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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.
Quark interactions with topological gluon configurations can induce chirality imbalance and local parity violation in quantum chromodynamics. This can lead to electric charge separation along the strong magnetic field in relativistic heavy-ion collisions – the chiral magnetic effect (CME). We report measurements by the STAR collaboration of a CME-sensitive observable in p + Au and d + Au collisions at 200 GeV, where the CME is not expected, using charge-dependent pair correlations relative to a third particle. We observe strong charge-dependent correlations similar to those measured in heavy-ion collisions. This bears important implications for the interpretation of the heavy-ion data.
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
The transversity distribution, which describes transversely polarized quarks in transversely polarized nucleons, is a fundamental component of the spin structure of the nucleon, and is only loosely constrained by global fits to existing semi-inclusive deep inelastic scattering (SIDIS) data. In transversely polarized p↑+p collisions it can be accessed using transverse polarization dependent fragmentation functions which give rise to azimuthal correlations between the polarization of the struck parton and the final state scalar mesons.This letter reports on spin dependent di-hadron correlations measured by the STAR experiment. The new dataset corresponds to 25 pb−1 integrated luminosity of p↑+p collisions at s=500 GeV, an increase of more than a factor of ten compared to our previous measurement at s=200 GeV. Non-zero asymmetries sensitive to transversity are observed at a Q2 of several hundred GeV and are found to be consistent with the former measurement and a model calculation. We expect that these data will enable an extraction of transversity with comparable precision to current SIDIS datasets but at much higher momentum transfers where subleading effects are suppressed.
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. 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 d𝜎∕dt agrees well with the results obtained at √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 𝜎f id el = 462.1 ± 0.9(stat.) ± 1.1(syst.) ± 11.6(scale) 𝜇b.
We report the measurement of K∗0 meson at midrapidity (|y|< 1.0) in Au+Au collisions at sNN−−−√~=~7.7, 11.5, 14.5, 19.6, 27 and 39 GeV collected by the STAR experiment during the RHIC beam energy scan (BES) program. The transverse momentum spectra, yield, and average transverse momentum of K∗0 are presented as functions of collision centrality and beam energy. The K∗0/K yield ratios are presented for different collision centrality intervals and beam energies. The K∗0/K ratio in heavy-ion collisions are observed to be smaller than that in small system collisions (e+e and p+p). The K∗0/K ratio follows a similar centrality dependence to that observed in previous RHIC and LHC measurements. The data favor the scenario of the dominance of hadronic re-scattering over regeneration for K∗0 production in the hadronic phase of the medium.
We report a measurement of cumulants and correlation functions of event-by-event proton multiplicity distributions from fixed-target Au+Au collisions at sNN−−−√ = 3 GeV measured by the STAR experiment. Protons are identified within the rapidity (y) and transverse momentum (pT) region −0.9<y<0 and 0.4<pT<2.0 GeV/c in the center-of-mass frame. A systematic analysis of the proton cumulants and correlation functions up to sixth-order as well as the corresponding ratios as a function of the collision centrality, pT, and y are presented. The effect of pileup and initial volume fluctuations on these observables and the respective corrections are discussed in detail. The results are compared to calculations from the hadronic transport UrQMD model as well as a hydrodynamic model. In the most central 5\% collisions, the value of proton cumulant ratio C4/C2 is negative, drastically different from the values observed in Au+Au collisions at higher energies. Compared to model calculations including Lattice QCD, a hadronic transport model, and a hydrodynamic model, the strong suppression in the ratio of C4/C2 at 3 GeV Au+Au collisions indicates an energy regime dominated by hadronic interactions.
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.
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.
Azimuthal anisotropy measurement of (multi-)strange hadrons in Au+Au collisions at √sNN = 54.4 GeV
(2023)
Azimuthal anisotropy of produced particles is one of the most important observables used to access the collective properties of the expanding medium created in relativistic heavy-ion collisions. In this paper, we present second (v2) and third (v3) order azimuthal anisotropies of K0S, ϕ, Λ, Ξ and Ω at mid-rapidity (|y|<1) in Au+Au collisions at sNN−−−√ = 54.4 GeV measured by the STAR detector. The v2 and v3 are measured as a function of transverse momentum and centrality. Their energy dependence is also studied. v3 is found to be more sensitive to the change in the center-of-mass energy than v2. Scaling by constituent quark number is found to hold for v2 within 10%. This observation could be evidence for the development of partonic collectivity in 54.4 GeV Au+Au collisions. Differences in v2 and v3 between baryons and anti-baryons are presented, and ratios of v3/v3/22 are studied and motivated by hydrodynamical calculations. The ratio of v2 of ϕ mesons to that of anti-protons (v2(ϕ)/v2(p¯)) shows centrality dependence at low transverse momentum, presumably resulting from the larger effects from hadronic interactions on anti-proton v2.
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.
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.
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.
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.
We report the beam energy and collision centrality dependence of fifth and sixth order cumulants (C5, C6) and factorial cumulants (κ5, κ6) of net-proton and proton distributions, from sNN−−−−√=3−200 GeV Au+Au collisions at RHIC. The net-proton cumulant ratios generally follow the hierarchy expected from QCD thermodynamics, except for the case of collisions at sNN−−−−√ = 3 GeV. C6/C2 for 0-40\% centrality collisions is increasingly negative with decreasing sNN−−−−√, while it is positive for the lowest sNN−−−−√ studied. These observed negative signs are consistent with QCD calculations (at baryon chemical potential, μB≤ 110 MeV) that include a crossover quark-hadron transition. In addition, for sNN−−−−√≥ 11.5 GeV, the measured proton κn, within uncertainties, does not support the two-component shape of proton distributions that would be expected from a first-order phase transition. Taken in combination, the hyper-order proton number fluctuations suggest that the structure of QCD matter at high baryon density, μB∼750 MeV (sNN−−−−√ = 3 GeV) is starkly different from those at vanishing μB∼20MeV (sNN−−−−√ = 200 GeV and higher).
We report on measurements of sequential Υ suppression in Au+Au collisions at sNN−−−√ = 200 GeV with the STAR detector at the Relativistic Heavy Ion Collider (RHIC) through both the dielectron and dimuon decay channels. In the 0-60% centrality class, the nuclear modification factors (RAA), which quantify the level of yield suppression in heavy-ion collisions compared to p+p collisions, for Υ(1S) and Υ(2S) are 0.40±0.03 (stat.)±0.03 (sys.)±0.09 (norm.) and 0.26±0.08 (stat.)±0.02 (sys.)±0.06 (norm.), respectively, while the upper limit of the Υ(3S) RAA is 0.17 at a 95% confidence level. This provides experimental evidence that the Υ(3S) is significantly more suppressed than the Υ(1S) at RHIC. The level of suppression for Υ(1S) is comparable to that observed at the much higher collision energy at the Large Hadron Collider. These results point to the creation of a medium at RHIC whose temperature is sufficiently high to strongly suppress excited Υ states.
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.
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.
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.
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.