Inclusive charged hadron elliptic flow in Au + Au collisions at sNN\sqrt{s_{NN}} = 7.7 - 39 GeV

A systematic study is presented for centrality, transverse momentum ($p_T$) and pseudorapidity ($\eta$) dependence of the inclusive charged hadron elliptic flow ($v_2$) at midrapidity($|\eta| < 1.0$) in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 19.6, 27 and 39 GeV. The results obtained with different methods, including correlations with the event plane reconstructed in a region separated by a large pseudorapidity gap and 4-particle cumulants ($v_2{4}$), are presented in order to investigate non-flow correlations and $v_2$ fluctuations. We observe that the difference between $v_2{2}$ and $v_2{4}$ is smaller at the lower collision energies. Values of $v_2$, scaled by the initial coordinate space eccentricity, $v_{2}/\varepsilon$, as a function of $p_T$ are larger in more central collisions, suggesting stronger collective flow develops in more central collisions, similar to the results at higher collision energies. These results are compared to measurements at higher energies at the Relativistic Heavy Ion Collider ($\sqrt{s_{NN}}$ = 62.4 and 200 GeV) and at the Large Hadron Collider (Pb + Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV). The $v_2(p_T)$ values for fixed $p_T$ rise with increasing collision energy within the $p_T$ range studied ($< 2 {\rm GeV}/c$). A comparison to viscous hydrodynamic simulations is made to potentially help understand the energy dependence of $v_{2}(p_{T})$. We also compare the $v_2$ results to UrQMD and AMPT transport model calculations, and physics implications on the dominance of partonic versus hadronic phases in the system created at Beam Energy Scan (BES) energies are discussed.Comment: 20 pages, 12 figures. Version accepted by PR

Observation of the antimatter helium-4 nucleus

High-energy nuclear collisions create an energy density similar to that of the universe microseconds after the Big Bang, and in both cases, matter and antimatter are formed with comparable abundance. However, the relatively short-lived expansion in nuclear collisions allows antimatter to decouple quickly from matter, and avoid annihilation. Thus, a high energy accelerator of heavy nuclei is an efficient means of producing and studying antimatter. The antimatter helium-4 nucleus ($^4\bar{He}$), also known as the anti-{\alpha} ($\bar{\alpha}$), consists of two antiprotons and two antineutrons (baryon number B=-4). It has not been observed previously, although the {\alpha} particle was identified a century ago by Rutherford and is present in cosmic radiation at the 10% level. Antimatter nuclei with B < -1 have been observed only as rare products of interactions at particle accelerators, where the rate of antinucleus production in high-energy collisions decreases by about 1000 with each additional antinucleon. We present the observation of the antimatter helium-4 nucleus, the heaviest observed antinucleus. In total 18 $^4\bar{He}$ counts were detected at the STAR experiment at RHIC in 10$^9$ recorded Au+Au collisions at center-of-mass energies of 200 GeV and 62 GeV per nucleon-nucleon pair. The yield is consistent with expectations from thermodynamic and coalescent nucleosynthesis models, which has implications beyond nuclear physics.Comment: 19 pages, 4 figures. Submitted to Nature. Under media embarg

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

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

Growth of Long Range Forward-Backward Multiplicity Correlations with Centrality in Au+Au Collisions at sNN\sqrt{s_{NN}} = 200 GeV

Forward-backward multiplicity correlation strengths have been measured with the STAR detector for Au+Au and $\textit{p+p}$ collisions at $\sqrt{s_{NN}}$ = 200 GeV. Strong short and long range correlations (LRC) are seen in central Au+Au collisions. The magnitude of these correlations decrease with decreasing centrality until only short range correlations are observed in peripheral Au+Au collisions. Both the Dual Parton Model (DPM) and the Color Glass Condensate (CGC) predict the existence of the long range correlations. In the DPM the fluctuation in the number of elementary (parton) inelastic collisions produces the LRC. In the CGC longitudinal color flux tubes generate the LRC. The data is in qualitative agreement with the predictions from the DPM and indicates the presence of multiple parton interactions.Comment: 6 pages, 3 figures The abstract has been slightly modifie

Strangeness Enhancement in Cu+Cu and Au+Au Collisions at \sqrt{s_{NN}} = 200 GeV

We report new STAR measurements of mid-rapidity yields for the $\Lambda$, $\bar{\Lambda}$, $K^{0}_{S}$, $\Xi^{-}$, $\bar{\Xi}^{+}$, $\Omega^{-}$, $\bar{\Omega}^{+}$ particles in Cu+Cu collisions at \sNN{200}, and mid-rapidity yields for the $\Lambda$, $\bar{\Lambda}$, $K^{0}_{S}$ particles in Au+Au at \sNN{200}. We show that at a given number of participating nucleons, the production of strange hadrons is higher in Cu+Cu collisions than in Au+Au collisions at the same center-of-mass energy. We find that aspects of the enhancement factors for all particles can be described by a parameterization based on the fraction of participants that undergo multiple collisions

K/pi Fluctuations at Relativistic Energies

We report results for $K/\pi$ fluctuations from Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6, 62.4, 130, and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider. Our results for $K/\pi$ fluctuations in central collisions show little dependence on the incident energies studied and are on the same order as results observed by NA49 at the Super Proton Synchrotron in central Pb+Pb collisions at $\sqrt{s_{NN}}$ = 12.3 and 17.3 GeV. We also report results for the collision centrality dependence of $K/\pi$ fluctuations as well as results for $K^{+}/\pi^{+}$, $K^{-}/\pi^{-}$, $K^{+}/\pi^{-}$, and $K^{-}/\pi^{+}$ fluctuations. We observe that the $K/\pi$ fluctuations scale with the multiplicity density, $dN/d\eta$, rather than the number of participating nucleons.Comment: 6 pages, 4 figure

Forward Neutral Pion Transverse Single Spin Asymmetries in p+p Collisions at \sqrt{s}=200 GeV

We report precision measurements of the Feynman-x dependence, and first measurements of the transverse momentum dependence, of transverse single spin asymmetries for the production of \pi^0 mesons from polarized proton collisions at \sqrt{s}=200 GeV. The x_F dependence of the results is in fair agreement with perturbative QCD model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the p_T dependence at fixed x_F are not consistent with pQCD-based calculations.Comment: 6 pages, 4 figure

Scales of processes involving binary light-heavy ions

Foram realizadas medidas de seções de choque para processos onde ocorre a emissão binária de fragmentos complexos, utilizando técnicas de coincidência cinemática, para o sistema 16O + 10B no intervalo de 22 MeV < EC.M. < 24.5 MeV com EC.M. ~ 20 KeV e 17 MeV < EC.M. < 25 MeV com EC.M. ~ 190 KeV e para o sistema 19F + 12C em 22 MeV < EC.M. < 24.5 MeV com EC.M. ~ 20 KeV. As funções de excitação obtidas apresentam fortes flutuações. As larguras médias destas flutuações foram obtidas através da análise das funções de correlação em energia. Para o sistema 16O + 10B foram obtidas larguras da ordem de 25 KeV e 350 KeV. Para o sistema 19F + 12C obteve-se larguras da ordem de 40 KeV. As larguras menores, para ambos os sistemas, apresentam comportamento aproximadamente constante, tanto em função da energia de excitação dos fragmentos no canal de saída quanto em função do ângulo de emissão destes fragmentos. Estas larguras sugerem que o tempo de vida do sistema intermediário, da ordem de 2 x 10-20 s, seja muito superior ao seu tempo de rotação, condizente com o previsto para um mecanismo de fusão-fissão. A largura maior obtida para o sistema 16O + 10B apresenta, no canal de espalhamento elástico, uma dependência clara com o ângulo de espalhamento. Esta largura, assim como a dependência angular observada, sugere que o tempo de vida do sistema intermediário seja levemente superior ao tempo de revolução do mesmo. Estes fatos sugerem que o mecanismo responsável por estas flutuações seja o \"orbiting\" nuclear. Foram feitas comparações com modelos onde há a formação de núcleo composto e \"orbiting\" nuclear e os resultados obtidos são consistentes com os dados experimentais.Binary decay cross section measurements have been performed for the system 16O + 10B at 22 MeV < EC.M. < 24.5 MeV (EC.M. ~ 20 KeV) and 17 MeV < EC.M. < 25 MeV (EC.M. ~ 190 KeV ) and for the system 19F + 12C at 22 MeV < EC.M. < 24.5 MeV (EC.M. ~ 20 KeV). The binary fragments were identified by the kinematic coincidence technique. The excitation functions for these systems show strong fluctuations and their average widths were obtained through the analysis of energy correlation functions. The small widths observed for the 16O + 10B (about 20 KeV) and 19F + 12C (about 40 KeV) systems show a constant behavior with the fragments excitation energy and scattering angle. These widths correspond to an intermediate system time scale of about 2 x 10-20 s which is larger than its revolution time. These features suggest the presence of the fusion-fission mechanism, where the compound nucleus spends a lot of time to acquire a relaxed form and thermal equilibrium. For the 16O + 10B system, larger widths (about 350 KeV) were also observed which are related to a faster process. These widths show a clear dependence with the scattering angle in the elastic scattering channel. These aspects are expected for a process where the time scale is comparable to the intermediate system revolution time, like an orbiting mechanism. Theoretical predictions for fusion-fission and orbiting mechanisms were compared to the experimental results and a good agreement was observed

Scales of processes involving binary light-heavy ions

Foram realizadas medidas de seções de choque para processos onde ocorre a emissão binária de fragmentos complexos, utilizando técnicas de coincidência cinemática, para o sistema 16O + 10B no intervalo de 22 MeV < EC.M. < 24.5 MeV com EC.M. ~ 20 KeV e 17 MeV < EC.M. < 25 MeV com EC.M. ~ 190 KeV e para o sistema 19F + 12C em 22 MeV < EC.M. < 24.5 MeV com EC.M. ~ 20 KeV. As funções de excitação obtidas apresentam fortes flutuações. As larguras médias destas flutuações foram obtidas através da análise das funções de correlação em energia. Para o sistema 16O + 10B foram obtidas larguras da ordem de 25 KeV e 350 KeV. Para o sistema 19F + 12C obteve-se larguras da ordem de 40 KeV. As larguras menores, para ambos os sistemas, apresentam comportamento aproximadamente constante, tanto em função da energia de excitação dos fragmentos no canal de saída quanto em função do ângulo de emissão destes fragmentos. Estas larguras sugerem que o tempo de vida do sistema intermediário, da ordem de 2 x 10-20 s, seja muito superior ao seu tempo de rotação, condizente com o previsto para um mecanismo de fusão-fissão. A largura maior obtida para o sistema 16O + 10B apresenta, no canal de espalhamento elástico, uma dependência clara com o ângulo de espalhamento. Esta largura, assim como a dependência angular observada, sugere que o tempo de vida do sistema intermediário seja levemente superior ao tempo de revolução do mesmo. Estes fatos sugerem que o mecanismo responsável por estas flutuações seja o \"orbiting\" nuclear. Foram feitas comparações com modelos onde há a formação de núcleo composto e \"orbiting\" nuclear e os resultados obtidos são consistentes com os dados experimentais.Binary decay cross section measurements have been performed for the system 16O + 10B at 22 MeV < EC.M. < 24.5 MeV (EC.M. ~ 20 KeV) and 17 MeV < EC.M. < 25 MeV (EC.M. ~ 190 KeV ) and for the system 19F + 12C at 22 MeV < EC.M. < 24.5 MeV (EC.M. ~ 20 KeV). The binary fragments were identified by the kinematic coincidence technique. The excitation functions for these systems show strong fluctuations and their average widths were obtained through the analysis of energy correlation functions. The small widths observed for the 16O + 10B (about 20 KeV) and 19F + 12C (about 40 KeV) systems show a constant behavior with the fragments excitation energy and scattering angle. These widths correspond to an intermediate system time scale of about 2 x 10-20 s which is larger than its revolution time. These features suggest the presence of the fusion-fission mechanism, where the compound nucleus spends a lot of time to acquire a relaxed form and thermal equilibrium. For the 16O + 10B system, larger widths (about 350 KeV) were also observed which are related to a faster process. These widths show a clear dependence with the scattering angle in the elastic scattering channel. These aspects are expected for a process where the time scale is comparable to the intermediate system revolution time, like an orbiting mechanism. Theoretical predictions for fusion-fission and orbiting mechanisms were compared to the experimental results and a good agreement was observed

Indications of Conical Emission of Charged Hadrons at the BNL Relativistic Heavy Ion Collider

Three-particle azimuthal correlation measurements with a high transverse momentum trigger particle are reported for pp, d+Au, and Au+Au collisions at 200 GeV by the STAR experiment. Dijet structures are observed in pp, d+Au and peripheral Au+Au collisions. An additional structure is observed in central Au+Au data, signaling conical emission of correlated charged hadrons. The conical emission angle is found to be 1.37 +- 0.02(stat) +0.06-0.07(syst), independent of pt.Comment: 7 pages, 4 figures, 1 tabl