Isobaric Collisions Research Articles

Energy dependence of heavy-ion initial condition in isobar collisions

Collisions of isobar nuclei, those with the same mass number but different structure parameters, provide a new way to probe the initial condition of the heavy ion collisions. Using transport model simulation of 96Ru+96Ru and 96Zr+96Zr collisions at two energies sNN=0.2 TeV and 5.02 TeV, where 96Ru and 96Zr nuclei have significantly different deformations and radial profiles, we identify sources of eccentricities contributing independently to the final state harmonic flow vn. The efficacy for flow generation differs among these sources, and explains the modest energy dependence of the isobar ratios of vn. Additionally, a significant component of vn is found to be uncorrelated with the eccentricity, but is instead generated dynamically during system evoluation. Experimental measurement of these ratios at the LHC energy and comparison with RHIC energy can provide insight into the collision-energy dependence of the initial condition.

Unveiling Baryon Charge Carriers through Charge Stopping in Isobar Collisions

Unveiling Baryon Charge Carriers through Charge Stopping in Isobar Collisions

Hard probes in isobar collisions as a probe of the neutron skin

We present an estimate of the yield of hard probes expected for collisions of the isobars 4496Ru and 4096Zr at collision energies reachable at RHIC and the LHC. These yields are proportional to the number of binary nucleon-nucleon interactions, which is characteristically different due to the presence of the large neutron skin in 4096Zr. This provides an independent opportunity to measure the difference between the neutron skin of 4496Ru and 4096Zr, which can provide an important constraint on the Equation of State of cold neutron-rich matter.

Exploring rapidity-even dipolar flow in isobaric collisions at RHIC

Employing the AMPT transport model, we investigate the response of the rapidity-even dipolar flow ( v1even ) and its associated Global Momentum Conservation (GMC) parameter K to structural disparities within 96Ru and 96Zr nuclei. We analyze Ru + Ru and Zr + Zr collisions at a center-of-mass energy of sNN = 200 GeV. Our analysis demonstrates that the eccentricity ε 1, v1even and K exhibit subtle yet discernible sensitivity to the input nuclear structure distinctions between 96Ru and 96Zr isobars. This observation suggests that measuring v1even and the GMC parameter in these isobaric collisions could serve as a means to fine-tune the comprehension of their nuclear structure disparities and offer insights to enhance the initial condition assumptions of theoretical models.

Upper limit on the chiral magnetic effect in isobar collisions at the Relativistic Heavy-Ion Collider

The chiral magnetic effect (CME) is a phenomenon that arises from the QCD anomaly in the presence of an external magnetic field. The experimental search for its evidence has been one of the key goals of the physics program of the Relativistic Heavy-Ion Collider. The STAR Collaboration has previously presented the results of a blind analysis of isobar collisions (Ru4496+Ru4496, Zr4096+Zr4096) in the search for the CME. The isobar ratio (Y) of CME-sensitive observable, charge separation scaled by elliptic anisotropy, is close to but systematically larger than the inverse multiplicity ratio, the naive background baseline. This indicates the potential existence of a CME signal and the presence of remaining nonflow background due to two- and three-particle correlations, which are different between the isobars. In this postblind analysis, we estimate the contributions from those nonflow correlations as a background baseline to Y, utilizing the isobar data as well as Heavy Ion Jet Interaction Generator simulations. This baseline is found consistent with the isobar ratio measurement, and an upper limit of 10% at 95% confidence level is extracted for the CME fraction in the charge separation measurement in isobar collisions at sNN=200 GeV. Published by the American Physical Society 2024

Search for baryon junctions in photonuclear processes and isobar collisions at RHIC

During the early development of quantum chromodynamics, it was proposed that baryon number could be carried by a non-perturbative Y-shaped topology of gluon fields, called the gluon junction, rather than by the valence quarks as in the QCD standard model. A puzzling feature of ultra-relativistic nucleus-nucleus collisions is the apparent substantial baryon excess in the mid-rapidity region that could not be adequately accounted for in most conventional models of quark and diquark transport. The transport of baryonic gluon junctions is predicted to lead to a characteristic exponential distribution of net-baryon density with rapidity and could resolve the puzzle. In this context we point out that the rapidity density of net-baryons near mid-rapidity indeed follows an exponential distribution with a slope of -0.61±0.03\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$-0.61\\pm 0.03$$\\end{document} as a function of beam rapidity in the existing global data from A+A collisions at AGS, SPS and RHIC energies. To further test if quarks or gluon junctions carry the baryon quantum number, we propose to study the absolute magnitude of the baryon vs. charge stopping in isobar collisions at RHIC. We also argue that semi-inclusive photon-induced processes (γ+p\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gamma +p$$\\end{document}/A) at RHIC kinematics provide an opportunity to search for the signatures of the baryon junction and to shed light onto the mechanisms of observed baryon excess in the mid-rapidity region in ultra-relativistic nucleus-nucleus collisions. Such measurements can be further validated in A+A collisions at the LHC and e+p\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$e+p$$\\end{document}/A collisions at the EIC.

Impact of initial fluctuations and nuclear deformations in isobar collisions

Impact of initial fluctuations and nuclear deformations in isobar collisions

Effect of electric and chiral magnetic conductivities on azimuthally fluctuating electromagnetic fields and observables in isobar collisions

Effect of electric and chiral magnetic conductivities on azimuthally fluctuating electromagnetic fields and observables in isobar collisions

Characterizing the initial- and final-state effects in isobaric collisions at energies available at the BNL Relativistic Heavy Ion Collider

Characterizing the initial- and final-state effects in isobaric collisions at energies available at the BNL Relativistic Heavy Ion Collider

Hyperon polarization in heavy ion collisions at STAR

In these proceedings, we present the measurements of global polarization for Λ, ¯Λ with the high-statistics data collected by the STAR experiment for isobar (Ru+Ru, Zr+Zr) collisions at √SNN = 200 GeV and Au+Au collisions at √SNN = 19.6, 27 GeV. These measurements allow us to study possible magnetic field driven effects through the polarization difference between Λ and ¯Λ and system size dependence of global polarization. Furthermore, we present the first measurements of Λ, ¯Λ hyperon local polarization in isobar collisions at √SNN = 200 GeV and Au+Au collisions at √SNN = 19.6, 27 GeV. Comparisons with previous measurements in Au+Au and Pb+Pb collisions at RHIC and LHC provide important insights into the collision system size and energy dependence of the vorticities. The local polarization measurements at lower beam energies can probe the predicted baryonic spin hall effect in a dense baryonic environment in heavy-ion collisions.

Tracing baryon and electric charge transport in isobar collisions

It is of fundamental interest to understand the carrier of conserved quantum charges within protons and nuclei at high energy. Preliminary data from isobar collisions at RHIC reveal a scaled net-baryon to net-electric charge ratio (B/ΔQ × ΔZ/A) at midrapidity between 1.2 and 2, consistent with string junction model predictions. Here, we compute the initial stage scaled net-baryon to net-electric charge ratio for isobar collisions. Our model incorporates a realization of the string junction model and models the nuclear structure. Our predictions identify the baseline expectations for such measurement and quantify the impact of the nuclear structure.

Measurements of charm quark interaction with the QGP in heavy-ion collisions at STAR

We report the measurements of D0 transverse momentum (pT) differential invariant spectra with pT < 8 GeV/c for 0–10%, 10–40%, and 40–80% centrality classes in isobar collisions (96Ru + 96Ru and 96Zr + 96Zr) at √SNN = 200 GeV with the STAR experiment. The strong suppression of D0 yield is observed for pT > 3 GeV/c in the central collisions, demonstrating that charm quarks suffer significant energy loss in the bulk QCD medium. In this proceeding, the measurements of D0 meson tagged jets at √SNN = 200 GeV in Au+Au collisions are reported as well. We present the nuclear modification factor, RCP, as a function of pT,Jet and zJet, and show the ratios of the D0 radial profile with 1<pT,D0 < 10 GeV/c. Comparisons to model calculations for D0 and D0-tagged jets are also discussed.

Background control and upper limit on the chiral magnetic effect in isobar collisions at √SNN = 200 GeV from STAR

The STAR Collaboration has reported results from a blind analysis of isobar collisions (4496Ru + 4496Ru , 4096Zr + 4096Zr) at √SNN = 200 GeV on the search for the chiral magnetic effect (CME). Significant differences were observed in the measured multiplicity (N) and elliptic anisotropy (v2) between the two isobar systems. In these proceedings, we present two post-blind analyses aimed at mitigating remaining background effects. The first involves employing an event weighting procedure to match the distributions in N and v2 and then comparing the CME-sensitive Δγ correlator and signed balance functions. The second analysis investigates the contributions of the two- and three-particle nonflow to the isobar ratio of Δγ/v2. The estimated background baseline is consistent with the isobar measurements, and an upper limit is extracted on the CME signal.

New insights into global spin alignment of vector mesons using relativistic heavy-ion collisions from STAR

In these proceedings, we present new measurements from STAR of the global spin alignment(ρ00) of ϕ mesons in Au+Au collisions at √SNN = 19.6, 14.6 GeV using BES-II data and of J/ψ mesons in isobar collisions 4496Ru + 4496Ru and 4096Zr + 4096Zr at √SNN = 200 GeV. The energy-dependent spin alignment for ϕ mesons from BES-II data is consistent with that published from BES-I and showcases a significant improvement in precision. We performed a new measurement assessing the rapidity dependence of ϕ meson ρ00, and our findings are consistent with theoretical predictions. The global spin alignment of J/ψ mesons with respect to first order event plane aligns with a value of 1/3 within the statistical error. Additionally, we discuss the procedure of measuring the global spin alignment of ρ0 mesons at RHIC and provide the projection for errors.

Hyperon Polarization along the Beam Direction Relative to the Second and Third Harmonic Event Planes in Isobar Collisions at sqrt[s_{NN}]=200 GeV.

The polarization of Λ and Λ[over ¯] hyperons along the beam direction has been measured relative to the second and third harmonic event planes in isobar Ru+Ru and Zr+Zr collisions at sqrt[s_{NN}]=200 GeV. This is the first experimental evidence of the hyperon polarization by the triangular flow originating from the initial density fluctuations. The amplitudes of the sine modulation for the second and third harmonic results are comparable in magnitude, increase from central to peripheral collisions, and show a mild p_{T} dependence. The azimuthal angle dependence of the polarization follows the vorticity pattern expected due to elliptic and triangular anisotropic flow, and qualitatively disagrees with most hydrodynamic model calculations based on thermal vorticity and shear induced contributions. The model results based on one of existing implementations of the shear contribution lead to a correct azimuthal angle dependence, but predict centrality and p_{T} dependence that still disagree with experimental measurements. Thus, our results provide stringent constraints on the thermal vorticity and shear-induced contributions to hyperon polarization. Comparison to previous measurements at RHIC and the LHC for the second-order harmonic results shows little dependence on the collision system size and collision energy.

Production of neutron-rich actinide isotopes in isobaric collisions via multinucleon transfer reactions

Production of neutron-rich actinide isotopes in isobaric collisions via multinucleon transfer reactions

Impact of nuclear structure on longitudinal flow decorrelations in high-energy isobar collisions

Fluctuations of harmonic flow along pseudorapidity, known as flow decorrelations, are an important probe of the initial state geometry of the quark-gluon plasma. The flow decorrelations are shown to be sensitive to the collective structure of the colliding nuclei, as revealed clearly by comparing 96Ru+96Ru collisions and 96Zr+96Zr collisions, which have different nuclear structures. The flow decorrelations in central collisions are mostly sensitive to nuclear deformations, while those in the mid-central collisions are primarily sensitive to differences in skin thickness between 96Ru and 96Zr. Longitudinal flow decorrelations in heavy-ion collisions are a new tool to probe the structure of colliding nuclei.

Inferring nuclear structure from heavy isobar collisions using Trajectum

Nuclei with equal number of baryons but varying proton number (isobars) have many commonalities, but differ in both electric charge and nuclear structure. Relativistic collisions of such isobars provide unique opportunities to study the variation of the magnetic field, provided the nuclear structure is well understood. In this work we simulate collisions using several state-of-the-art parametrizations of the ^{96}_{40}4096Zr and ^{96}_{44}4496Ru isobars and show that a comparison with the exciting STAR measurement [arXiv:2109.00131] of ultrarelativistic collisions can uniquely identify the structure of both isobars. This not only provides an urgently needed understanding of the structure of the Zirconium and Ruthenium isobars, but also paves the way for more detailed studies of nuclear structure using relativistic heavy ion collisions.

Probing nuclear structure with mean transverse momentum in relativistic isobar collisions

Probing nuclear structure with mean transverse momentum in relativistic isobar collisions

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.