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At the origin of the Universe, an asymmetry between the amount of created matter and antimatter led to the matter-dominated Universe as we know it today. The origins of this asymmetry remain unknown so far. High-energy nuclear collisions create conditions similar to the Universe microseconds after the Big Bang, with comparable amounts of matter and antimatter16. Much of the created antimatter escapes the rapidly expanding fireball without annihilating, making such collisions an effective experimental tool to create heavy antimatter nuclear objects and to study their properties7-14, hoping to shed some light on the existing questions on the asymmetry between matter and antimatter. Here we report the observation of the antimatter hypernucleus 4H, composed of a Λ, an antiproton and two antineutrons. The discovery was made through its two-body decay after production in ultrarelativistic heavy-ion collisions by the STAR experiment at the Relativistic Heavy Ion Collider15,16. In total, 15.6 candidate 4H antimatter hypernuclei are obtained with an estimated background count of 6.4. The lifetimes of the antihypernuclei 3H and 4H are measured and compared with the lifetimes of their corresponding hypernuclei, testing the symmetry between matter and antimatter. Various production yield ratios among (anti)hypernuclei (hypernuclei and/or antihypernuclei) and (anti)nuclei (nuclei and/or antinuclei) are also measured and compared with theoretical model predictions, shedding light on their production mechanisms.

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 (\\({^{96}_{44}\\text{Ru}}+{^{96}_{44}\\text{Ru}}\\), \\({^{96}_{40}\\text{Zr}}+{^{96}_{40}\\text{Zr}}\\)) 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 post-blind 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 \\(\\sqrt{s_{\\rm NN}}=200\\) GeV.

We report the first measurements of cumulants, up to \\(4^{th}\\) order, of deuteron number distributions and proton-deuteron correlations in Au+Au collisions recorded by the STAR experiment in phase-I of Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider. Deuteron cumulants, their ratios, and proton-deuteron mixed cumulants are presented for different collision centralities covering a range of center-of-mass energy per nucleon pair \\(\\sqrt{s_{NN}}\\)~=~7.7 to 200~GeV. It is found that the cumulant ratios at lower collision energies favor a canonical ensemble over a grand canonical ensemble in thermal models. An anti-correlation between proton and deuteron multiplicity is observed across all collision energies and centralities, consistent with the expectation from global baryon number conservation. The UrQMD model coupled with a phase-space coalescence mechanism qualitatively reproduces the collision-energy dependence of cumulant ratios and proton-deuteron correlations.

Flow coefficients (\\(v_2\\) and \\(v_3\\)) are measured in high-multiplicity \\(p\\)+Au, \\(d\\)+Au, and \\(^{3}\\)He\\(+\\)Au collisions at a center-of-mass energy of \\(\\sqrt{s_{_{\\mathrm{NN}}}\\) = 200 GeV using the STAR detector. The measurements utilize two-particle correlations with a pseudorapidity requirement of \\(|\\eta| <\\) 0.9 and a pair gap of \\(|\\Delta\\eta|>1.0\\). The primary focus is on analysis methods, particularly the subtraction of non-flow contributions. Four established non-flow subtraction methods are applied to determine \\(v_n\\), validated using the HIJING event generator. \\(v_n\\) values are compared across the three collision systems at similar multiplicities; this comparison cancels the final state effects and isolates the impact of initial geometry. While \\(v_2\\) values show differences among these collision systems, \\(v_3\\) values are largely similar, consistent with expectations of subnucleon fluctuations in the initial geometry. The ordering of \\(v_n\\) differs quantitatively from previous measurements using two-particle correlations with a larger rapidity gap, which, according to model calculations, can be partially attributed to the effects of longitudinal flow decorrelations. The prospects for future measurements to improve our understanding of flow decorrelation and subnucleonic fluctuations are also discussed.

We report the systematic measurement of protons and light nuclei production in Au+Au collisions at \\(\\sqrt{s_{\\mathrm{NN}}}\\) = 3 GeV by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The transverse momentum (\\(p_{T}\\)) spectra of protons (\\(p\\)), deuterons (\\(d\\)), tritons (\\(t\\)), \\(^{3}\\mathrm{He}\\), and \\(^{4}\\mathrm{He}\\) are measured from mid-rapidity to target rapidity for different collision centralities. We present the rapidity and centrality dependence of particle yields (\\(dN/dy\\)), average transverse momentum (\\(\\langle p_{T}\\rangle\\)), yield ratios (\\(d/p\\), \\(t/p\\),\\(^{3}\\mathrm{He}/p\\), \\(^{4}\\mathrm{He}/p\\)), as well as the coalescence parameters (\\(B_2\\), \\(B_3\\)). The 4\\(\\pi\\) yields for various particles are determined by utilizing the measured rapidity distributions, \\(dN/dy\\). Furthermore, we present the energy, centrality, and rapidity dependence of the compound yield ratios (\\(N_{p} \\times N_{t} / N_{d}^{2}\\)) and compare them with various model calculations. The physics implications of those results on the production mechanism of light nuclei and on QCD phase structure are discussed.

With the STAR experiment at the BNL Relativisic Heavy Ion Collider, we characterize \\(\\sqrt{s_\\mathrm{NN}}\\) = 200 GeV p+Au collisions by event activity (EA) measured within the pseudorapidity range \\(eta\\) \\(in\\) [-5, -3.4] in the Au-going direction and report correlations between this EA and hard- and soft- scale particle production at midrapidity (\\(\\eta\\) \\(\\in\\) [-1, 1]). At the soft scale, charged particle production in low-EA p+Au collisions is comparable to that in p+p collisions and increases monotonically with increasing EA. At the hard scale, we report measurements of high transverse momentum (pT) jets in events of different EAs. In contrast with the soft particle production, high-pT particle production and EA are found to be inversely related. To investigate whether this is a signal of jet quenching in high-EA events, we also report ratios of pT imbalance and azimuthal separation of dijets in high- and low-EA events. Within our measurement precision, no significant differences are observed, disfavoring the presence of jet quenching in the highest 30% EA p+Au collisions at \\(\\sqrt{s_\\mathrm{NN}}\\) = 200 GeV.

We report on the charged-particle multiplicity dependence of net-proton cumulant ratios up to sixth order from \\(\\sqrt{s}=200\\) GeV \\(p\\)+\\(p\\) collisions at the Relativistic Heavy Ion Collider (RHIC). The measured ratios \\(C_{4}/C_{2}\\), \\(C_{5}/C_{1}\\), and \\(C_{6}/C_{2}\\) decrease with increased charged-particle multiplicity and rapidity acceptance. Neither the Skellam baselines nor PYTHIA8 calculations account for the observed multiplicity dependence. In addition, the ratios \\(C_{5}/C_{1}\\) and \\(C_{6}/C_{2}\\) approach negative values in the highest-multiplicity events, which implies that thermalized QCD matter may be formed in \\(p\\)+\\(p\\) collisions.

For the search of the chiral magnetic effect (CME), STAR previously presented the results from isobar collisions (\\({^{96}_{44}\\text{Ru}}+{^{96}_{44}\\text{Ru}}\\), \\({^{96}_{40}\\text{Zr}}+{^{96}_{40}\\text{Zr}}\\)) obtained through a blind analysis. The ratio of results in Ru+Ru to Zr+Zr collisions for the CME-sensitive charge-dependent azimuthal correlator (\\(\\Delta\\gamma\\)), normalized by elliptic anisotropy (\\(v_{2}\\)), was observed to be close to but systematically larger than the inverse multiplicity ratio. The background baseline for the isobar ratio, \\(Y = \\frac{(\\Delta\\gamma/v_{2})^{\\text{Ru}}}{(\\Delta\\gamma/v_{2})^{\\text{Zr}}}\\), is naively expected to be \\(\\frac{(1/N)^{\\text{Ru}}}{(1/N)^{\\text{Zr}}}\\); however, genuine two- and three-particle correlations are expected to alter it. We estimate the contributions to \\(Y\\) from those correlations, utilizing both the isobar data and HIJING simulations. After including those contributions, we arrive at a final background baseline for \\(Y\\), which is consistent with the isobar data. We extract an upper limit for the CME fraction in the \\(\\Delta\\gamma\\) measurement of approximately \\(10\\%\\) at a \\(95\\%\\) confidence level on in isobar collisions at \\(\\sqrt{s_{\\text{NN}}} = 200\\) GeV, with an expected \\(15\\%\\) difference in their squared magnetic fields.

At the origin of the Universe, asymmetry between the amount of created matter and antimatter led to the matter-dominated Universe as we know today. The origins of this asymmetry remain not completely understood yet. High-energy nuclear collisions create conditions similar to the Universe microseconds after the Big Bang, with comparable amounts of matter and antimatter. Much of the created antimatter escapes the rapidly expanding fireball without annihilating, making such collisions an effective experimental tool to create heavy antimatter nuclear objects and study their properties, hoping to shed some light on existing questions on the asymmetry between matter and antimatter. Here we report the first observation of the antimatter hypernucleus \\hbox{\\(^4_{\\bar{\\Lambda}}\\overline{\\hbox{H}}\\)}, composed of a \\(\\bar{\\Lambda}\\) , an antiproton and two antineutrons. The discovery was made through its two-body decay after production in ultrarelativistic heavy-ion collisions by the STAR experiment at the Relativistic Heavy Ion Collider. In total, 15.6 candidate \\hbox{\\(^4_{\\bar{\\Lambda}}\\overline{\\hbox{H}}\\)} antimatter hypernuclei are obtained with an estimated background count of 6.4. The lifetimes of the antihypernuclei \\hbox{\\(^3_{\\bar{\\Lambda}}\\overline{\\hbox{H}}\\)} and \\hbox{\\(^4_{\\bar{\\Lambda}}\\overline{\\hbox{H}}\\)} are measured and compared with the lifetimes of their corresponding hypernuclei, testing the symmetry between matter and antimatter. Various production yield ratios among (anti)hypernuclei and (anti)nuclei are also measured and compared with theoretical model predictions, shedding light on their production mechanisms.

We report results on an elastic cross section measurement in proton-proton collisions at a center-of-mass energy \\(\\sqrt{s}=510\\) GeV, obtained with the Roman Pot setup of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The elastic differential cross section is measured in the four-momentum transfer squared range \\(0.23 \\leq -t \\leq 0.67\\) GeV\\(^2\\). We find that a constant slope \\(B\\) does not fit the data in the aforementioned \\(t\\) range, and we obtain a much better fit using a second-order polynomial for \\(B(t)\\). The \\(t\\) dependence of \\(B\\) is determined using six subintervals of \\(t\\) in the STAR measured \\(t\\) range, and is in good agreement with the phenomenological models. The measured elastic differential cross section \\(\\mathrm{d}\\sigma/\\mathrm{dt}\\) agrees well with the results obtained at \\(\\sqrt{s} = 546\\) GeV for proton--antiproton collisions by the UA4 experiment. We also determine that the integrated elastic cross section within the STAR \\(t\\)-range is \\(\\sigma^\\mathrm{fid}_\\mathrm{el} = 462.1 \\pm 0.9 (\\mathrm{stat.}) \\pm 1.1 (\\mathrm {syst.}) \\pm 11.6 (\\mathrm {scale})\\)~\\(\\mu\\mathrm{b}\\).