Relativistic Heavy Ion Collider Data Research Articles

Evidence of Hexadecapole Deformation in Uranium-238 at the Relativistic Heavy Ion Collider.

State-of-the-art hydrodynamic simulations of the quark-gluon plasma are unable to reproduce the elliptic flow of particles observed at the BNL Relativistic Heavy Ion Collider (RHIC) in relativistic ^{238}U+^{238}U collisions when they rely on information obtained from low-energy experiments for the implementation of deformation in the colliding ^{238}U ions. We show that this is due to an inappropriate treatment of well-deformed nuclei in the modeling of the initial conditions of the quark-gluon plasma. Past studies have identified the deformation of the nuclear surface with that of the nuclear volume, though these are different concepts. In particular, a volume quadrupole moment can be generated by both a surface hexadecapole and a surface quadrupole moment. This feature was so far neglected in the modeling of heavy-ion collisions, and is particularly relevant for nuclei like ^{238}U, which is both quadrupole deformed and hexadecapole deformed. With rigorous input from Skyrme density functional calculations, we show that correcting for such effects in the implementation of nuclear deformations in hydrodynamic simulations restores agreement with BNL RHIC data. This brings consistency to the results of nuclear experiments across energy scales, and demonstrates the impact of the hexadecapole deformation of ^{238}U on high-energy collisions.

Determining the jet transport coefficient q̂ from inclusive hadron suppression measurements using Bayesian parameter estimation

We report a new determination of q̂, the jet transport coefficient of the quark-gluon plasma. We use the JETSCAPE framework, which incorporates a novel multistage theoretical approach to in-medium jet evolution and Bayesian inference for parameter extraction. The calculations, based on the Matter and Lbt jet quenching models, are compared to experimental measurements of inclusive hadron suppression in Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and Pb+Pb collisions at the CERN Large Hadron Collider (LHC). The correlation of experimental systematic uncertainties is accounted for in the parameter extraction. The functional dependence of q̂ on jet energy or virtuality and medium temperature is based on a perturbative picture of in-medium scattering, with components reflecting the different regimes of applicability of Matter and Lbt. In the multistage approach, the switch between Matter and Lbt is governed by a virtuality scale Q0. Comparison of the posterior model predictions to the RHIC and LHC hadron suppression data shows reasonable agreement, with moderate tension in limited regions of phase space. The distribution of q̂/T3 extracted from the posterior distributions exhibits weak dependence on jet momentum and medium temperature T, with 90% credible region (CR) depending on the specific choice of model configuration. The choice of Matter+Lbt, with switching at virtuality Q0, has 90% CR of 2<q̂/T3<4 for pT,jet>40 GeV/c. The value of Q0, determined here for the first time, is in the range 2.0–2.7 GeV.9 MoreReceived 23 February 2021Accepted 21 July 2021DOI:https://doi.org/10.1103/PhysRevC.104.024905©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHydrodynamic modelsJets & heavy flavor physicsParticle & resonance productionRelativistic heavy-ion collisionsTechniquesBayesian methodsNuclear Physics

W-boson production in polarized proton-proton collisions at RHIC through next-to-next-to-leading order in perturbative QCD

We perform a study of W-boson production in polarized proton-proton collisions through next-to-next-to-leading order (NNLO) in perturbative QCD. This calculation is required to extend the extraction of polarized parton distribution functions to NNLO accuracy. We present differential distributions at s=510 GeV, relevant for comparison to measurements from the Relativistic Heavy Ion Collider (RHIC). The NNLO QCD corrections significantly reduce the scale dependence of the cross section. We compare the longitudinal single-spin asymmetries as a function of lepton pseudorapidity to RHIC data. The asymmetries exhibit excellent stability under perturbative QCD corrections.

Simultaneous description of hadron and jet suppression in heavy-ion collisions

We present a global fit to all data on the suppression of high-energy jets and high-energy hadrons in the most central heavy-ion collisions at the CERN Large Hadron Collider (LHC) for two different collision energies, within a hybrid strong-weak coupling quenching model. Even though the measured suppression factors for hadrons and jets differ significantly from one another and appear to asymptote to different values in the high-energy limit, we obtain a simultaneous description of all these data after constraining the value of a single model parameter. We use our model to investigate the origin of the difference between the observed suppression of jets and hadrons and relate it, quantitatively, to the observed modification of the jet fragmentation function in jets that have been modified by passage through the medium produced in heavy-ion collisions. In particular, the observed increase in the fraction of hard fragments in medium-modified jets, which indicates that jets with the fewest hardest fragments lose the least energy, corresponds quantitatively to the observed difference between the suppression of hadrons and jets. We argue that a harder fragmentation pattern for jets with a given energy after quenching is a generic feature of any mechanism for the interaction between jets and the medium that they traverse that yields a larger suppression for wider jets. We also compare the results of our global fit to LHC data to measurements of the suppression of high-energy hadrons in BNL Relativistic Heavy Ion Collider (RHIC) collisions, and find that with its parameter chosen to fit the LHC data, our model is inconsistent with the RHIC data at the $3\ensuremath{\sigma}$ level, suggesting that hard probes interact more strongly with the less hot quark-gluon plasma produced at RHIC.

Evolving charge correlations in a hybrid model with both hydrodynamics and hadronic Boltzmann descriptions

Correlations related to local charge conservation provide insight into the creation and evolution of up, down and strange charges in the quark-gluon plasma. Here, the evolution of charge correlations is overlaid onto a hydrodynamic calculation for the regions where temperature exceed 155 MeV, then transferred and carried through a microscopic model of the hadronic stage. Thus, for the first time, charge correlations are evolved consistently with a full state-of-the-art description of a heavy-ion collision. The charge correlations are projected onto charge balance functions, which characterize such correlations in the final state, and are presented as a function of relative rapidity and relative azimuthal angle for Au/Au collisions. The role of the hadronic stage is investigated. Calculation of the contribution to charge-separation observables related to the chiral magnetic effect are also presented. Calculations are compared to data from the STAR Collaboration at RHIC (Relativistic Heavy Ion Collider) data when possible.

Measurements of double-helicity asymmetries in inclusiveJ/ψproduction in longitudinally polarizedp+pcollisions ats=510GeV

We report the double helicity asymmetry, $A_{LL}^{J/\psi}$, in inclusive $J/\psi$ production at forward rapidity as a function of transverse momentum $p_T$ and rapidity $|y|$. The data analyzed were taken during $\sqrt{s}=510$ GeV longitudinally polarized $p$$+$$p$ collisions at the Relativistic Heavy Ion Collider (RHIC) in the 2013 run using the PHENIX detector. At this collision energy, $J/\psi$ particles are predominantly produced through gluon-gluon scatterings, thus $A_{LL}^{J/\psi}$ is sensitive to the gluon polarization inside the proton. We measured $A_{LL}^{J/\psi}$ by detecting the decay daughter muon pairs $\mu^+ \mu^-$ within the PHENIX muon spectrometers in the rapidity range $1.2<|y|<2.2$. In this kinematic range, we measured the $A_{LL}^{J/\psi}$ to be $0.012 \pm 0.010$~(stat)~$\pm$~$0.003$(syst). The $A_{LL}^{J/\psi}$ can be expressed to be proportional to the product of the gluon polarization distributions at two distinct ranges of Bjorken $x$: one at moderate range $x \approx 0.05$ where recent RHIC data of jet and $\pi^0$ double helicity spin asymmetries have shown evidence for significant gluon polarization, and the other one covering the poorly known small-$x$ region $x \approx 2\times 10^{-3}$. Thus our new results could be used to further constrain the gluon polarization for $x< 0.05$.

Probing QCD phase structure using baryon multiplicity distribution

The canonical partition functions $Z_n$ and the number distributions $P_n$ which are obervable in experiments, are related by a single parameter, the fugacities $\xi=\exp(\mu/T)$. With the charge parity invariance, $Z_n$ and $\xi$ can be determined. Thermodynamic quantities such as the number density susceptibility and the kurtosis are then calculated from the grand canonical partition function $Z(\xi,T)=\sum Z_n(T) \xi^n$, ($n=-N_{\rm max},\cdots, N_{\rm max}$), for any chemical potential $\mu$, although the region over which the results are reliable for these quantities is constrained by $N_{\rm max}$. We then calculate the Lee-Yang zeros, which are the zeros of $Z(\xi)$ in the complex fugacity plane, as poles of $d\log Z(\xi)/d\xi$ by using the Cauchy integral theorem. With the help of a multiple precision library, this method provides any precision required without misidentification of the zeros. We analyse $Z_n$ from the net-proton number distributions recently measured at the Relativistic Heavy Ion Collider (RHIC) by assuming the net-proton number is approximately propotional to that of the baryon after the freeze-out, and calculate the moments. We also evaluate the Lee-Yang zero structures obtained from RHIC data and compare them with those obtained from lattice quantum chromodynamics (QCD) calculations. Possible regions of QCD phase transition lines are estimated from the thermodynamics quantities and the Lee-Yang zeros. We discuss how the limited $N_{\rm max}$ in both experimental and numerical studies affects the reliability of the thermodynamic results and Lee-Yang zeros.

Transverse Single-Spin Asymmetries in Proton-Proton Collisions Within Collinear Factorization

We provide a new analysis within collinear factorization of transverse single-spin asymmetries (TSSAs) in high transverse momentum charged and neutral pion production in pp collisions at the Relativistic Heavy Ion Collider (RHIC). This study incorporates the so-called twist-3 fragmentation term and shows that one can describe RHIC data through this mechanism. Moreover, by fixing other non-perturbative inputs through extractions of transverse momentum dependent functions in e+e- → h1h2X and semi-inclusive deep-inelastic scattering (SIDIS), we provide for the first time a consistency between certain spin/azimuthal asymmetries in all three reactions (i.e., pp, e+e-, and SIDIS).

ANin proton-proton collisions and the role of twist-3 fragmentation

We review and give an update on the current status of what causes transverse single-spin asymme- tries (TSSAs) in semi-inclusive processes where a single hadron is detected in the final state, especially those involving proton-proton (pp) collisions. In particular, we provide a new analysis within collinear factorization of TSSAs in high transverse momentum charged and neutral pion production in ppcollisions at the Relativistic Heavy Ion Collider (RHIC). This study incorporates the so-called twist-3 fragmentation term and shows that one can describe RHIC data through this mechanism. Moreover, by fixing other non-perturbative inputs through extractions of transverse momentum dependent functions in e + e − → h1h2X and semi-inclusive deep-inelastic scattering (SIDIS), we provide for the first time a consistency between certain spin/azimuthal asymmetries in all three reactions (i.e., pp, e + e − , and SIDIS).

A critical review of RHIC experimental results

The relativistic heavy-ion collider (RHIC) was constructed to achieve an asymptotic state of nuclear matter in heavy-ion collisions, a near-ideal gas of deconfined quarks and gluons denoted quark–gluon plasma or QGP. RHIC collisions are indeed very different from the hadronic processes observed at the Bevalac and AGS, but high-energy elementary-collision mechanisms are also non-hadronic. The two-component model (TCM) combines measured properties of elementary collisions with the Glauber eikonal model to provide an alternative asymptotic limit for A–A collisions. RHIC data have been interpreted to indicate formation of a strongly-coupled QGP (sQGP) or "perfect liquid". In this review, I consider the experimental evidence that seems to support such conclusions and alternative evidence that may conflict with those conclusions and suggest different interpretations.

Evidence for polarization of gluons in the proton.

We discuss the impact of recent high-statistics Relativistic Heavy Ion Collider data on the determination of the gluon polarization in the proton in the context of a global QCD analysis of polarized parton distributions. We find evidence for a nonvanishing polarization of gluons in the region of momentum fraction and at the scales mostly probed by the data. Although information from low momentum fractions is presently lacking, this finding is suggestive of a significant contribution of gluon spin to the proton spin, thereby limiting the amount of orbital angular momentum required to balance the proton spin budget.

Hard probe of geometry and fluctuations in heavy ion collisions atsNN=0.2, 2.76, and 5.5 TeV

Background: A significant quenching of high energy jets was observed in the heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) facility, and is now confirmed at the CERN Large Hadron Collider (LHC) facility. The RHIC plus LHC era provides a unique opportunity to study the jet-medium interaction that leads to the jet quenching, and the medium itself at different collision energies (medium temperatures).Purpose: We study the azimuthal anisotropy of jet quenching, to seek constraints on different models featuring distinct path-length and density dependences for jet energy loss, and to gain a better understanding of the medium.Methods: The models are fixed by using the RHIC data, and then applied to study the LHC case. A set of harmonic (Fourier) coefficients ${v}_{n}$ are extracted from the jet azimuthal anisotropy on a event-by-event basis.Results: The second harmonics ${v}_{2}$, mostly driven by the medium's geometry, can be used to differentiate jet quenching models. Other harmonics are also compared with the LHC (2.76 TeV) data. The predictions for future LHC (5.5 TeV) run are presented.Conclusions: We find that a too strong path-length dependence (e.g., cubic) is ruled out by the LHC ${v}_{2}$ data, while the model with a strong near-${T}_{c}$ enhancement for the jet-medium interaction describes the data very well. It is worth pointing out that the latter model expects a less color-opaque medium at LHC.

Challenging the utility of third-order azimuth harmonics in the description of ultrarelativistic heavy-ion collisions

In recent years it has become conventional practice to include higher-order cylindrical harmonics in the phenomenological description of two-particle angular correlations from ultra-relativistic heavy-ion collisions. These model elements, whose dependence on relative azimuth angle has the form $\cos[m(\phi_1-\phi_2)]$ where $m > 2$, were introduced to support a hydrodynamic flow interpretation of the same-side ($|\phi_1-\phi_2| < \pi/2$) 2D peak in the correlations. Previous studies have shown that the $m > 2$ harmonics are not required by the data, that they destabilize the fitting models, and that their net effect is to decompose the same-side peak into two components, one being dependent on and the other being independent of relative pseudorapidity. Thus we are lead to question whether descriptions of angular correlation data including higher-order harmonics inform our understanding of the same-side peak or heavy-ion collisions in general. Results from analysis of two-dimensional angular correlation data from the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC) show that the RHIC data do not exclude a single-Gaussian hypothesis for the same-side peak. We find that the net effect of including the $m = 3$ harmonic or azimuth sextupole in the fitting model is the inclusion of small non-Gaussian dependence in the mathematical description of the same-side peak. Those non-Gaussian effects are systematically insignificant and can be accommodated by minor perturbations to the same-side 2D Gaussian peak model, which act locally at small relative azimuth. We also demonstrate that the 0-1% 2D angular correlation data for 2.76 TeV Pb+Pb collisions from ATLAS, which display an away-side double peak on azimuth, do not require a sextupole and exclude a positive same-side sextupole.

Alternative scenarios of relativistic heavy-ion collisions. II. Particle production

Particle production in relativistic collisions of heavy nuclei is analyzed in a wide range of incident energies 2.7 GeV $\ensuremath{\le}\sqrt{{s}_{NN}}\ensuremath{\le}$ 62.4 GeV. The analysis is performed within the three-fluid model employing three different equations of state (EoSs): a purely hadronic EoS, an EoS with the first-order phase transition, and an EoS with a smooth crossover transition. It is found that the hadronic scenario fails to reproduce experimental yields of antibaryons (strange and nonstrange), starting already from lower Super Proton Synchrotron (SPS) energies, i.e., $\sqrt{{s}_{NN}}&gt;$ 5 GeV. Moreover, at energies above the top SPS one, i.e., $\sqrt{{s}_{NN}}&gt;$ 17.4 GeV, the midrapidity densities predicted by the hadronic scenario considerably exceed the available Relativistic Heavy-Ion Collider data on all species. At the same time the deconfinement-transition scenarios reasonably agree (to a various extent) with all the data. The present analysis demonstrates certain advantage of the deconfinement-transition EoSs. However, all scenarios fail to reproduce the strangeness enhancement in the incident energy range near 30$A$ GeV (i.e., a horn anomaly in the ${K}^{+}/{\ensuremath{\pi}}^{+}$ ratio) and yields of $\ensuremath{\phi}$ mesons at 20$A$--40$A$ GeV.

Time evolution of the luminosity of colliding heavy-ion beams in BNL Relativistic Heavy Ion Collider and CERN Large Hadron Collider

We have studied the time evolution of the heavy ion luminosity and bunch intensities in the Relativistic Heavy Ion Collider (RHIC), at BNL, and in the Large Hadron Collider (LHC), at CERN. First, we present measurements from a large number of RHIC stores (from Run 7), colliding 100 GeV/nucleon Au beams without stochastic cooling. These are compared with two different calculation methods. The first is a simulation based on multi-particle tracking taking into account collisions, intrabeam scattering, radiation damping, and synchrotron and betatron motion. In the second, faster, method, a system of ordinary differential equations with terms describing the corresponding effects on emittances and bunch populations is solved numerically. Results of the tracking method agree very well with the RHIC data. With the faster method, significant discrepancies are found since the losses of particles diffusing out of the RF bucket due to intrabeam scattering are not modeled accurately enough. Finally, we use both methods to make predictions of the time evolution of the future Pb beams in the LHC at injection and collision energy. For this machine, the two methods agree well.

Early anisotropic hydrodynamics and thermalization and Hanbury-Brown–Twiss puzzles in the BNL Relativistic Heavy Ion Collider (RHIC)

We address the problem if the early thermalization and HBT puzzles in relativistic heavy-ion collisions may be solved by the assumption that the early dynamics of the produced matter is locally anisotropic. The hybrid model describing the purely transverse hydrodynamic evolution followed by the perfect-fluid hydrodynamic stage is constructed. The transition from the transverse to perfect-fluid hydrodynamics is described by the Landau matching conditions applied at a fixed proper time. The global fit to the RHIC data reproduces the soft hadronic observables (the pion, kaon, and the proton spectra, the pion and kaon elliptic flow, and the pion HBT radii) with the accuracy of about 20%. These results indicate that the assumption of the very fast thermalization may be relaxed. In addition, the presented model suggests that a large part of the inconsistencies between the theoretical and experimental HBT results may be removed.

Baryon stopping and saturation physics in relativistic collisions

We investigate baryon transport in relativistic heavy-ion collisions at energies reached at the CERN Super Proton Synchrotron, BNL Relativistic Heavy-Ion Collider (RHIC), and CERN LHC in the model of saturation. An analytical scaling law is derived within the color glass condensate framework based on small-coupling QCD. Transverse momentum spectra, net-baryon rapidity distributions and their energy, mass and centrality dependences are well described. In a comparison with RHIC data in Au + Au collisions at sqrt (s_NN) = 62.4 GeV and 200 GeV, the gradual approach to the gluon saturation regime is investigated, and limits for the saturation-scale exponent are determined. Predictions for net-baryon rapidity spectra and the mean rapidity loss in central Pb + Pb collisions at LHC energies of sqrt (s_NN) = 5.52 TeV are made.

Soft Open Charm Production in Heavy-Ion Collisions

The effects of strong longitudinal color electric fields on the open charm production in nucleus-nucleus (A+A) collisions at 200A GeV are investigated within the framework of the HIJING/BB[over ] v2.0 model. A threefold increase of the effective string tension due to in-medium effects in A+A collisions results in a sizable ( approximately 60%-70%) enhancement of the total charm production cross sections (sigma (cc)(NN)). The nuclear modification factor shows a suppression at moderate transverse momentum (p(T)) consistent with BNL Relativistic Heavy Ion Collider data. At Large Hadron Collider energies the model predicts an increase of sigma (cc)(NN) by approximately an order of magnitude.

Baryon Stopping as a New Probe of Geometric Scaling

We suggest using net-baryon rapidity distributions in central relativistic heavy-ion collisions at energies reached at the CERN Super Proton Synchrotron, BNL Relativistic Heavy-Ion Collider (RHIC), and CERN LHC in order to probe saturation physics. Within the color glass condensate framework based on small-coupling QCD, net-baryon rapidity distributions are shown to exhibit geometric scaling. In a comparison with RHIC data in Au+Au collisions at sqrt[sNN=62.4 GeV and 200 GeV the gradual approach to the gluon saturation regime is investigated. Predictions for net-baryon rapidity spectra and the mean rapidity loss in central Pb+Pb collisions at LHC energies of sqrt[sNN=5.52 TeV are made.

The “perfect” fluid quenches jets almost perfectly

The QCD matter produced in nuclear collisions at the Relativistic Heavy Ion Collider (RHIC) has been found to have a very low shear viscosity, which is close to the lower bound allowed by unitarity. The matter has also been found to strongly suppress the emission of energetic hadrons. This phenomenon, called “jet quenching” is interpreted to be the result of a large energy loss by the precursor parton on its path through the dense matter, primarily due to gluon radiation. I discuss how the two phenomena are related. The RHIC data suggest, in some scenarios of jet quenching, that the quark–gluon plasma created in nuclear collisions is characterized by strong coupling, but still admits a quasi-particle description.