35 research outputs found
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Charm- and Bottom-Quark Production in AuAu Collisions at = 200 GeV
The invariant yield of electrons from open-heavy-flavor decays for GeV/ at midrapidity in AuAu collisions at = 200 GeV has been measured by the PHENIX experiment at the Relativistic Heavy Ion Collider. A displaced-vertex analysis with the PHENIX silicon-vertex detector enables extraction of the fraction of charm and bottom hadron decays and unfolding of the invariant yield of parent charm and bottom hadrons. The nuclear-modification factors for electrons from charm and bottom hadron decays and heavy-flavor hadrons show both a centrality and a quark-mass dependence, indicating suppression in the quark-gluon plasma produced in these collisions that is medium sized and quark-mass dependent
Charm- and Bottom-Quark Production in AuAu Collisions at = 200 GeV
The invariant yield of electrons from open-heavy-flavor decays for GeV/ at midrapidity in AuAu collisions at = 200 GeV has been measured by the PHENIX experiment at the Relativistic Heavy Ion Collider. A displaced-vertex analysis with the PHENIX silicon-vertex detector enables extraction of the fraction of charm and bottom hadron decays and unfolding of the invariant yield of parent charm and bottom hadrons. The nuclear-modification factors for electrons from charm and bottom hadron decays and heavy-flavor hadrons show both a centrality and a quark-mass dependence, indicating suppression in the quark-gluon plasma produced in these collisions that is medium sized and quark-mass dependent
Measurement of charged pion double spin asymmetries at midrapidity in longitudinally polarized collisions at = 510 GeV
International audienceThe PHENIX experiment at the Relativistic Heavy Ion Collider has measured the longitudinal double spin asymmetries, ALL, for charged pions at midrapidity (|η|<0.35) in longitudinally polarized p+p collisions at s=510ââGeV. These measurements are sensitive to the gluon spin contribution to the total spin of the proton in the parton momentum fraction x range between 0.04 and 0.09. One can infer the sign of the gluon polarization from the ordering of pion asymmetries with charge alone. The asymmetries are found to be consistent with global quantum-chromodynamics fits of deep-inelastic scattering and data at s=200ââGeV, which show a nonzero positive contribution of gluon spin to the proton spin
and production at forward rapidity in + collisions at GeV
International audienceThe PHENIX experiment at the Relativistic Heavy Ion Collider has measured the differential cross section, mean transverse momentum, mean transverse momentum squared of inclusive J/Ï, and cross section ratio of Ï(2S) to J/Ï at forward rapidity in p+p collisions at s=510ââGeV via the dimuon decay channel. Comparison is made to inclusive J/Ï cross sections measured at s=200ââGeV and 2.76â13 TeV. The result is also compared to leading-order nonrelativistic QCD calculations coupled to a color-glass-condensate description of the low-x gluons in the proton at low transverse momentum (pT) and to next-to-leading order nonrelativistic QCD calculations for the rest of the pT range. These calculations overestimate the data at low pT. While consistent with the data within uncertainties above â3ââGeV/c, the calculations are systematically below the data. The total cross section times the branching ratio is BR dÏppJ/Ï/dy(1.2<|y|<2.2,0<pT<10ââGeV/c)=54.3±0.5(stat)±5.5(syst)âânb
Measurement of at forward and backward rapidity in , l, u, and HeAu collisions at
International audienceCharmonium is a valuable probe in heavy-ion collisions to study the properties of the quark gluon plasma, and is also an interesting probe in small collision systems to study cold nuclear matter effects, which are also present in large collision systems. With the recent observations of collective behavior of produced particles in small system collisions, measurements of the modification of charmonium in small systems have become increasingly relevant. We present the results of J/Ï measurements at forward and backward rapidity in various small collision systems, p+p, p+Al, p+Au, and He3+Au, at sNN=200 GeV. The results are presented in the form of the observable RAB, the nuclear modification factor, a measure of the ratio of the J/Ï invariant yield compared to the scaled yield in p+p collisions. We examine the rapidity, transverse momentum, and collision centrality dependence of nuclear effects on J/Ï production with different projectile sizes p and He3, and different target sizes Al and Au. The modification is found to be strongly dependent on the target size, but to be very similar for p+Au and He3+Au. However, for 0%â20% central collisions at backward rapidity, the modification factor for He3+Au is found to be smaller than that for p+Au, with a mean fit to the ratio of 0.89±0.03(stat)±0.08(syst), possibly indicating final state effects due to the larger projectile size
Nonprompt direct-photon production in AuAu collisions at GeV
The measurement of the direct-photon spectrum from AuAu collisions at GeV is presented by the PHENIX collaboration using the external-photon-conversion technique for 0%--93% central collisions in a transverse-momentum () range of 0.8--10 GeV/. An excess of direct photons, above prompt-photon production from hard-scattering processes, is observed for GeV/. Nonprompt direct photons are measured by subtracting the prompt component, which is estimated as -scaled direct photons from collisions at 200 GeV, from the direct-photon spectrum. Results are obtained for GeV/ and suggest that the spectrum has an increasing inverse slope from to 0.4 GeV/ with increasing , which indicates a possible sensitivity of the measurement to photons from earlier stages of the evolution of the collision. In addition, like the direct-photon production, the -integrated nonprompt direct-photon yields also follow a power-law scaling behavior as a function of collision-system size. The exponent, , for the nonprompt component is found to be consistent with 1.1 with no apparent dependence
Study of -meson production in Al, Au, Au, and HeAu collisions at GeV
Small nuclear collisions are mainly sensitive to cold-nuclear-matter effects; however, the collective behavior observed in these collisions shows a hint of hot-nuclear-matter effects. The identified-particle spectra, especially the mesons which contain strange and antistrange quarks and have a relatively small hadronic-interaction cross section, are a good tool to study these effects. The PHENIX experiment has measured mesons in a specific set of small collision systems Al, Au, and HeAu, as well as Au [Phys. Rev. C {\bf 83}, 024909 (2011)], at GeV. The transverse-momentum spectra and nuclear-modification factors are presented and compared to theoretical-model predictions. The comparisons with different calculations suggest that quark-gluon plasma may be formed in these small collision systems at GeV. However, the volume and the lifetime of the produced medium may be insufficient for observing strangeness-enhancement and jet-quenching effects. Comparison with calculations suggests that the main production mechanisms of mesons at midrapidity may be different in Al versus HeAu collisions at GeV. While thermal quark recombination seems to dominate in HeAu collisions, fragmentation seems to be the main production mechanism in Al collisions
Measurement of Direct-Photon Cross Section and Double-Helicity Asymmetry at GeV in Collisions
We present the measurement of the cross section and double-helicity asymmetry of direct-photon production in collisions at GeV. The measurement has been performed at midrapidity () with the PHENIX detector at the Relativistic Heavy Ion Collider. Direct photons are dominantly produced by the quark-gluon scattering at relativistic energies. Direct photons are produced from the initial partonic hard scattering and do not interact via the strong force. Therefore, this measurement provides a clean and direct access to the gluons in the polarized proton in the gluon-momentum-fraction range