137 research outputs found

    An Experimental Exploration of the QCD Phase Diagram: The Search for the Critical Point and the Onset of De-confinement

    Full text link
    The QCD phase diagram lies at the heart of what the RHIC Physics Program is all about. While RHIC has been operating very successfully at or close to its maximum energy for almost a decade, it has become clear that this collider can also be operated at lower energies down to 5 GeV without extensive upgrades. An exploration of the full region of beam energies available at the RHIC facility is imperative. The STAR detector, due to its large uniform acceptance and excellent particle identification capabilities, is uniquely positioned to carry out this program in depth and detail. The first exploratory beam energy scan (BES) run at RHIC took place in 2010 (Run 10), since several STAR upgrades, most importantly a full barrel Time of Flight detector, are now completed which add new capabilities important for the interesting physics at BES energies. In this document we discuss current proposed measurements, with estimations of the accuracy of the measurements given an assumed event count at each beam energy.Comment: 59 pages, 78 figure

    Observation of charge-dependent azimuthal correlations and possible local strong parity violation in heavy ion collisions

    Get PDF
    Parity-odd domains, corresponding to non-trivial topological solutions of the QCD vacuum, might be created during relativistic heavy-ion collisions. These domains are predicted to lead to charge separation of quarks along the orbital momentum of the system created in non-central collisions. To study this effect, we investigate a three particle mixed harmonics azimuthal correlator which is a \P-even observable, but directly sensitive to the charge separation effect. We report measurements of this observable using the STAR detector in Au+Au and Cu+Cu collisions at sNN\sqrt{s_{NN}}=200 and 62~GeV. The results are presented as a function of collision centrality, particle separation in rapidity, and particle transverse momentum. A signal consistent with several of the theoretical expectations is detected in all four data sets. We compare our results to the predictions of existing event generators, and discuss in detail possible contributions from other effects that are not related to parity violation.Comment: 17 pages, 14 figures, as accepted for publication in Physical Review C

    Studies of di-jet survival and surface emission bias in Au+Au collisions via angular correlations with respect to back-to-back leading hadrons

    Get PDF
    We report first results from an analysis based on a new multi-hadron correlation technique, exploring jet-medium interactions and di-jet surface emission bias at RHIC. Pairs of back-to-back high transverse momentum hadrons are used for triggers to study associated hadron distributions. In contrast with two- and three-particle correlations with a single trigger with similar kinematic selections, the associated hadron distribution of both trigger sides reveals no modification in either relative pseudo-rapidity or relative azimuthal angle from d+Au to central Au+Au collisions. We determine associated hadron yields and spectra as well as production rates for such correlated back-to-back triggers to gain additional insights on medium properties.Comment: By the STAR Collaboration. 6 pages, 2 figure

    Longitudinal Spin Transfer to Λ\Lambda and Λˉ\bar{\Lambda} Hyperons in Polarized Proton-Proton Collisions at s\sqrt{s} = 200 GeV

    Get PDF
    The longitudinal spin transfer, DLLD_{LL}, from high energy polarized protons to Λ\Lambda and Λˉ\bar{\Lambda} hyperons has been measured for the first time in proton-proton collisions at s=200GeV\sqrt{s} = 200 \mathrm{GeV} with the STAR detector at RHIC. The measurements cover pseudorapidity, η\eta, in the range η<1.2|\eta| < 1.2 and transverse momenta, pTp_\mathrm{T}, up to 4GeV/c4 \mathrm{GeV}/c. The longitudinal spin transfer is found to be DLL=0.03±0.13(stat)±0.04(syst)D_{LL}= -0.03\pm 0.13(\mathrm{stat}) \pm 0.04(\mathrm{syst}) for inclusive Λ\Lambda and DLL=0.12±0.08(stat)±0.03(syst)D_{LL} = -0.12 \pm 0.08(\mathrm{stat}) \pm 0.03(\mathrm{syst}) for inclusive Λˉ\bar{\Lambda} hyperons with =0.5 = 0.5 and =3.7GeV/c = 3.7 \mathrm{GeV}/c. The dependence on η\eta and pTp_\mathrm{T} is presented.Comment: 5 pages, 4 figure

    Inclusive pi^0, eta, and direct photon production at high transverse momentum in p+p and d+Au collisions at sqrt(s_NN) = 200 GeV

    Get PDF
    We report a measurement of high-p_T inclusive pi^0, eta, and direct photon production in p+p and d+Au collisions at sqrt(s_NN) = 200 GeV at midrapidity (0 gamma gamma were detected in the Barrel Electromagnetic Calorimeter of the STAR experiment at the Relativistic Heavy Ion Collider. The eta -> gamma gamma decay was also observed and constituted the first eta measurement by STAR. The first direct photon cross section measurement by STAR is also presented, the signal was extracted statistically by subtracting the pi^0, eta, and omega(782) decay background from the inclusive photon distribution observed in the calorimeter. The analysis is described in detail, and the results are found to be in good agreement with earlier measurements and with next-to-leading order perturbative QCD calculations.Comment: 28 pages, 30 figures, 6 tables, the updated version that was accepted by Phys. Rev.

    Charged and strange hadron elliptic flow in Cu+Cu collisions at sNN\sqrt{s_{NN}} = 62.4 and 200 GeV

    Get PDF
    We present the results of an elliptic flow analysis of Cu+Cu collisions recorded with the STAR detector at 62.4 and 200GeV. Elliptic flow as a function of transverse momentum is reported for different collision centralities for charged hadrons and strangeness containing hadrons KS0K_{S}^{0}, Λ\Lambda, Ξ\Xi, ϕ\phi in the midrapidity region eta<1.0|eta|<1.0. Significant reduction in systematic uncertainty of the measurement due to non-flow effects has been achieved by correlating particles at midrapidity, η<1.0|\eta|<1.0, with those at forward rapidity, 2.5<η<4.02.5<|\eta|<4.0. We also present azimuthal correlations in p+p collisions at 200 GeV to help estimating non-flow effects. To study the system-size dependence of elliptic flow, we present a detailed comparison with previously published results from Au+Au collisions at 200 GeV. We observe that v2v_{2}(pTp_{T}) of strange hadrons has similar scaling properties as were first observed in Au+Au collisions, i.e.: (i) at low transverse momenta, pT<2GeV/cp_T<2GeV/c, v2v_{2} scales with transverse kinetic energy, mTmm_{T}-m, and (ii) at intermediate pTp_T, 2<pT<4GeV/c2<p_T<4GeV/c, it scales with the number of constituent quarks, nqn_q. We have found that ideal hydrodynamic calculations fail to reproduce the centrality dependence of v2v_{2}(pTp_{T}) for KS0K_{S}^{0} and Λ\Lambda. Eccentricity scaled v2v_2 values, v2/ϵv_{2}/\epsilon, are larger in more central collisions, suggesting stronger collective flow develops in more central collisions. The comparison with Au+Au collisions which go further in density shows v2/ϵv_{2}/\epsilon depend on the system size, number of participants NpartN_{part}. This indicates that the ideal hydrodynamic limit is not reached in Cu+Cu collisions, presumably because the assumption of thermalization is not attained.Comment: 18 pages, 14 figure

    Measurements of Dihadron Correlations Relative to the Event Plane in Au+Au Collisions at sNN=200\sqrt{s_{NN}}=200 GeV

    Full text link
    Dihadron azimuthal correlations containing a high transverse momentum (\pt) trigger particle are sensitive to the properties of the nuclear medium created at RHIC through the strong interactions occurring between the traversing parton and the medium, i.e. jet-quenching. Previous measurements revealed a strong modification to dihadron azimuthal correlations in Au+Au collisions with respect to \pp\ and \dAu\ collisions. The modification increases with the collision centrality, suggesting a path-length dependence to the jet-quenching effect. This paper reports STAR measurements of dihadron azimuthal correlations in mid-central (20-60\%) Au+Au collisions at \snn=200~GeV as a function of the trigger particle's azimuthal angle relative to the event plane, \phis=|\phit-\psiEP|. The azimuthal correlation is studied as a function of both the trigger and associated particle \pt. The subtractions of the combinatorial background and anisotropic flow, assuming Zero Yield At Minimum (\zyam), are described. The away-side correlation is strongly modified, and the modification varies with \phis, which is expected to be related to the path-length that the away-side parton traverses. The pseudo-rapidity (\deta) dependence of the near-side correlation, sensitive to long range \deta correlations (the ridge), is also investigated. The ridge and jet-like components of the near-side correlation are studied as a function of \phis. The ridge appears to drop with increasing \phis while the jet-like component remains approximately constant. ...Comment: 50 pages, 39 figures, 6 table

    Identified high-pTp_{T} spectra in Cu+Cu collisions at sNN\sqrt{s_{NN}}=200 GeV

    Get PDF
    We report new results on identified (anti)proton and charged pion spectra at large transverse momenta (3<pTp_{T}<10 GeV/c) from Cu+Cu collisions at sNN\sqrt{s_{NN}}=200 GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). This study explores the system size dependence of two novel features observed at RHIC with heavy ions: the hadron suppression at high-pTp_{T} and the anomalous baryon to meson enhancement at intermediate transverse momenta. Both phenomena could be attributed to the creation of a new form of QCD matter. The results presented here bridge the system size gap between the available pp and Au+Au data, and allow the detailed exploration for the on-set of the novel features. Comparative analysis of all available 200 GeV data indicates that the system size is a major factor determining both the magnitude of the hadron spectra suppression at large transverse momenta and the relative baryon to meson enhancement.Comment: Submitted to Phys. Rev. C, 9 pages, 5 figure

    Studying Parton Energy Loss in Heavy-Ion Collisions via Direct-Photon and Charged-Particle Azimuthal Correlations

    Get PDF
    Charged-particle spectra associated with direct photon (γdir\gamma_{dir} ) and π0\pi^0 are measured in pp+pp and Au+Au collisions at center-of-mass energy sNN=200\sqrt{s_{_{NN}}}=200 GeV with the STAR detector at RHIC. A hower-shape analysis is used to partially discriminate between γdir\gamma_{dir} and π0\pi^0. Assuming no associated charged particles in the γdir\gamma_{dir} direction (near side) and small contribution from fragmentation photons (γfrag\gamma_{frag}), the associated charged-particle yields opposite to γdir\gamma_{dir} (away side) are extracted. At mid-rapidity (η<0.9|\eta|<0.9) in central Au+Au collisions, charged-particle yields associated with γdir\gamma_{dir} and π0\pi^0 at high transverse momentum (8<pTtrig<168< p_{T}^{trig}<16 GeV/cc) are suppressed by a factor of 3-5 compared with pp + pp collisions. The observed suppression of the associated charged particles, in the kinematic range η<1|\eta|<1 and 3<pTassoc<163< p_{T}^{assoc} < 16 GeV/cc, is similar for γdir\gamma_{dir} and π0\pi^0, and independent of the γdir\gamma_{dir} energy within uncertainties. These measurements indicate that the parton energy loss, in the covered kinematic range, is insensitive to the parton path length.Comment: submitted to Phys. Rev. Lett, 6 pages, 4 figure
    corecore