We report precision measurements of hypernuclei $^3_ΛH$ and $^4_ΛH$ lifetimes obtained from Au + Au collisions at $\sqrt{sNN}$ = 3.0 GeV and 7.2 GeV collected by the STAR experiment at the ...Relativistic Heavy Ion Collider, and the first measurement of $^3_ΛH$ and $^4_ΛH$ midrapidity yields in Au + Au collisions at $\sqrt{sNN}$ = 3.0 GeV. $^3_ΛH$ and $^4_ΛH$, 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 $^3_ΛH$ and 218 ± 6 (stat) ± 13 (syst) ps for $^4_ΛH$. The pT-integrated yields of $^3_ΛH$ and $^4_ΛH$ are presented in different centrality and rapidity intervals. It is observed that the shape of the rapidity distribution of $4_ΛH$ is different for 0%–10% and 10%–50% centrality collisions. Thermal model calculations, using the canonical ensemble for strangeness, describes the $^3_ΛH$ yield well, while underestimating the $^4_ΛH$ 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 $^3_ΛH$ and $^4_ΛH$ 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 chiral magnetic effect (CME) refers to charge separation along a strong magnetic field due to imbalanced chirality of quarks in local parity and charge-parity violating domains in quantum ...chromodynamics. The experimental measurement of the charge separation is made difficult by the presence of a major background from elliptic azimuthal anisotropy. This background and the CME signal have different sensitivities to the spectator and participant planes, and could thus be determined by measurements with respect to these planes. We report such measurements in Au+Au collisions at a nucleon-nucleon center-of-mass energy of 200 GeV at the Relativistic Heavy-Ion Collider. It is found that the charge separation, with the flow background removed, is consistent with zero in peripheral (large impact parameter) collisions. Some indication of finite CME signals is seen with a significance of 1–3 standard deviations in mid-central (intermediate impact parameter) collisions. Furthermore, significant residual background effects may, however, still be present.
We report on the W and Z/γ∗ differential and total cross sections as well as the W+/W− and (W+ + W−)/(Z/γ∗) cross section ratios measured by the STAR experiment at RHIC in p + p collisions at √s = ...500 GeV and 510 GeV. The cross sections and their ratios are sensitive to quark and antiquark parton distribution functions. In particular, at leading order, the W cross section ratio is sensitive to the d/u ratio. These measurements were taken at high Q2 ∼ M2W, M2Z and can serve as input into global analyses to provide constraints on the sea quark distributions. The results presented here combine three STAR datasets from 2011, 2012, and 2013, accumulating an integrated luminosity of 350 pb−1. We also assess the expected impact that our W+ / W− cross section ratios will have on various quark distributions, and find sensitivity to the u − d and d/u distributions.
The STAR collaboration reports a measurement of the transverse single-spin asymmetries, AN , for neutral pions produced in polarized proton collisions with protons ( pp ), with aluminum nuclei ( pAl ...) and with gold nuclei ( pAu ) at a nucleon-nucleon center-of-mass energy of 200 GeV. Neutral pions are observed in the forward direction relative to the transversely polarized proton beam, in the pseudorapidity region 2.7<η<3.8 . Results are presented for π0 s observed in the STAR forward meson spectrometer electromagnetic calorimeter in narrow Feynman x ( xF ) and transverse momentum ( pT ) bins, spanning the range 0.17<xF<0.81 and 1.7<pT<6.0 GeV/c . For fixed xF<0.47 , the asymmetries are found to rise with increasing transverse momentum. For larger xF , the asymmetry flattens or falls as pT increases. Parametrizing the ratio r ( A ) ≡AN ( pA ) / AN ( pp ) =AP over the kinematic range, the ratio r ( A ) is found to depend only weakly on A , with ( P ) =-0.027±0.005 . No significant difference in P is observed between the low- pT region, pT<2.5 GeV/c , where gluon saturation effects may play a role, and the high- pT region, pT>2.5 GeV/c . It is further observed that the value of AN is significantly larger for events with a large- pT isolated π0 than for events with a nonisolated π0 accompanied by additional jetlike fragments. The nuclear dependence r ( A ) is similar for isolated and nonisolated π0 events.
We report a new measurement of the production cross section for inclusive electrons from open heavy-flavor hadron decays as a function of transverse momentum (pT) at midrapidity (|y| < 0.7) in p + p ...collisions at √s = 200 GeV. Overall, the result is presented for 2.5 < PT < 10 GeV/c with an improved precision above 6 GeV/c with respect to the previous measurements, providing more constraints on perturbative QCD calculations. Moreover, this measurement also provides a high-precision reference for measurements of nuclear modification factors for inclusive electrons from open-charm and -bottom hadron decays in heavy-ion collisions.
The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-π0s produced at forward pseudorapidities (2.6<η<4.0) in p+p, p+Al, and p+Au collisions at a center-of-mass ...energy of 200 GeV. We observe a clear suppression of the correlated yields of back-to-back π0 pairs in p+Al and p+Au collisions compared to the p+p data. The observed suppression of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. Furthermore, the larger suppression found in p+Au relative to p+Al collisions exhibits a dependence of the saturation scale $Q^{2}_{s}$ on the mass number A . A linear scaling of the suppression with A1/3 is observed with a slope of -0.09±0.01.
In this work, we report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at $\sqrt{s_{NN}}$ =3.0 GeV, measured by the STAR experiment in the kinematic ...acceptance of rapidity (y) and transverse momentum (pT) within -0.5 < y < 0 and 0.4 < pT < 2.0 GeV/c. In the most central 0–5% collisions, a proton cumulant ratio is measured to be C4/C2 = –0:85 ± 0:09 (stat:)±0:82 (syst:), which is 2σ below the Poisson baseline with respect to both the statistical and systematic uncertainties. The hadronic transport UrQMD model reproduces our C4/C2 in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in C4/C2 is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3 GeV.
Global polarization of Ξ and Ω hyperons has been measured for the first time in Au+Au collisions at $\sqrt{s_{NN}}$=200 GeV. The measurements of the $Ξ^{-}$ and $\overline{Ξ}^{+}$ hyperon ...polarization have been performed by two independent methods, via analysis of the angular distribution of the daughter particles in the parity violating weak decay Ξ→Λ+π, as well as by measuring the polarization of the daughter Λ hyperon, polarized via polarization transfer from its parent. The polarization, obtained by combining the results from the two methods and averaged over $Ξ^{-}$ and $\overline{Ξ}^{+}$, is measured to be $\langle{P_{Ξ}}\rangle$=0.47±0.10(stat)±0.23(syst)% for the collision centrality 20%-80%. The $\langle{P_{Ξ}}\rangle$ is found to be slightly larger than the inclusive Λ polarization and in reasonable agreement with a multiphase transport model. The $\langle{P_{Ξ}}\rangle$ is found to follow the centrality dependence of the vorticity predicted in the model, increasing toward more peripheral collisions. Finally, the global polarization of Ω, $\langle{P_{Ω}}\rangle$=1.11±0.87(stat)±1.97(syst)% was obtained by measuring the polarization of daughter Λ in the decay Ω→Λ+K, assuming the polarization transfer factor $C_{ΩΛ}=1$.
We report on new measurements of inclusive J/ψ polarization at mid-rapidity in p+p collisions at √s = 200 GeV by the STAR experiment at RHIC. The polarization parameters, λθ, λφ, and λθφ, are ...measured as a function of transverse momentum (pT) in both the Helicity and CollinsSoper (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 non-relativistic 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 new measurements of inclusive J/ψ polarization at mid-rapidity in p+p collisions at $\sqrt{s}$ = 200 GeV by the STAR experiment at RHIC. The polarization parameters, λθ, λφ, and λθφ, are ...measured as a function of transverse momentum (pT) in both the Helicity and CollinsSoper (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 non-relativistic QCD gives the best overall description of the experimental results, even though other models cannot be ruled out due to experimental uncertainties.
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