Refine
Year of publication
Document Type
- Article (240)
- Preprint (190)
- Working Paper (1)
Language
- English (431)
Has Fulltext
- yes (431)
Is part of the Bibliography
- no (431)
Keywords
- BESIII (17)
- e +-e − Experiments (13)
- Branching fraction (9)
- Particle and Resonance Production (7)
- Quarkonium (6)
- Spectroscopy (6)
- Hadronic decays (5)
- Branching fractions (4)
- Charm Physics (4)
- Exotics (4)
Institute
- Physik (321)
- Frankfurt Institute for Advanced Studies (FIAS) (103)
- Biodiversität und Klima Forschungszentrum (BiK-F) (1)
- Center for Financial Studies (CFS) (1)
- House of Finance (HoF) (1)
- Institut für Ökologie, Evolution und Diversität (1)
- Medizin (1)
- Sustainable Architecture for Finance in Europe (SAFE) (1)
- Wirtschaftswissenschaften (1)
In relativistic heavy-ion collisions, a global spin polarization, PH, of Λ and Λ¯ hyperons along the direction of the system angular momentum was discovered and measured across a broad range of collision energies and demonstrated a trend of increasing PH with decreasing sNN−−−√. A splitting between Λ and Λ¯ polarization may be possible due to their different magnetic moments in a late-stage magnetic field sustained by the quark-gluon plasma which is formed in the collision. The results presented in this study find no significant splitting at the collision energies of sNN−−−√=19.6 and 27 GeV in the RHIC Beam Energy Scan Phase II using the STAR detector, with an upper limit of PΛ¯−PΛ<0.24% and PΛ¯−PΛ<0.35%, respectively, at a 95% confidence level. We derive an upper limit on the naïve extraction of the late-stage magnetic field of B<9.4⋅1012 T and B<1.4⋅1013 T at sNN−−−√=19.6 and 27 GeV, respectively, although more thorough derivations are needed. Differential measurements of PH were performed with respect to collision centrality, transverse momentum, and rapidity. With our current acceptance of |y|<1 and uncertainties, we observe no dependence on transverse momentum and rapidity in this analysis. These results challenge multiple existing model calculations following a variety of different assumptions which have each predicted a strong dependence on rapidity in this collision-energy range.
We report a new measurement of the production of electrons from open heavy-flavor hadron decays (HFEs) at mid-rapidity (|y|< 0.7) in Au+Au collisions at sNN−−−√=200 GeV. Invariant yields of HFEs are measured for the transverse momentum range of 3.5<pT<9 GeV/c in various configurations of the collision geometry. The HFE yields in head-on Au+Au collisions are suppressed by approximately a factor of 2 compared to that in p+p collisions scaled by the average number of binary collisions, indicating strong interactions between heavy quarks and the hot and dense medium created in heavy-ion collisions. Comparison of these results with models provides additional tests of theoretical calculations of heavy quark energy loss in the quark-gluon plasma.
Density fluctuations near the QCD critical point can be probed via an intermittency analysis in relativistic heavy-ion collisions. We report the first measurement of intermittency in Au+Au collisions at √sNN = 7.7-200 GeV measured by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The scaled factorial moments of identified charged hadrons are analyzed at mid-rapidity and within the transverse momentum phase space. We observe a power-law behavior of scaled factorial moments in Au+Au collisions and a decrease in the extracted scaling exponent (ν) from peripheral to central collisions. The ν is consistent with a constant for different collisions energies in the mid-central (10-40%) collisions. Moreover, the ν in the 0-5% most central Au+Au collisions exhibits a non-monotonic energy dependence that reaches a minimum around √sNN = 27 GeV. The physics implications on the QCD phase structure are discussed.
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.
A decisive experimental test of the Chiral Magnetic Effect (CME) is considered one of the major scientific goals at the Relativistic Heavy-Ion Collider (RHIC) towards understanding the nontrivial topological fluctuations of the Quantum Chromodynamics vacuum. In heavy-ion collisions, the CME is expected to result in a charge separation phenomenon across the reaction plane, whose strength could be strongly energy dependent. The previous CME searches have been focused on top RHIC energy collisions. In this Letter, we present a low energy search for the CME in Au+Au collisions at sNN−−−√=27 GeV. We measure elliptic flow scaled charge-dependent correlators relative to the event planes that are defined at both mid-rapidity |η|<1.0 and at forward rapidity 2.1<|η|<5.1. We compare the results based on the directed flow plane (Ψ1) at forward rapidity and the elliptic flow plane (Ψ2) at both central and forward rapidity. The CME scenario is expected to result in a larger correlation relative to Ψ1 than to Ψ2, while a flow driven background scenario would lead to a consistent result for both event planes. In 10-50\% centrality, results using three different event planes are found to be consistent within experimental uncertainties, suggesting a flow driven background scenario dominating the measurement. We obtain an upper limit on the deviation from a flow driven background scenario at the 95\% confidence level. This work opens up a possible road map towards future CME search with the high statistics data from the RHIC Beam Energy Scan Phase-II.
The linear and mode-coupled contributions to higher-order anisotropic flow are presented for Au+Au collisions at √sN N = 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 initialstate 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 η s (T ,μB ).
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.
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.
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.
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.
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.
Investigation of the linear and mode-coupled flow harmonics in Au+Au collisions at √sNN = 200 GeV
(2020)
Flow harmonics (vn) of the Fourier expansion for the azimuthal distributions of hadrons are commonly employed to quantify the azimuthal anisotropy of particle production relative to the collision symmetry planes. While lower order Fourier coefficients (v2 and v3) are more directly related to the corresponding eccentricities of the initial state, the higher-order flow harmonics (vn>3) can be induced by a modecoupled response to the lower-order anisotropies, in addition to a linear response to the same-order anisotropies. These higher-order flow harmonics and their linear and mode-coupled contributions can be used to more precisely constrain the initial conditions and the transport properties of the medium in theoretical models. The multiparticle azimuthal cumulant method is used to measure the linear and mode-coupled contributions in the higher-order anisotropic flow, the mode-coupled response coefficients, and the correlations of the event plane angles for charged particles as functions of centrality and transverse momentum in Au+Au collisions at nucleon-nucleon center-of-mass energy √sN N= 200 GeV. The results are compared to similar LHC measurements as well as to several viscous hydrodynamic calculations with varying initial conditions.
Measurement of groomed jet substructure observables in p+p collisions at √s = 200 GeV with STAR
(2020)
In this letter, measurements of the shared momentum fraction (zg) and the groomed jet radius (Rg), as defined in the SoftDrop algorithm, are reported in p+p collisions at √s = 200 GeV collected by the STAR experiment. These substructure observables are differentially measured for jets of varying resolution parameters from R = 0.2 − 0.6 in the transverse momentum range 15 < pT,jet < 60 GeV/c. These studies show that, in the pT,jet range accessible at √s = 200 GeV and with increasing jet resolution parameter and jet transverse momentum, the zg distribution asymptotically converges to the DGLAP splitting kernel for a quark radiating a gluon. The groomed jet radius measurements reflect a momentum-dependent narrowing of the jet structure for jets of a given resolution parameter, i.e., the larger the pT,jet, the narrower the first splitting. For the first time, these fully corrected measurements are compared to Monte Carlo generators with leading order QCD matrix elements and leading log in the parton shower, and to state-of-the-art theoretical calculations at next-to-leading-log accuracy. We observe that PYTHIA 6 with parameters tuned to reproduce RHIC measurements is able to quantitatively describe data, whereas PYTHIA 8 and HERWIG 7, tuned to reproduce LHC data, are unable to provide a simultaneous description of both zg and Rg, resulting in opportunities for fine parameter tuning of these models for p+p collisions at RHIC energies. We also find that the theoretical calculations without non-perturbative corrections are able to qualitatively describe the trend in data for jets of large resolution parameters at high pT,jet, but fail at small jet resolution parameters and low jet transverse momenta.
We report results on the total and elastic cross sections in proton-proton collisions at √s = 200 GeV obtained with the Roman Pot setup of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The elastic differential cross section was measured in the squared four-momentum transfer range 0.045 ≤ −t ≤ 0.135 GeV2. The value of the exponential slope parameter B of the elastic differential cross section dσ/dt ∼ e−Bt in the measured −t range was found to be B = 14.32 ± 0.09(stat.)+0.13 −0.28(syst.) GeV−2. The total cross section σtot, obtained from extrapolation of the dσ/dt to the optical point at −t = 0, is σtot = 54.67 ± 0.21(stat.)+1.28 −1.38(syst.) mb. We also present the values of the elastic cross section σel = 10.85 ± 0.03(stat.)+0..49 −0.41(syst.) mb, the elastic cross section integrated within the STAR t-range σ det el = 4.05 ± 0.01(stat.)+0.18−0.17(syst.) mb, and the inelastic cross section σinel = 43.82 ± 0.21(stat.)+1.37−1.44(syst.) mb. The results are compared with the world data
We report a high precision measurement of the transverse single spin asymmetry AN at the center of mass energy √s=200 GeV in elastic proton–proton scattering by the STAR experiment at RHIC. The AN was measured in the four-momentum transfer squared t range 0.003⩽|t|⩽0.035 (GeV/c)2, the region of a significant interference between the electromagnetic and hadronic scattering amplitudes. The measured values of AN and its t-dependence are consistent with a vanishing hadronic spin-flip amplitude, thus providing strong constraints on the ratio of the single spin-flip to the non-flip amplitudes. Since the hadronic amplitude is dominated by the Pomeron amplitude at this √s, we conclude that this measurement addresses the question about the presence of a hadronic spin flip due to the Pomeron exchange in polarized proton–proton elastic scattering.
STAR's measurements of directed flow (v1) around midrapidity for π±, K±, K0S, p and p¯ in Au + Au collisions at $\sqrtsNN = 200$ GeV are presented. A negative v1(y) slope is observed for most of produced particles (π±, K±, K0S and p¯). The proton v1(y) slope is found to be much closer to zero compared to antiprotons. A sizable difference is seen between v1 of protons and antiprotons in 5-30% central collisions. The v1 excitation function is presented. Comparisons to model calculations (RQMD, UrQMD, AMPT, QGSM with parton recombination, and a hydrodynamics model with a tilted source) are made. Anti-flow alone cannot explain the centrality dependence of the difference between the v1(y) slopes of protons and antiprotons.
STAR's measurements of directed flow (v1) around midrapidity for π±, K±, K0S, p and p¯ in Au + Au collisions at $\sqrtsNN = 200$ GeV are presented. A negative v1(y) slope is observed for most of produced particles (π±, K±, K0S and p¯). In 5-30% central collisions a sizable difference is present between the v1(y) slope of protons and antiprotons, with the former being consistent with zero within errors. The v1 excitation function is presented. Comparisons to model calculations (RQMD, UrQMD, AMPT, QGSM with parton recombination, and a hydrodynamics model with a tilted source) are made. For those models which have calculations of v1 for both pions and protons, none of them can describe v1(y) for pions and protons simultaneously. The hydrodynamics model with a tilted source as currently implemented cannot explain the centrality dependence of the difference between the v1(y) slopes of protons and antiprotons.
Rapidity-odd directed flow measurements at midrapidity are presented for Λ, Λ¯, K±, K0s and ϕ at sNN−−−−√= 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV in Au+Au collisions recorded by the STAR detector at the Relativistic Heavy Ion Collider. These measurements greatly expand the scope of data available to constrain models with differing prescriptions for the equation of state of quantum chromodynamics. Results show good sensitivity for testing a picture where flow is assumed to be imposed before hadron formation and the observed particles are assumed to form via coalescence of constituent quarks. The pattern of departure from a coalescence-inspired sum-rule can be a valuable new tool for probing the collision dynamics.
Fluctuations of conserved quantities such as baryon number, charge, and strangeness are sensitive to the correlation length of the hot and dense matter created in relativistic heavy-ion collisions and can be used to search for the QCD critical point. We report the first measurements of the moments of net-kaon multiplicity distributions in Au+Au collisions at √sNN = 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4, and 200 GeV. The collision centrality and energy dependence of the mean (M), variance (σ 2), skewness (S), and kurtosis (κ) for net-kaon multiplicity distributions as well as the ratio σ 2/M and the products Sσ and κσ 2 are presented. Comparisons are made with Poisson and negative binomial baseline calculations as well as with UrQMD, a transport model (UrQMD) that does not include effects from the QCD critical point. Within current uncertainties, the net-kaon cumulant ratios appear to be monotonic as a function of collision energy.
Transverse spin transfer to Λ and ¯Λ hyperons in polarized proton-proton collisions at √𝑠=200 GeV
(2018)
The transverse spin transfer from polarized protons to Λ and Λ¯ hyperons is expected to provide sensitivity to the transversity distribution of the nucleon and to the transversely polarized fragmentation functions. We report the first measurement of the transverse spin transfer to Λ and Λ¯ along the polarization direction of the fragmenting quark, DTT, in transversely polarized proton-proton collisions at s√=200GeV with the STAR detector at RHIC. The data correspond to an integrated luminosity of 18pb−1 and cover the pseudorapidity range |η|<1.2 and transverse momenta pT up to 8GeV/c. The dependence on pT and η are presented. The DTT results are found to be comparable with a model prediction, and are also consistent with zero within uncertainties.