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Measurements of mass and Λ binding energy of 4ΛH and 4ΛHe in Au+Au collisions at sNN−−−√=3 GeV are presented, with an aim to address the charge symmetry breaking (CSB) problem in hypernuclei systems with atomic number A = 4. The Λ binding energies are measured to be 2.22±0.06(stat.)±0.14(syst.) MeV and 2.38±0.13(stat.)±0.12(syst.) MeV for 4ΛH and 4ΛHe, respectively. The measured Λ binding-energy difference is 0.16±0.14(stat.)±0.10(syst.) MeV for ground states. Combined with the γ-ray transition energies, the binding-energy difference for excited states is −0.16±0.14(stat.)±0.10(syst.) MeV, which is negative and comparable to the value of the ground states within uncertainties. These new measurements on the Λ binding-energy difference in A = 4 hypernuclei systems are consistent with the theoretical calculations that result in ΔB4Λ(1+exc)≈−ΔB4Λ(0+g.s.)<0 and present a new method for the study of CSB effect using relativistic heavy-ion collisions.
We report precision measurements of hypernuclei 3ΛH and 4ΛH lifetimes obtained from Au+Au collisions at \snn = 3.0\,GeV and 7.2\,GeV collected by the STAR experiment at RHIC, and the first measurement of 3ΛH and 4ΛH mid-rapidity yields in Au+Au collisions at \snn = 3.0\,GeV. The lifetimes are measured to be 221±15(stat.)±19(syst.)\,ps for 3ΛH and 218±6(stat.)±13(syst.)\,ps for 4ΛH. The pT-integrated yields of 3ΛH and 4ΛH are presented in different centrality and rapidity intervals. It is observed that the shape of the rapidity distribution of 4ΛH is different for 0--10\% and 10--50\% centrality collisions. Thermal model calculations, using the canonical ensemble for strangeness, describes the 3ΛH yield well, while underestimating the 4ΛH yield. Transport models, combining baryonic mean-field and coalescence (JAM) or utilizing dynamical cluster formation via baryonic interactions (PHQMD) for light nuclei and hypernuclei production, approximately describe the measured 3ΛH and 4ΛH yields.
We report precision measurements of hypernuclei 3ΛH and 4ΛH lifetimes obtained from Au+Au collisions at \snn = 3.0\,GeV and 7.2\,GeV collected by the STAR experiment at RHIC, and the first measurement of 3ΛH and 4ΛH mid-rapidity yields in Au+Au collisions at \snn = 3.0\,GeV. 3ΛH and 4ΛH, being the two simplest bound states composed of hyperons and nucleons, are cornerstones in the field of hypernuclear physics. Their lifetimes are measured to be 221±15(stat.)±19(syst.)\,ps for 3ΛH and 218±6(stat.)±13(syst.)\,ps for 4ΛH. The pT-integrated yields of 3ΛH and 4ΛH are presented in different centrality and rapidity intervals. It is observed that the shape of the rapidity distribution of 4ΛH is different for 0--10\% and 10--50\% centrality collisions. Thermal model calculations, using the canonical ensemble for strangeness, describes the 3ΛH yield well, while underestimating the 4ΛH yield. Transport models, combining baryonic mean-field and coalescence (JAM) or utilizing dynamical cluster formation via baryonic interactions (PHQMD) for light nuclei and hypernuclei production, approximately describe the measured 3ΛH and 4ΛH yields. Our measurements provide means to precisely assess our understanding of the fundamental baryonic interactions with strange quarks, which can impact our understanding of more complicated systems involving hyperons, such as the interior of neutron stars or exotic hypernuclei.
We report precision measurements of hypernuclei 3ΛH and 4ΛH lifetimes obtained from Au+Au collisions at \snn = 3.0\,GeV and 7.2\,GeV collected by the STAR experiment at RHIC, and the first measurement of 3ΛH and 4ΛH mid-rapidity yields in Au+Au collisions at \snn = 3.0\,GeV. 3ΛH and 4ΛH, being the two simplest bound states composed of hyperons and nucleons, are cornerstones in the field of hypernuclear physics. Their lifetimes are measured to be 221±15(stat.)±19(syst.)\,ps for 3ΛH and 218±6(stat.)±13(syst.)\,ps for 4ΛH. The pT-integrated yields of 3ΛH and 4ΛH are presented in different centrality and rapidity intervals. It is observed that the shape of the rapidity distribution of 4ΛH is different for 0--10\% and 10--50\% centrality collisions. Thermal model calculations, using the canonical ensemble for strangeness, describes the 3ΛH yield well, while underestimating the 4ΛH yield. Transport models, combining baryonic mean-field and coalescence (JAM) or utilizing dynamical cluster formation via baryonic interactions (PHQMD) for light nuclei and hypernuclei production, approximately describe the measured 3ΛH and 4ΛH yields. Our measurements provide means to precisely assess our understanding of the fundamental baryonic interactions with strange quarks, which can impact our understanding of more complicated systems involving hyperons, such as the interior of neutron stars or exotic hypernuclei.
The STAR collaboration presents jet substructure measurements related to both the momentum fraction and the opening angle within jets in p+p and Au+Au collisions at sNN−−−√=200 GeV. The substructure observables include SoftDrop groomed momentum fraction (zg), groomed jet radius (Rg), and subjet momentum fraction ((zSJ)) and opening angle ((θSJ)). The latter observable is introduced for the first time. Fully corrected subjet measurements are presented for p+p collisions and are compared to leading order Monte Carlo models. The subjet θSJ distributions reflect the jets leading opening angle and are utilized as a proxy for the resolution scale of the medium in Au+Au collisions. We compare data from Au+Au collisions to those from p+p which are embedded in minimum-bias Au+Au events in order to include the effects of detector smearing and the heavy-ion collision underlying event. The subjet observables are shown to be more robust to the background than zg and (Rg).
We observe no significant modifications of the subjet observables within the two highest-energy, back-to-back jets, resulting in a distribution of opening angles and the splittings that are vacuum-like. We also report measurements of the differential di-jet momentum imbalance (AJ) for jets of varying θg. We find no qualitative differences in energy loss signatures for varying angular scales in the range 0.1<θSJ<0.3, leading to the possible interpretation that energy loss in this population of high momentum di-jet pairs, is due to soft medium-induced gluon radiation from a single color-charge as it traverses the medium.
The STAR collaboration presents jet substructure measurements related to both the momentum fraction and the opening angle within jets in \pp and \AuAu collisions at \sqrtsn =200 GeV. The substructure observables include SoftDrop groomed momentum fraction (\zg), groomed jet radius (\rg), and subjet momentum fraction (\zsj) and opening angle (\tsj). The latter observable is introduced for the first time. Fully corrected subjet measurements are presented for \pp collisions and are compared to leading order Monte Carlo models. The subjet \tsj~distributions reflect the jets leading opening angle and are utilized as a proxy for the resolution scale of the medium in \AuAu collisions. We compare data from \AuAu collisions to those from \pp which are embedded in minimum-bias \AuAu events in order to include the effects of detector smearing and the heavy-ion collision underlying event. The subjet observables are shown to be more robust to the background than \zg~and \rg.
We observe no significant modifications of the subjet observables within the two highest-energy, back-to-back jets, resulting in a distribution of opening angles and the splittings that are vacuum-like. We also report measurements of the differential di-jet momentum imbalance (AJ) for jets of varying \tsj. We find no qualitative differences in energy loss signatures for varying angular scales in the range 0.1< \tsj <0.3, leading to the possible interpretation that energy loss in this population of high momentum di-jet pairs, is due to soft medium-induced gluon radiation from a single color-charge as it traverses the medium.
A data-driven method was applied to Au+Au collisions at √sNN = 200 GeV made with the STAR detector at RHIC to isolate pseudorapidity distance η-dependent and η-independent correlations by using two- and four-particle azimuthal cumulant measurements. We identified a η-independent component of the correlation, which is dominated by anisotropic flow and flow fluctuations. It was also found to be independent of η within the measured range of pseudorapidity |η| < 1. In 20–30% central Au+Au collisions, the relative flow fluctuation was found to be 34%±2%(stat.)±3%(sys.) for particles with transverse momentum pT less than 2 GeV/c. The η-dependent part, attributed to nonflow correlations, is found to be 5% ± 2%(sys.) relative to the flow of the measured second harmonic cumulant at |η| > 0.7.
We report the first multi-differential measurements of strange hadrons of K −, φ and − yields as well as the ratios of φ/K − and φ/− in Au+Au collisions at √sNN = 3 GeV with the STAR experiment fixed target configuration at RHIC. The φ mesons and − hyperons are measured through hadronic decay channels, φ → K + K − and Ξ− → Λπ−. Collision centrality and rapidity dependence of the transverse momentum spectra for these strange hadrons are presented. The 4π yields and ratios are compared to thermal model and hadronic transport model predictions. At this collision energy, thermal model with grand canonical ensemble (GCE) under-predicts the φ/K − and φ/− ratios while the result of canonical ensemble (CE) calculations reproduce φ/K −, with the correlation length rc ∼ 2.7 fm, and φ/−, rc ∼ 4.2 fm, for the 0-10% central collisions. Hadronic transport models including high mass resonance decays could also describe the ratios. While thermal calculations with GCE work well for strangeness production in high energy collisions, the change to CE at 3 GeV implies a rather different medium property at high baryon density.
We report on a polarization measurement of inclusive J/ψ mesons in the di-electron decay channel at mid-rapidity at 2 < pT < 6 GeV/c in p + p collisions at √s = 200 GeV. Data were taken with the STAR detector at RHIC. The J/ψ polarization measurement should help to distinguish between different models of the J/ψ production mechanism since they predict different pT dependences of the J/ψ polarization. In this analysis, J/ψ polarization is studied in the helicity frame. The polarization parameter λθ measured at RHIC becomes smaller towards high pT , indicating more longitudinal J/ψ polarization as pT increases. The result is compared with predictions of presently available models.
Measurements of mass and Λ binding energy of 4ΛH and 4ΛHe in Au+Au collisions at sNN−−−√=3 GeV are presented, with an aim to address the charge symmetry breaking (CSB) problem in hypernuclei systems with atomic number A = 4. The Λ binding energies are measured to be 2.22±0.06(stat.)±0.14(syst.) MeV and 2.38±0.13(stat.)±0.12(syst.) MeV for 4ΛH and 4ΛHe, respectively. The measured Λ binding-energy difference is 0.16±0.14(stat.)±0.10(syst.) MeV for ground states. Combined with the γ-ray transition energies, the binding-energy difference for excited states is −0.16±0.14(stat.)±0.10(syst.) MeV, which is negative and comparable to the value of the ground states within uncertainties. These new measurements on the Λ binding-energy difference in A = 4 hypernuclei systems are consistent with the theoretical calculations that result in ΔB4Λ(1+exc)≈−ΔB4Λ(0+g.s.)<0 and present a new method for the study of CSB effect using relativistic heavy-ion collisions.
We present the first inclusive measurements of the invariant and SoftDrop jet mass in proton-proton collisions at s√=200 GeV at STAR. The measurements are fully corrected for detector effects, and reported differentially in both the jet transverse momentum and jet radius parameter. We compare the measurements to established leading-order Monte Carlo event generators and find that STAR-tuned PYTHIA-6 reproduces the data, while LHC tunes of PYTHIA-8 and HERWIG-7 do not agree with the data, providing further constraints on parameter tuning. Finally, we observe that SoftDrop grooming, for which the contribution of wide-angle non-perturbative radiation is suppressed, shifts the jet mass distributions into closer agreement with the partonic jet mass as determined by both PYTHIA-8 and a next-to-leading-logarithmic accuracy perturbative QCD calculation. These measurements complement recent LHC measurements in a different kinematic region, as well as establish a baseline for future jet mass measurements in heavy-ion collisions at RHIC.
We report a systematic measurement of cumulants, Cn, for net-proton, proton and antiproton, and correlation functions, κn, for proton and antiproton multiplicity distributions up to the fourth order in Au+Au collisions at sNN−−−√ = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4 and 200 GeV. The Cn and κn are presented as a function of collision energy, centrality and kinematic acceptance in rapidity, y, and transverse momentum, pT. The data were taken during the first phase of the Beam Energy Scan (BES) program (2010 -- 2017) at the Relativistic Heavy Ion Collider (RHIC) facility. The measurements are carried out at midrapidity (|y|< 0.5) and transverse momentum 0.4 < pT < 2.0 GeV/c, using the STAR detector at RHIC. We observe a non-monotonic energy dependence (sNN−−−√ = 7.7 -- 62.4 GeV) of the net-proton C4/C2 with the significance of 3.1σ for the 0-5\% central Au+Au collisions. This is consistent with the expectations of critical fluctuations in a QCD-inspired model. Thermal and transport model calculations show a monotonic variation with sNN−−−√. For the multiparticle correlation functions, we observe significant negative values for a two-particle correlation function, κ2, of protons and antiprotons, which are mainly due to the effects of baryon number conservation. Furthermore, it is found that the four-particle correlation function, κ4, of protons plays a role in determining the energy dependence of proton C4/C1 below 19.6 GeV, which cannot be solely understood by the negative values of κ2 for protons.
We report a systematic measurement of cumulants, Cn, for net-proton, proton and antiproton multiplicity distributions, and correlation functions, κn, for proton and antiproton multiplicity distributions up to the fourth order in Au+Au collisions at sNN−−−√ = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4 and 200 GeV. The Cn and κn are presented as a function of collision energy, centrality and kinematic acceptance in rapidity, y, and transverse momentum, pT. The data were taken during the first phase of the Beam Energy Scan (BES) program (2010 -- 2017) at the BNL Relativistic Heavy Ion Collider (RHIC) facility. The measurements are carried out at midrapidity (|y|< 0.5) and transverse momentum 0.4 < pT < 2.0 GeV/c, using the STAR detector at RHIC. We observe a non-monotonic energy dependence (sNN−−−√ = 7.7 -- 62.4 GeV) of the net-proton C4/C2 with the significance of 3.1σ for the 0-5\% central Au+Au collisions. This is consistent with the expectations of critical fluctuations in a QCD-inspired model. Thermal and transport model calculations show a monotonic variation with sNN−−−√. For the multiparticle correlation functions, we observe significant negative values for a two-particle correlation function, κ2, of protons and antiprotons, which are mainly due to the effects of baryon number conservation. Furthermore, it is found that the four-particle correlation function, κ4, of protons plays a role in determining the energy dependence of proton C4/C1 below 19.6 GeV, which cannot be understood by the effect of baryon number conservation.
We report high-precision measurements of the longitudinal double-spin asymmetry, 𝐴𝐿𝐿, for midrapidity inclusive jet and dijet production in polarized 𝑝𝑝 collisions at a center-of-mass energy of √𝑠=200 GeV. The new inclusive jet data are sensitive to the gluon helicity distribution, Δ𝑔(𝑥,𝑄2), for gluon momentum fractions in the range from 𝑥≃0.05 to 𝑥≃0.5, while the new dijet data provide further constraints on the 𝑥 dependence of Δ𝑔(𝑥,𝑄2). The results are in good agreement with previous measurements at √𝑠=200 GeV and with recent theoretical evaluations of prior world data. Our new results have better precision and thus strengthen the evidence that Δ𝑔(𝑥,𝑄2) is positive for 𝑥>0.05.
The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at mid-rapidity |yee|<1 in minimum-bias Au+Au collisions at sNN−−−−√ = 19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened ρ spectral function for Mee<1.1 GeV/c2. The integrated dielectron excess yield at sNN−−−−√ = 19.6 GeV for 0.4<Mee<0.75 GeV/c2, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In+In collisions at sNN−−−−√ = 17.3 GeV. For sNN−−−−√ = 200 GeV, the normalized excess yield in central collisions is higher than that at sNN−−−−√ = 17.3 GeV and increases from peripheral to central collisions. These measurements indicate that the lifetime of the hot, dense medium created in central Au+Au collisions at sNN−−−−√ = 200 GeV is longer than those in peripheral collisions and at lower energies.
Di-hadron correlations with identified leading hadrons in 200 GeV Au+Au and d+Au collisions at STAR
(2015)
The STAR collaboration presents for the first time two-dimensional di-hadron correlations with identified leading hadrons in 200 GeV central Au+Au and minimum-bias d+Au collisions to explore hadronization mechanisms in the quark gluon plasma. The enhancement of the jet-like yield for leading pions in Au+Au data with respect to the d+Au reference and the absence of such an enhancement for leading non-pions (protons and kaons) are discussed within the context of a quark recombination scenario. The correlated yield at large angles, specifically in the \emph{ridge region}, is found to be significantly higher for leading non-pions than pions. The consistencies of the constituent quark scaling, azimuthal harmonic model and a mini-jet modification model description of the data are tested, providing further constraints on hadronization.
Measurement of inclusive J/ψ polarization in p + p collisions at √s=200 GeV by the STAR experiment
(2020)
We report on new measurements of inclusive 𝐽/𝜓 polarization at midrapidity in 𝑝+𝑝 collisions at √𝑠=200 GeV by the STAR experiment at the Relativistic Heavy Ion Collider. The polarization parameters, 𝜆𝜃, 𝜆𝜙, and 𝜆𝜃𝜙, are measured as a function of transverse momentum (𝑝T) in both the helicity and Collins-Soper (CS) reference frames within 𝑝T<10 GeV/𝑐. Except for 𝜆𝜃 in the CS frame at the highest measured 𝑝T, all three polarization parameters are consistent with 0 in both reference frames without any strong 𝑝T dependence. Several model calculations are compared with data, and the one using the Color Glass Condensate effective field theory coupled with nonrelativistic QCD gives the best overall description of the experimental results, even though other models cannot be ruled out due to experimental uncertainties.
Measurement of inclusive charged-particle jet production in Au + Au collisions at √sNN=200 GeV
(2020)
The STAR Collaboration at the Relativistic Heavy Ion Collider reports the first measurement of inclusive jet production in peripheral and central Au+Au collisions at √𝑠𝑁𝑁=200 GeV. Jets are reconstructed with the anti-𝑘𝑇 algorithm using charged tracks with pseudorapidity |𝜂|<1.0 and transverse momentum 0.2<𝑝ch
𝑇,jet<30 GeV/𝑐, with jet resolution parameter 𝑅=0.2, 0.3, and 0.4. The large background yield uncorrelated with the jet signal is observed to be dominated by statistical phase space, consistent with a previous coincidence measurement. This background is suppressed by requiring a high-transverse-momentum (high-𝑝𝑇) leading hadron in accepted jet candidates. The bias imposed by this requirement is assessed, and the 𝑝𝑇 region in which the bias is small is identified. Inclusive charged-particle jet distributions are reported in peripheral and central Au+Au collisions for 5<𝑝ch
𝑇,jet<25 GeV/𝑐 and 5<𝑝ch
𝑇,jet<30 GeV/𝑐, respectively. The charged-particle jet inclusive yield is suppressed for central Au+Au collisions, compared to both the peripheral Au+Au yield from this measurement and to the 𝑝𝑝 yield calculated using the PYTHIA event generator. The magnitude of the suppression is consistent with that of inclusive hadron production at high 𝑝𝑇 and that of semi-inclusive recoil jet yield when expressed in terms of energy loss due to medium-induced energy transport. Comparison of inclusive charged-particle jet yields for different values of 𝑅 exhibits no significant evidence for medium-induced broadening of the transverse jet profile for 𝑅 <0.4 in central Au+Au collisions. The measured distributions are consistent with theoretical model calculations that incorporate jet quenching.
Measurement of inclusive charged-particle jet production in Au+Au collisions at √sNN = 200 GeV
(2021)
The STAR Collaboration at the Relativistic Heavy Ion Collider reports the first measurement of inclusive jet production in peripheral and central Au+Au collisions at sNN−−−−√=200 GeV. Jets are reconstructed with the anti-kT algorithm using charged tracks with pseudorapidity |η|<1.0 and transverse momentum 0.2<pchT,jet<30 GeV/c, with jet resolution parameter R=0.2, 0.3, and 0.4. The large background yield uncorrelated with the jet signal is observed to be dominated by statistical phase space, consistent with a previous coincidence measurement. This background is suppressed by requiring a high-transverse-momentum (high-pT) leading hadron in accepted jet candidates. The bias imposed by this requirement is assessed, and the pT region in which the bias is small is identified. Inclusive charged-particle jet distributions are reported in peripheral and central Au+Au collisions for 5<pchT,jet<25 GeV/c and 5<pchT,jet<30 GeV/c, respectively. The charged-particle jet inclusive yield is suppressed for central Au+Au collisions, compared to both the peripheral Au+Au yield from this measurement and to the pp yield calculated using the PYTHIA event generator. The magnitude of the suppression is consistent with that of inclusive hadron production at high pT, and that of semi-inclusive recoil jet yield when expressed in terms of energy loss due to medium-induced energy transport. Comparison of inclusive charged-particle jet yields for different values of R exhibits no significant evidence for medium-induced broadening of the transverse jet profile for R<0.4 in central Au+Au collisions. The measured distributions are consistent with theoretical model calculations that incorporate jet quenching.
We report on the measurement of the Central Exclusive Production of charged particle pairs h+h− (h = π, K, p) with the STAR detector at RHIC in proton-proton collisions at √s = 200 GeV. The charged particle pairs produced in the reaction pp → p′ + h+h− + p′ are reconstructed from the tracks in the central detector and identified using the specific energy loss and the time of flight method, while the forward-scattered protons are measured in the Roman Pot system. Exclusivity of the event is guaranteed by requiring the transverse momentum balance of all four final-state particles. Differential cross sections are measured as functions of observables related to the central hadronic final state and to the forward-scattered protons. They are measured in a fiducial region corresponding to the acceptance of the STAR detector and determined by the central particles’ transverse momenta and pseudorapidities as well as by the forward-scattered protons’ momenta. This fiducial region roughly corresponds to the square of the four-momentum transfers at the proton vertices in the range 0.04 GeV2 < −t1, −t2 < 0.2 GeV2, invariant masses of the charged particle pairs up to a few GeV and pseudorapidities of the centrally-produced hadrons in the range |η| < 0.7. The measured cross sections are compared to phenomenological predictions based on the Double Pomeron Exchange (DPE) model. Structures observed in the mass spectra of π+π− and K+K− pairs are consistent with the DPE model, while angular distributions of pions suggest a dominant spin-0 contribution to π+π− production. For π+π− production, the fiducial cross section is extrapolated to the Lorentz-invariant region, which allows decomposition of the invariant mass spectrum into continuum and resonant contributions. The extrapolated cross section is well described by the continuum production and at least three resonances, the f0(980), f2(1270) and f0(1500), with a possible small contribution from the f0(1370). Fits to the extrapolated differential cross section as a function of t1 and t2 enable extraction of the exponential slope parameters in several bins of the invariant mass of π+π− pairs. These parameters are sensitive to the size of the interaction region.