after the collision.

Measurement of nuclear modification factors of ϒ(1S), ϒ(2S), and ϒ(3S) mesons in PbPb collisions at [formula omitted] TeV

The cross sections for ϒ(1S), ϒ(2S), and ϒ(3S) production in lead–lead (PbPb) and proton–proton (pp) collisions at sNN=5.02 TeV have been measured using the CMS detector at the LHC. The nuclear modification factors, RAA, derived from the PbPb-to-pp ratio of yields for each state, are studied as functions of meson rapidity and transverse momentum, as well as PbPb collision centrality. The yields of all three states are found to be significantly suppressed, and compatible with a sequential ordering of the suppression, RAA(ϒ(1S))>RAA(ϒ(2S))>RAA(ϒ(3S)). The suppression of ϒ(1S) is larger than that seen at sNN=2.76TeV, although the two are compatible within uncertainties. The upper limit on the RAA of ϒ(3S) integrated over pT, rapidity and centrality is 0.096 at 95% confidence level, which is the strongest suppression observed for a quarkonium state in heavy ion collisions to date.