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.

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.

The nuclear equation of state (EOS) is at the center of numerous theoretical and experimental efforts in nuclear physics. With advances in microscopic theories for nuclear interactions, the availability of experiments probing nuclear matter under conditions not reached before, endeavors to develop sophisticated and reliable transport simulations to interpret these experiments, and the advent of multi-messenger astronomy, the next decade will bring new opportunities for determining the nuclear matter EOS, elucidating its dependence on density, temperature, and isospin asymmetry. Among controlled terrestrial experiments, collisions of heavy nuclei at intermediate beam energies (from a few tens of MeV/nucleon to about 25 GeV/nucleon in the fixed-target frame) probe the widest ranges of baryon density and temperature, enabling studies of nuclear matter from a few tenths to about 5 times the nuclear saturation density and for temperatures from a few to well above a hundred MeV, respectively. Collisions of neutron-rich isotopes further bring the opportunity to probe effects due to the isospin asymmetry. However, capitalizing on the enormous scientific effort aimed at uncovering the dense nuclear matter EOS, both at RHIC and at FRIB as well as at other international facilities, depends on the continued development of state-of-the-art hadronic transport simulations. This white paper highlights the essential role that heavy-ion collision experiments and hadronic transport simulations play in understanding strong interactions in dense nuclear matter, with an emphasis on how these efforts can be used together with microscopic approaches and neutron star studies to uncover the nuclear EOS.

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.

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.

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.

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.

Abstract 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 $$ \sqrt{s_{\textrm{NN}}} $$ = 3 GeV with the STAR experiment at RHIC. K S 0 $$ {K}_S^0 $$ 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 $$ {K}_S^0 $$ 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.

With the STAR experiment at the BNL Relativistic Heavy Ion Collider, we characterize root s(NN) = 200 GeV p + Au collisions by event activity (EA) measured within the pseudorapidity range eta is an element of [-5,-3.4] in the Au-going direction and report correlations between this EA and hard- and soft-scale particle production at midrapidity (eta is an element of [-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 (p(T)) jets in events of different EAs. In contrast with the soft particle production, high-p(T) 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 p(T) 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 + v Au collisions at root s(NN) = 200 GeV.

We measure triangular flow relative to the reaction plane at 3 GeV center-of-mass energy in Au+Au collisions at the BNL Relativistic Heavy Ion Collider. A significant v_3 signal for protons is observed, which increases for higher rapidity, higher transverse momentum, and more peripheral collisions. The triangular flow is essentially rapidity-odd with a slope at mid-rapidity, dv_3/dy|_(y=0), opposite in sign compared to the slope for directed flow. No significant v_3 signal is observed for charged pions and kaons. Comparisons with models suggest that a mean field potential is required to describe these results, and that the triangular shape of the participant nucleons is the result of stopping and nuclear geometry.

We report the systematic measurement of protons and light nuclei production in Au+Au collisions at root s(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), He-3, and He-4 have been measured from midrapidity to target rapidity for different collision centralities. We present the rapidity and centrality dependence of particle yields (dN/dy), average transverse momentum (< p(T)>), yield ratios (d/p, t/p, He-3/p, He-4/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 x N-t/N-d(2)) and compare them with various model calculations. The physics implications of these results on the production mechanism of light nuclei and the QCD phase structure are discussed.

The high luminosity Time-of-Flight (TOF) systems call for timing detectors with both good time resolution and adequate rate capability, which have been fulfilled by the Multigap Resistive Plate Chamber (MRPC) assembled with low-resistive glass electrodes. However, recent tests and operations with high rate MRPCs have given evidences to the luminosity effects, such as the dark current rise and related higher noise rate, which may vitiate the stability in experimental operations. This work discusses the results from observations and tests, and infer the fishline spacers as a source of dark current and noise problems. Then, a sealed MRPC with mylar spacers is proposed, developed and tested. The prototype showed good luminosity tolerance during the X-ray test, and its performances of 95% efficiency and 80 ps time resolution have been validated in cosmic tests.

The future fixed target high-rate compressed baryonic matter (CBM) experiment is one of the experimental pillars of the Facility for Antiproton and Ion Research (FAIR) located in Darmstadt/Germany. In order to provide an excellent particle identification (PID) of charged hadrons, the CBM-time-of-flight (TOF) group has developed a concept of a 120 m ^2 large TOF wall with a system time resolution below 80 ps based on multi-gap resistive plate chambers (MRPC). Currently, timing MRPC systems are operated with a gas mixture based on tetrafluoroethane (R134a, C _2 H _2 F _4 ) with additions of few vol _6 ) or/and isobutane (i-C _4 H _10 ). Unfortunately, these gas mixtures have a high global warming potential (GWP) in the order of 1500, and therefore, strategies to reduce the environmental impact have to be developed. The various possibilities, including studies on eco-friendly gases, and the considered strategy by the CBM-TOF group will be elaborated in this article.

This corrects the article DOI: 10.1103/PhysRevLett.126.162301.

We report on new measurements of elliptic flow (v(2)) of electrons from heavy-flavor hadron decays at mid-rapidity (|y| < 0.8) in Au+Au collisions at vs(NN) = 27 and 54.4 GeV from the STAR experiment. Heavy-flavor decay electrons (eHF) in Au+Au collisions at vs(NN) = 54.4 GeV exhibit a non-zero v(2) in the transverse momentum (p(T)) region of p(T) < 2 GeV/c with the magnitude comparable to that at vs(NN) = 200 GeV. The measured e(HF) v(2) at 54.4 GeV is also consistent with the expectation of their parent charm hadron v(2) following number-of-constituent-quark scaling as other light and strange flavor hadrons at this energy. These suggest that charm quarks gain significant collectivity through the evolution of the QCD medium and may reach local thermal equilibrium in Au+Au collisions at vs(NN) = 54.4 GeV. The measured e(HF) v(2) in Au+Au collisions at vs(NN) = 27 GeV is consistent with zero within large uncertainties. The energy dependence of v(2) for different flavor particles (p, f, D-0/e(HF)) shows an indication of quark mass hierarchy in reaching thermalization in high-energy nuclear collisions.(C) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons .org /licenses /by /4 .0/). Funded by SCOAP(3).