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Observation of directed flow of hypernuclei Λ³H and Λ⁴H in √sNN = 3 GeV Au+Au collisions at RHIC
(2022)
We report here the first observation of directed flow (v1) of the hypernuclei 3ΛH and 4ΛH in mid-central Au+Au collisions at sNN−−−√ = 3 GeV at RHIC. These data are taken as part of the beam energy scan program carried out by the STAR experiment. From 165 million good events in 5-40% centrality, about 8400 3ΛH and 5200 4ΛH candidates are reconstructed through two- and three-body decay channels. We observe that these hypernuclei exhibit significant directed flow. Comparing to that of light nuclei, it is found that the mid-rapidity v1 slopes of 3ΛH and 4ΛH follow baryon number scaling, implying that the coalescence is the dominant mechanism for these hypernuclei production in such collisions.
The STAR experiment at RHIC reports new measurements of jet quenching based on the semi-inclusive distribution of charged-particle jets recoiling from direct photon (γdir) and neutral pion (π0) triggers in p+p and central Au+Au collisions at √sNN=200 GeV, for triggers in the range 9<EtrigT<20 GeV. The datasets have integrated luminosities of 3.9 nb−1 for Au+Au and 23 pb−1 for p+p collisions. Jets are reconstructed using the anti-kT algorithm with resolution parameters R=0.2 and 0.5. The large uncorrelated jet background in central Au+Au collisions is corrected using a mixed-event approach, which enables precise charged-particle jet measurements at low transverse momentum pchT,jet and large R. Recoil-jet distributions are reported in the range pchT,jet<25 GeV. Comparison of the distributions measured in p+p and Au+Au collisions reveals strong medium-induced jet yield suppression for R=0.2, with markedly less suppression for R=0.5. These data provide new insight into the mechanisms underlying jet quenching, and the angular dependence of medium-induced jet-energy transport.
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-I) program at the Relativistic Heavy Ion Collider (RHIC). 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 sNN−−−√ = 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.
Elliptic flow of heavy-flavor decay electrons in Au+Au collisions at √sNN = 27 and 54.4 GeV at RHIC
(2023)
We report on new measurements of elliptic flow (v2) of electrons from heavy-flavor hadron decays at mid-rapidity (|y|<0.8) in Au+Au collisions at sNN−−−√ = 27 and 54.4 GeV from the STAR experiment. Heavy-flavor decay electrons (eHF) in Au+Au collisions at sNN−−−√ = 54.4 GeV exhibit a non-zero v2 in the transverse momentum (pT) region of pT< 2 GeV/c with the magnitude comparable to that at sNN−−−√=200 GeV. The measured eHF v2 at 54.4 GeV is also consistent with the expectation of their parent charm hadron v2 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 sNN−−−√=54.4 GeV. The measured eHF v2 in Au+Au collisions at sNN−−−√= 27 GeV is consistent with zero within large uncertainties. The energy dependence of v2 for different flavor particles (π,ϕ,D0/eHF) shows an indication of quark mass hierarchy in reaching thermalization in high-energy nuclear collisions.
The longitudinal and transverse spin transfers to Λ (Λ¯¯¯¯) 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, DLL, and the transverse spin transfer coefficient, DTT, to Λ and Λ¯¯¯¯ in polarized proton-proton collisions at √s = 200 GeV by the STAR experiment at RHIC. The data set 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 data sets have about twice the statistics of previous results and cover a kinematic range of |ηΛ(Λ¯¯¯¯)| < 1.2 and transverse momentum pT,Λ(Λ¯¯¯¯) up to 8 GeV/c. We also report the first measurements of the hyperon spin transfer coefficients DLL and DTT as a function of the fractional jet momentum z carried by the hyperon, which can provide more direct constraints on the polarized fragmentation functions.
The STAR Collaboration presents measurements of the semi-inclusive distribution of charged-particle jets recoiling from energetic direct-photon γdir and neutral-pion (π0) triggers in p+p and central Au+Au collisions at √sNN=200 GeV over a broad kinematic range, for jet resolution parameters R=0.2 and 0.5. Medium-induced jet yield suppression is observed to be larger for R=0.2 than for 0.5, reflecting the angular range of jet energy redistribution due to quenching. The magnitude of suppression is similar for γdir- and π0-triggered data, which constrains the color-charge and path-length dependence of jet quenching. Theoretical model calculations incorporating jet quenching do not fully describe the measurements.
In heavy-ion collision experiments, the global collectivity of final-state particles can be quantified by anisotropic flow coefficients (vn). The first-order flow coefficient, also referred to as the directed flow (v1), 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 (η), 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<|η|<5.1) and high-statistics BES-II data enables us to extend the v1 measurement to the forward and backward η regions. In this paper, we present the measurement of v1 over a wide η range in Au+Au collisions at √sNN= 19.6 and 27 GeV using the STAR EPD. The results of the analysis at √sNN=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 v1 at large η as was observed experimentally. The model comparison also indicates v1 at large η might be sensitive to the QGP phase transition.
The differential cross section for Z0 production, measured as a function of the boson's transverse momentum (pT), provides important constraints on the evolution of the transverse momentum dependent parton distribution functions (TMDs). The transverse single spin asymmetry (TSSA) of the Z0 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 Z0/γ∗ differential cross section as a function of its pT in p+p collisions at a center-of-mass energy of 510 GeV, together with the Z0/γ∗ total cross section. We also report the measurement of Z0/γ∗ TSSA in transversely polarized p+p collisions at 510 GeV.
Elliptic flow of heavy-flavor decay electrons in Au+Au collisions at √sNN = 27 and 54.4 GeV at RHIC
(2023)
We report on new measurements of elliptic flow (v2) of electrons from heavy-flavor hadron decays at mid-rapidity (|y|<0.8) in Au+Au collisions at sNN−−−√ = 27 and 54.4\,GeV from the STAR experiment. Heavy-flavor decay electrons (eHF) in Au+Au collisions at sNN−−−√ = 54.4\,GeV exhibit a non-zero v2 in the transverse momentum (pT) region of pT< 2\,GeV/c with the magnitude comparable to that at sNN−−−√=200 \,GeV. The measured eHF v2 at 54.4\,GeV is also consistent with the expectation of their parent charm hadron v2 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 sNN−−−√=54.4\,GeV. The measured eHF v2 in Au+Au collisions at sNN−−−√= 27\,GeV is consistent with zero within large uncertainties. The energy dependence of v2 for different flavor particles (π,ϕ,D0/eHF) shows an indication of quark mass hierarchy in reaching thermalization in high-energy nuclear collisions.
The chiral magnetic wave (CMW) has been theorized to propagate in the deconfined nuclear medium formed in high-energy heavy-ion collisions, and to cause a difference in elliptic flow (v2) between negatively and positively charged hadrons. Experimental data consistent with the CMW have been reported by the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC), based on the charge asymmetry dependence of the pion v2 from Au+Au collisions at sNN−−−√ = 27 to 200 GeV. In this comprehensive study, we present the STAR measurements of elliptic flow and triangular flow of charged pions, along with the v2 of charged kaons and protons, as a function of charge asymmetry in Au+Au collisions at sNN−−−√ = 27, 39, 62.4 and 200 GeV. The slope parameters extracted from the linear dependence of the v2 difference on charge asymmetry for different particle species are reported and compared in different centrality intervals. In addition, the slopes of v2 for charged pions in small systems, \textit{i.e.}, p+Au and d+Au at sNN−−−√ = 200 GeV, are also presented and compared with those in large systems, \textit{i.e.}, Au+Au at sNN−−−√ = 200 GeV and U+U at 193 GeV. Our results provide new insights for the possible existence of the CMW, and further constrain the background contributions in heavy-ion collisions at RHIC energies.
Flow coefficients (v2 and v3) are measured in high-multiplicity p+Au, d+Au, and 3He+Au collisions at a center-of-mass energy of √sNN = 200 GeV using the STAR detector. The measurements are conducted using two-particle correlations with a pseudorapidity requirement of |η|< 0.9 and a pair gap of |Δη|>1.0. The primary focus of this paper is on the analysis procedures and methods employed, especially the subtraction of non-flow contributions. Four well-established non-flow subtraction methods are applied to determine vn, and their validity is verified using the HIJING event generator. The vn values are compared across the three collision systems at similar multiplicities, which allows for cancellation of final state effects and isolation of the impact of the initial geometry. While the v2 values display differences among these collision systems, the v3 values are largely similar, consistent with the expectations of subnucleon fluctuations in the initial geometry. The ordering of vn differs quantitatively from previous measurements obtained using two-particle correlations with a larger rapidity gap; this difference could be partially attributed to the effects of flow decorrelations in the rapidity direction.
Measurements of exclusive J/ψ, ψ(2s), and electron-positron (e+e−) pair photoproduction in Au+Au ultra-peripheral collisions are reported by the STAR experiment at √sNN=200 GeV. We report several first measurements at the Relativistic Heavy-Ion Collider, which are i) J/ψ photoproduction with large momentum transfer up to 2.2 (GeV/c)2, ii) coherent J/ψ photoproduction associated with neutron emissions from nuclear breakup, iii) the rapidity dependence of incoherent J/ψ photoproduction, iv) the ψ(2s) photoproduction cross section at mid-rapidity, and v) e+e− pair photoproduction up to high invariant mass of 6 GeV/c2. For measurement ii), the coherent J/ψ total cross section of γ∗+Au→J/ψ+Au as a function of the center-of-mass energy Wγ∗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/ψ 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.
Atomic nuclei are self-organized, many-body quantum systems bound by strong nuclear forces within femtometer-scale space. These complex systems manifest a diverse set of shapes~, traditionally explored via non-invasive spectroscopic techniques at low energies. Their instantaneous shapes, obscured by long-timescale quantum fluctuations, are considered not directly observable at low energy. We introduce a complementary method, collective flow assisted nuclear shape imaging, to image the nuclear global shape by colliding them at ultrarelativistic speeds and analyzing the collective response of outgoing debris. This technique captures a collision-specific snapshot of the spatial matter distribution in the nuclei, which, through the hydrodynamic expansion, leaves imprints on the particle momentum distribution patterns observed in detectors. We benchmark this method in collisions of ground state Uranium-238 nuclei, known for its elongated, axial-symmetric shape. Our findings, while confirming an overall deformation broadly consistent with prior low-energy experiments, also indicate a small deviation from axial symmetry in the nuclear ground state. This approach marks a new way of imaging nuclei, especially those with uncertain shape characteristics, and refines initial conditions in high-energy nuclear collisions. It tackles the important issue of nuclear structure evolution across various energy scales.
For the search of the chiral magnetic effect (CME), STAR previously presented the results from isobar collisions (9644Ru+9644Ru, 9640Zr+9640Zr) obtained through a blind analysis. The ratio of results in Ru+Ru to Zr+Zr collisions for the CME-sensitive charge-dependent azimuthal correlator (Δγ), normalized by elliptic anisotropy (v2), was observed to be close to but systematically larger than the inverse multiplicity ratio. The background baseline for the isobar ratio, Y=(Δγ/v2)Ru(Δγ/v2)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 Δγ measurement of approximately 10% at a 95% confidence level on in isobar collisions at sNN−−−√=200 GeV.
In a Quark-Gluon Plasma (QGP), the fundamental building blocks of matter, quarks and gluons, are under extreme conditions of temperature and density. A QGP could exist in the early stages of the Universe, and in various objects and events in the cosmos. The thermodynamic and hydrodynamic properties of the QGP are described by Quantum Chromodynamics (QCD) and can be studied in heavy-ion collisions. Despite being a key thermodynamic parameter, the QGP temperature is still poorly known. Thermal lepton pairs (e+e− and μ+μ−) are ideal penetrating probes of the true temperature of the emitting source, since their invariant-mass spectra suffer neither from strong final-state interactions nor from blue-shift effects due to rapid expansion. Here we measure the QGP temperature using thermal e+e− production at the Relativistic Heavy Ion Collider (RHIC). The average temperature from the low-mass region (in-medium ρ0 vector-meson dominant) is (1.99±0.24)×1012 K, consistent with the chemical freeze-out temperature from statistical models and the phase transition temperature from LQCD. The average temperature from the intermediate mass region (above the ρ0 mass, QGP dominant) is significantly higher at (3.40±0.55)×1012 K. This work provides essential experimental thermodynamic measurements to map out the QCD phase diagram and understand the properties of matter under extreme conditions.
With the STAR experiment at RHIC, we characterize √sNN = 200 GeV p+Au collisions by event activity (EA) measured within the pseudorapidity range η∈[−5,−3.4] in the Au-going direction and report correlations between this EA and hard- and soft-scale particle production at mid-rapidity (η∈[−1,1]). At the soft scale, charged particle production in low-EA p+Au collisions is comparable to that in \pp 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 to 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 √sNN = 200 GeV.
We measure triangular flow relative to the reaction plane at 3 GeV center-of-mass energy in Au+Au collisions at RHIC. A significant v3 signal is observed for protons, whose magnitude increases for higher rapidity, higher transverse momentum, and more peripheral collisions. The triangular flow is essentially rapidity-odd with a rapidity slope at mid-rapidity, dv3/dy|(y=0), opposite in sign compared to the slope for directed flow. No significant v3 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 results on an elastic cross section measurement in proton-proton collisions at a center-of-mass energy 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. 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 dσ/dt agrees well with the results obtained at 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 σfidel=462.1±0.9(stat.)±1.1(syst.)±11.6(scale) μb.
The STAR Collaboration presents measurements of the semi-inclusive distribution of charged-particle jets recoiling from energetic direct-photon γdir and neutral-pion (π0) triggers in p+p and central Au+Au collisions at sNN−−−√=200 GeV over a broad kinematic range, for jet resolution parameters R=0.2 and 0.5. Medium-induced jet yield suppression is observed to be larger for R=0.2 than for 0.5, reflecting the angular range of jet energy redistribution due to quenching. The magnitude of suppression is similar for γdir- and π0-triggered data, which constrains the color-charge and path-length dependence of jet quenching. Theoretical model calculations incorporating jet quenching do not fully describe the measurements.
The linear and mode-coupled contributions to higher-order anisotropic flow are presented for Au+Au collisions at sNN−−−√ = 27, 39, 54.4, and 200 GeV and compared to similar measurements for Pb+Pb collisions at the Large Hadron Collider (LHC). The coefficients and the flow harmonics' correlations, which characterize the linear and mode-coupled response to the lower-order anisotropies, indicate a beam energy dependence consistent with an influence from the specific shear viscosity (η/s). In contrast, the dimensionless coefficients, mode-coupled response coefficients, and normalized symmetric cumulants are approximately beam-energy independent, consistent with a significant role from initial-state effects. These measurements could provide unique supplemental constraints to (i) distinguish between different initial-state models and (ii) delineate the temperature (T) and baryon chemical potential (μB) dependence of the specific shear viscosity ηs(T,μB).