To meet the demands of the Beam Energy Scan phase-II (BES-II) program, the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) developed a dual real-time framework consisting of a High Level Trigger (HLT) and an Express Data Production system (xProduction). The HLT operates online within the Data Acquisition (DAQ) chain on a dedicated multi-core CPU cluster with the option to offload compute-intensive kernels to Xeon Phi coprocessors. It uses parallelized algorithms, such as the Cellular Automaton (CA) Track Finder, to perform rapid tracking, vertexing, and event filtering. This allows it to select events of interest in real time and provide immediate feedback on detector and beam conditions. In contrast, the xProduction workflow runs concurrently and independently of the DAQ loop. It applies near offline-quality calibration and reconstruction within hours of data collection. The xProduction input is the express data stream, whose content can be enriched by HLT trigger/priority selections under DAQ/HLT resource constraints, and it uses the STAR calibration/conditions framework, incorporating online calibration/QA information when available. This enables early preliminary physics analysis, including the reconstruction of rare signals, such as hyperons and hypernuclei. It also provides collaboration-wide access to analysis-ready datasets. Together, the HLT and xProduction systems form a complementary architecture: the HLT performs online event selection while the xProduction chain delivers high-quality results within a short amount of time. This integrated framework has enabled the prompt reconstruction of the 5He hypernucleus with high statistical significance and the efficient processing of hundreds of millions of heavy-ion collision events. Its demonstrated scalability and robustness establish a model for future high-luminosity experiments requiring both online event filtering and rapid access to analysis-quality data.

The system of magnetic measurements in large volumes based on Hall probes has been developed in the Institute for High Energy Physics (IHEP). An example of its practical use to measure the topography of the spectrometric magnet’s field of the SPASCHARM experiment at the U-70 accelerator complex is described. In these measurements, the special attention has been paid to calibrations, monitoring the stability of parameters, and of the accuracy in positioning the probes in the magnet aperture and in their orientation. Further development of the system and the study of its capabilities for improving the accuracy, reducing the measurement time and, in general, increasing its flexibility in applications are also presented.

Data taking has been started at the first stage of the SPASCHARM experiment on systematic study of the spin effects in strong interactions. The current setup configuration is described, and the detector characteristics attained during the beam data taking in the runs of 2018–2022 are listed.

In heavy-ion collision experiments, the global collectivity of final-state particles can be quantified by anisotropic flow coefficients (nu(n)). The first-order flow coefficient, also referred to as the directed flow (nu(1)), describes the collective sideward motion of produced particles and nuclear fragments in heavy-ion collisions. It carries information on the very early stage of the collision, especially at large pseudorapidity (eta), where it is believed to be generated during the nuclear passage time. Directed flow therefore probes the onset of bulk collective dynamics during thermalization, providing valuable experimental guidance to models of the pre-equilibrium stage. In 2018, the Event Plane Detector (EPD) was installed in STAR and used for the Beam Energy Scan phase-II (BES-II) data taking. The combination of EPD (2.1 < |eta| < 5.1) and high-statistics BES-II data enables us to extend the nu(1) measurement to the forward and backward eta regions. In this paper, we present the measurement of nu(1) over a wide. range in Au+Au collisions at root s(NN) = 19.6 and 27 GeV using the STAR EPD. The results of the analysis at root s(NN) = 19.6 GeV exhibit excellent consistency with the previous PHOBOS measurement, while elevating the precision of the overall measurement. The increased precision of the measurement also revealed finer structures in heavy-ion collisions, including a potential observation of the first-order event-plane decorrelation. Multiple physics models were compared to the experimental results. Only a transport model and a three-fluid hybrid model can reproduce a sizable nu(1) at large. as was observed experimentally. The model comparison also indicates nu(1) at large. might be sensitive to the QGP phase transition.

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

Angular distributions of charged particles relative to jet axes are studied in v sNN = 200 GeV Au+Au collisions as a function of the jet orientation with respect to the event plane. This differential study tests the expected path-length dependence of energy loss experienced by a hard-scattered parton as it traverses the hot and dense medium formed in heavy-ion collisions. A second-order event plane is used in the analysis as an experimental estimate of the reaction plane formed by the collision impact parameter and the beam direction. Charged-particle jets with 15 < p(T, jet) < 20 and 20 < p(T, jet) < 40 GeV/c were reconstructed with the anti-k(T) algorithm with radius parameter setting of R = 0.4 in the 20-50% centrality bin to maximize the initial-state eccentricity of the interaction region. The reaction plane fit method is implemented to remove the flow-modulated background with better precision than prior methods. Yields and widths of jet-associated charged-hadron distributions are extracted in three angular bins between the jet axis and the event plane. The event-plane (EP) dependence is further quantified by ratios of the associated yields in different EP bins. No dependence on orientation of the jet axis with respect to the event plane is seen within the uncertainties in the kinematic regime studied. This finding is consistent with a similar experimental observation by ALICE in root sNN = 2.76 TeV Pb-Pb collision data.

First results on Alpha-dependence measurements in inclusive K-S(0) meson production in pi(-)Alpha interactions (Alpha=C, Al, Cu, Sn, and W) are presented at approximate to 26.5 GeV/c. The ratios of the differential cross sections of four of these nuclei to the differential cross section on aluminum were measured and the dependence of these cross sections on the atomic of the nucleus was studied. The measurements were carried out in the kinematic region of the Feynman variable 0.2 < x(F) < 0.8 and transverse momentum p(T) < 1.2 GeV/c. K-S(0) mesons were detected in the decay mode K-S(0)->pi(+)pi(-) at the SPASCHARM experimental setup using negative charged beams at beamline 14 of the U-70 accelerator complex.

We report on the charged-particle multiplicity dependence of net-proton cumulant ratios up to sixth order from s = 200 GeV p+p collisions at the Relativistic Heavy Ion Collider (RHIC). The measured ratios C4/C2, C5/C1, and C6/C2 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 C5/C1 and C6/C2 approach negative values in the highest-multiplicity events, which implies that thermalized QCD matter may be formed in p+p collisions.

Measurements of exclusive J/psi, psi(2s), and electron-positron (e(+) e(-)) pair photoproduction in Au + Au ultraperipheral collisions are reported by the STAR experiment at root s(NN) = 200 GeV. We report several first measurements at the BNL Relativistic Heavy Ion Collider, which are (i) J/psi photoproduction with large momentum transfer up to 2.2 (GeV/c)(2), (ii) coherent J/psi photoproduction associated with neutron emissions from nuclear breakup, (iii) the rapidity dependence of incoherent J/psi photoproduction, (iv) the psi(2s) photoproduction cross section at midrapidity, and (v) e(+) e(-) pair photoproduction up to high invariant mass of 6 GeV/c(2). For measurement (ii), the coherent J/psi total cross section of gamma + Au -> J/psi + Au as a function of the center-of-mass energy W-gamma N has been obtained without photon energy ambiguities. The data are quantitatively compared with the Monte Carlo models STARlight, Sartre, BeAGLE, and theoretical calculations of gluon saturation with color glass condensate, nuclear shadowing with leading twist approximation, quantum electrodynamics, and the next-to-leading-order perturbative QCD. At the photon-nucleon center-of-mass energy of 25.0 GeV, the coherent and incoherent J/psi cross sections of Au nuclei are found to be 71% +/- 10% and 36% +/- 7%, respectively, of that of free protons. These data provide an important experimental constraint for nuclear parton distribution functions and a unique opportunity to advance the understanding of the nuclear modification effect at the top RHIC energy.

The longitudinal and transverse spin transfers to Lambda (Lambda) hyperons in polarized proton-proton collisions are expected to be sensitive to the helicity and transversity distributions, respectively, of (anti)strange quarks in the proton, and to the corresponding polarized fragmentation functions. We report improved measurements of the longitudinal spin transfer coefficient, D-LL, and the transverse spin transfer coefficient, D-TT,D- to Lambda and Lambda in polarized proton-proton collisions at root s=200 GeV by the STAR experiment at RHIC. The dataset includes longitudinally polarized proton-proton collisions with an integrated luminosity of 52 pb(-1), and transversely polarized proton-proton collisions with a similar integrated luminosity. Both datasets have about twice the statistics of previous results and cover a kinematic range of |eta(Lambda (Lambda))|<1.2 and transverse momentum p(T,Lambda( (Lambda)) up to 8 GeV/c. We also report the first measurements of the hyperon spin transfer coefficients D-LL and D-TT as a function of the fractional jet momentum z carried by the hyperon, which can provide more direct constraints on the polarized fragmentation functions.

We report the first measurements of cumulants, up to 4th 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 root 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.

The deconfined quark-gluon plasma (QGP) created in relativistic heavy-ion collisions enables the exploration of the fundamental properties of matter under extreme conditions. Noncentral collisions can produce strong magnetic fields on the order of 1018 G, which offers a probe into the electrical conductivity of the QGP. In particular, quarks and antiquarks carry opposite charges and receive contrary electromagnetic forces that alter their momenta. This phenomenon can be manifested in the collective motion of final -state particles, specifically in the rapidity-odd directed flow, denoted as v1oy thorn . Here, we present the charge-dependent measurements of dv1=dy near midrapidities for pi ⠂, K ⠂, and pop over bar thorn in Au thorn Au and isobar (96 44Ru thorn 9644Ru and 9640Zr thorn 96 40Zr) collisions at ffiffiffiffiffiffiffi p 1/4 200 GeV, and in Au thorn Au collisions at sNN 27 GeV, recorded by the STAR detector at the Relativistic Heavy Ion Collider. The combined dependence of the v1 signal on collision system, particle species, and collision centrality can be qualitatively and semiquantitatively understood as several effects on constituent quarks. While the results in central events can be explained by the u and d quarks transported from initial -state nuclei, those in peripheral events reveal the impacts of the electromagnetic field on the QGP. Our data put valuable constraints on the electrical conductivity of the QGP in theoretical calculations.

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 antimatter(1-6). 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 properties(7-14), 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 4/Lambda(H) over bar, composed of a (Lambda) over bar, 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(15,16). In total, 15.6 candidate 4/Lambda(H) over bar antimatter hypernuclei are obtained with an estimated background count of 6.4. The lifetimes of the antihypernuclei 3/Lambda(H) over bar and 4/Lambda(H) over tilde 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 differential cross section for Z(0) production, measured as a function of the boson's transverse momentum (p(T)), provides important constraints on the evolution of the transverse momentum dependent parton distribution functions (TMDs). The transverse single spin asymmetry (TSSA) of the Z(0) is sensitive to one of the polarized TMDs, the Sivers function, which is predicted to have the opposite sign in p + p -> W/Z + X from that which enters in semi-inclusive deep inelastic scattering. In this Letter, the STAR Collaboration reports the first measurement of the Z(0)/gamma* differential cross section as a function of its p(T) in p + p collisions at a center-of-mass energy of 510 GeV, together with the Z(0)/gamma* total cross section. We also report the measurement of Z(0)/gamma* TSSA in transversely polarized p + p collisions at 510 GeV.

For the search of the chiral magnetic effect (CME), STAR previously presented the results from isobar collisions (Ru-96(44) + Ru-96(44), Zr-96(40) + Zr-96(40)) 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 = (Delta gamma/v(2))(Ru)/(Delta gamma/v(2))(Zr), is naively expected to be (1/N)(Ru)/(1/N)(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 root S-NN = 200 GeV, with an expected 15% difference in their squared magnetic fields.

Wve report results on an elastic cross section measurement in proton-proton collisions at a center-of-mass energy root 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 <= -t <= 0.67 GeV2. This is the only measurement of the proton-proton elastic cross section in this t range for collision energies above the Intersecting Storage Rings (ISR) and below the Large Hadron Collider (LHC) colliders. 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). This is the first measurement below the LHC energies for which the non-constant behavior B(t) is observed. The t dependence of B is also 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 d sigma/dt agrees well with the results obtained at root s = 540 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(fid) (el) = 462.1 +/- 0.9(stat.) +/- 1.1(syst.) +/- 11.6(scale) mu b.

First results on 𝐴-dependence measurements in inclusive K0𝑆-meson production in 兀− 𝐴-interactions (𝐴 = C, Cu, Al, Sn, W) are presented at 26.5 GeV/c. The ratios of the differential cross sections of four of these nuclei to the differential cross section on aluminum were measured and the dependence of these cross-sections on the atomic number of the nucleus was studied. The measurements were carried out in the kinematic region of the Feynman variable 0.2 < 𝐹 < 0.8 and transverse momentum 𝑝𝑇 < 1.2 GeV/c. K0𝑆-mesons were detected in the decay mode K0𝑆→ 兀+兀−-at the SPASCHARM experimental setup using negative charged beams at beamline 14 of the U-70 accelerator complex.

First results on the polarization of Λ hyperons inclusively produced on 26.5-GeV/c K^ - and π^ - beams are presented. The measurements were carried out using the SPASCHARM setup at the U-70 accelerator facility (Protvino, Russia) on nuclear targets in 2021 and 2022. The polarization of an Λ hyperon on the π^ - beam does not exceed several percent in most of the studied kinematic region. The data obtained on the K^ - -meson beam exhibit a significant positive polarization at large values of the Feynman variable xF and transverse momentum pT measured for the first time on nuclei.

We report multi-differential measurements of strange hadron production ranging from mid- to target-rapidity in Au+Au collisions at a center-of-momentum energy per nucleon pair of √(s_NN) = 3 GeV with the STAR experiment at RHIC. K_S^0 meson and Λ hyperon yields are measured via their weak decay channels. Collision centrality and rapidity dependences of the transverse momentum spectra and particle ratios are presented. Particle mass and centrality dependence of the average transverse momenta of Λ and K_S^0 are compared with other strange particles, providing evidence of the development of hadronic rescattering in such collisions. The 4π yields of each of these strange hadrons show a consistent centrality dependence. Discussions on radial flow, the strange hadron production mechanism, and properties of the medium created in such collisions are presented together with results from hadronic transport and thermal model calculations.