28 research outputs found
Disentangling centrality bias and final-state effects in the production of high- using direct in Au collisions at GeV
International audiencePHENIX presents a simultaneous measurement of the production of direct and in Au collisions at GeV over a range of 7.5 to 18 GeV/ for different event samples selected by event activity, i.e. charged-particle multiplicity detected at forward rapidity. Direct-photon yields are used to empirically estimate the contribution of hard-scattering processes in the different event samples. Using this estimate, the average nuclear-modification factor is , consistent with unity for minimum-bias (MB) Au events. For event classes with moderate event activity, is consistent with the MB value within 5% uncertainty. These results confirm that the previously observed enhancement of high- production found in small-system collisions with low event activity is a result of a bias in interpreting event activity within the Glauber framework. In contrast, for the top 5% of events with the highest event activity, is suppressed by 20% relative to the MB value with a significance of , which may be due to final-state effects
Measurement of nuclear modification at backward and forward rapidity in , Al, and Au collisions at GeV
International audienceSuppression of the nuclear-modification factor has been seen as a trademark signature of final-state effects in large collision systems for decades. In small systems, the nuclear modification was attributed to cold-nuclear-matter effects until the observation of strong differential suppression of the state in collisions suggested the presence of final-state effects. Results of and measurements in the dimuon decay channel are presented here for , Al, and Au collision systems at GeV. The results are predominantly shown in the form of the nuclear-modification factor, , the ratio of the invariant yield per nucleon-nucleon collision in collisions of proton on target nucleus to that in collisions. Measurements of the and nuclear-modification factor are compared with shadowing and transport-model predictions, as well as to complementary measurements at Large-Hadron-Collider energies
Transverse-single-spin asymmetries of charged pions at midrapidity in transversely polarized collisions at GeV
International audienceIn 2015, the PHENIX Collaboration has measured single-spin asymmetries for charged pions in transversely polarized p+p collisions at the center-of-mass energy of s=200 GeV. The pions were detected at central rapidities of |η|<0.35. The single-spin asymmetries are consistent with zero for each charge individually, as well as consistent with the previously published neutral-pion asymmetries in the same rapidity range. However, they show a slight indication of charge-dependent differences which may suggest a flavor dependence in the underlying mechanisms that create these asymmetries
Improving constraints on gluon spin-momentum correlations in transversely polarized protons via midrapidity open-heavy-flavor electrons in collisions at GeV
Polarized proton-proton collisions provide leading-order access to gluons, presenting an opportunity to constrain gluon spin-momentum correlations within transversely polarized protons and enhance our understanding of the three-dimensional structure of the proton. Midrapidity open-heavy-flavor production at GeV is dominated by gluon-gluon fusion, providing heightened sensitivity to gluon dynamics relative to other production channels. Transverse single-spin asymmetries of electrons and positrons from heavy-flavor hadron decays are measured at midrapidity using the PHENIX detector at the Relativistic Heavy Ion Collider. These charge-separated measurements are sensitive to gluon correlators that can in principle be related to gluon orbital angular momentum via model calculations. Explicit constraints on gluon correlators are extracted for two separate models, one of which had not been constrained previously
Transverse single-spin asymmetry of charged hadrons at forward and backward rapidity in polarized +, +Al, and +Au collisions at GeV
International audienceReported here are transverse single-spin asymmetries () in the production of charged hadrons as a function of transverse momentum () and Feynman- () in polarized +, +Al, and +Au collisions at GeV. The measurements have been performed at forward and backward rapidity () in + collisions, whereas the +Al and +Au results show smaller asymmetries. This finding provides new opportunities to investigate the origin of transverse single-spin asymmetries and a tool to study nuclear effects in + collisions
Transverse single spin asymmetries of forward neutrons in , Al and Au collisions at GeV as a function of transverse and longitudinal momenta
International audienceIn 2015 the PHENIX collaboration at the Relativistic Heavy Ion Collider recorded p+p, p+Al, and p+Au collision data at center of mass energies of sNN=200 GeV with the proton beam(s) transversely polarized. At very forward rapidities η>6.8 relative to the polarized proton beam, neutrons were detected either inclusively or in (anti)correlation with detector activity related to hard collisions. The resulting single spin asymmetries, that were previously reported, have now been extracted as a function of the transverse momentum of the neutron as well as its longitudinal momentum fraction xF. The explicit kinematic dependence, combined with the correlation information allows for a closer look at the interplay of different mechanisms suggested to describe these asymmetries, such as hadronic interactions or electromagnetic interactions in ultraperipheral collisions, UPC. Events that are correlated with a hard collision indeed display a mostly negative asymmetry that increases in magnitude as a function of transverse momentum with only little dependence on xF. In contrast, events that are not likely to have emerged from a hard collision display positive asymmetries for the nuclear collisions with a kinematic dependence that resembles that of a UPC based model. Because the UPC interaction depends strongly on the charge of the nucleus, those effects are very small for p+p collisions, moderate for p+Al collisions, and large for p+Au collisions
Transverse single-spin asymmetry of midrapidity and mesons in +Au and +Al collisions at 200 GeV
International audiencePresented are the first measurements of the transverse single-spin asymmetries () for neutral pions and eta mesons in +Au and +Al collisions at GeV in the pseudorapidity range 0.35 with the PHENIX detector at the Relativistic Heavy Ion Collider. The asymmetries are consistent with zero, similar to those for midrapidity neutral pions and eta mesons produced in + collisions. These measurements show no evidence of additional effects that could potentially arise from the more complex partonic environment present in proton-nucleus collisions
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
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