We report a high precision measurement of the transverse single spin asymmetry A N at the center of mass energy s = 200 GeV in elastic protonaproton scattering by the STAR experiment at RHIC. The A N ...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 A N 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 protonaproton elastic scattering.
We report a high precision measurement of the transverse single spin asymmetry $A_N$ at $\sqrt{s}=200$ GeV in elastic proton-proton scattering by the STAR experiment at RHIC. The $A_N$ was measured ...in the four-momentum transfer $t$ range $0.003 \leqslant |t| \leqslant 0.035$ $\GeVcSq$, the region of a significant interference between the electromagnetic and hadronic scattering amplitudes. The measured values of $A_N$ and its $t$-dependence are consistent with the absence of a 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 $\sqrt{s}$, 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.
We present first measurements of the evolution of the differential transverse momentum correlation function, C, with collision centrality in Au + Au interactions at inline image. This observable ...exhibits a strong dependence on collision centrality that is qualitatively similar to that of number correlations previously reported. We use the observed longitudinal broadening of the near-side peak of C with increasing centrality to estimate the ratio of the shear viscosity to entropy density, I./s, of the matter formed in central Au + Au interactions. We obtain an upper limit estimate of I./s that suggests that the produced medium has a small viscosity per unit entropy.
We report transverse momentum ($p_{T} \leq15$ GeV/$c$) spectra of $\pi^{\pm}$, $K^{\pm}$, $p$, $\bar{p}$, $K_{S}^{0}$, and $\rho^{0}$ at mid-rapidity in p+p and Au+Au collisions at $\sqrt{s_{_{NN}}}$ ...= 200 GeV. Perturbative QCD calculations are consistent with $\pi^{\pm}$ spectra in p+p collisions but do not reproduce $K$ and $p(\bar{p})$ spectra. The observed decreasing antiparticle-to-particle ratios with increasing $p_T$ provide experimental evidence for varying quark and gluon jet contributions to high-$p_T$ hadron yields. The relative hadron abundances in Au+Au at $p_{T}{}^{>}_{\sim}8$ GeV/$c$ are measured to be similar to the p+p results, despite the expected Casimir effect for parton energy loss.
The contribution of B meson decays to non-photonic electrons, which are mainly produced by the semi-leptonic decays of heavy flavor mesons, in p+p collisions at $\sqrt{s} =$ 200 GeV has been measured ...using azimuthal correlations between non-photonic electrons and hadrons. The extracted B decay contribution is approximately 50% at a transverse momentum of $p_{T} \geq 5$ GeV/c. These measurements constrain the nuclear modification factor for electrons from B and D meson decays. The result indicates that B meson production in heavy ion collisions is also suppressed at high pT even under the extreme case for the ratio of B to D contributions to non-photonic electrons.
The general concepts in the critical phenomena related with the notions of “scaling” and “universality” are considered. Behavior of various systems near a phase transition is displayed. Search for ...clear signatures of the phase transition of the nuclear matter and location of the critical point in heavy-ion collisions (
HIC
) is discussed. The experimental data on inclusive spectra measured in
HIC
at
RHIC
and
SPS
over a wide range of energies
s
N N
1/2
= 9–200 GeV are analyzed in the framework of
z
-scaling. A microscopic scenario of the constituent interactions is presented. Dependence of the energy loss on the momentum of the produced hadron, energy and centrality of the collision is studied. Self-similarity of the constituent interactions described in terms of momentum fractions is used to characterize the nuclear medium by “specific heat” and colliding nuclei by fractal dimensions. Preferable kinematical regions to search for signatures of the phase transition of the nuclear matter produced in
HIC
are discussed. Discontinuity of the “specific heat” is assumed to be a signature of the phase transition and the critical point.
We present the first measurements of identified hadron production, azimuthal anisotropy, and pion interferometry from Au+Au collisions below the nominal injection energy at the BNL Relativistic ...Heavy-Ion Collider (RHIC) facility. The data were collected using the large acceptance solenoidal tracker at RHIC (STAR) detector at {radical}s{sub NN} = 9.2 GeV from a test run of the collider in the year 2008. Midrapidity results on multiplicity density dN/dy in rapidity y, average transverse momentum p{sub T}, particle ratios, elliptic flow, and Hanbury-Brown-Twiss (HBT) radii are consistent with the corresponding results at similar {radical}s{sub NN} from fixed-target experiments. Directed flow measurements are presented for both midrapidity and forward-rapidity regions. Furthermore the collision centrality dependence of identified particle dN/dy, p{sub T}, and particle ratios are discussed. These results also demonstrate that the capabilities of the STAR detector, although optimized for {radical}s{sub NN} = 200 GeV, are suitable for the proposed QCD critical-point search and exploration of the QCD phase diagram at RHIC.
We present the results of an elliptic flow, v{sub 2}, analysis of Cu+Cu collisions recorded with the solenoidal tracker detector (STAR) at the BNL Relativistic Heavy Ion Collider at {radical}s{sub ...NN} = 62.4 and 200 GeV. Elliptic flow as a function of transverse momentum, v{sub 2}(p{sub T}), is reported for different collision centralities for charged hadrons h{sup {+-}} and strangeness-ontaining hadrons K{sub S}{sup 0}, {Lambda}, {Xi}, and {phi} in the midrapidity region |{eta}| < 1.0. Significant reduction in systematic uncertainty of the measurement due to nonflow effects has been achieved by correlating particles at midrapidity, |{eta}| < 1.0, with those at forward rapidity, 2.5 < |{eta}| < 4.0. We also present azimuthal correlations in p+p collisions at {radical}s = 200 GeV to help in estimating nonflow effects. To study the system-size dependence of elliptic flow, we present a detailed comparison with previously published results from Au+Au collisions at {radical}s{sub NN} = 200 GeV. We observe that v{sub 2}(p{sub T}) of strange hadrons has similar scaling properties as were first observed in Au+Au collisions, that is, (i) at low transverse momenta, p{sub T} < 2 GeV/c, v{sub 2} scales with transverse kinetic energy, m{sub T}-m, and (ii) at intermediate p{sub T}, 2 < p{sub T} < 4 GeV/c, it scales with the number of constituent quarks, n{sub q}. We have found that ideal hydrodynamic calculations fail to reproduce the centrality dependence of v{sub 2}(p{sub T}) for K{sub S}{sup 0} and {Lambda}. Eccentricity scaled v{sub 2} values, v{sub 2}/{var_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 that v{sub 2}/{var_epsilon} depends on the system size, that is, the number of participants N{sub part}. This indicates that the ideal hydrodynamic limit is not reached in Cu+Cu collisions, presumably because the assumption of thermalization is not attained.
We report new results on identified (anti)proton and charged pion spectra at large transverse momenta (3 < p{sub T} < 10 GeV/c) from Cu+Cu collisions at {radical}s{sub 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-p{sub 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 for a detailed exploration of the onset 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.
Measurements of the differential cross-section and the transverse single-spin asymmetry, A_N, vs. x_F for pi0 and eta mesons are reported for 0.4 < x_F < 0.75 at an average pseudorapidity of 3.68. A ...data sample of approximately 6.3 pb^{-1} was analyzed, which was recorded during p+p collisions at sqrt{s} = 200 GeV by the STAR experiment at RHIC. The average transverse beam polarization was 56%. The cross-section for pi0 is consistent with a perturbative QCD prediction, and the eta/pi0 cross-section ratio agrees with previous mid-rapidity measurements. For 0.55 < x_F < 0.75, A_N for eta (0.210 +- 0.056) is 2.2 standard deviations larger than A_N for pi0 (0.081 +- 0.016).
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