92 research outputs found
Measurements of azimuthal anisotropy and charged-particle multiplicity in Au collisions at 200, 62.4, 39, and 19.6 GeV
International audienceWe present measurements of the elliptic flow (v2) as a function of transverse momentum (pT), pseudorapidity (η), and centrality in d+Au collisions at sNN=200, 62.4, 39, and 19.6 GeV. The beam-energy scan of d+Au collisions provides a testing ground for the onset of flow signatures in small collision systems. We measure a nonzero v2 signal at all four collision energies, which, at midrapidity and low pT, is consistent with predictions from viscous hydrodynamic models. Comparisons with calculations from parton transport models (based on the ampt Monte Carlo generator) show good agreement with the data at midrapidity to forward (d-going) rapidities and low pT. At backward (Au-going) rapidities and pT>1.5GeV/c, the data diverges from ampt calculations of v2 relative to the initial geometry, indicating the possible dominance of nongeometry related correlations, referred to as nonflow. We also present measurements of the charged-particle multiplicity (dNch/dη) as a function of η in central d+Au collisions at the same energies. We find that in d+Au collisions at sNN=200 GeV the v2 scales with dNch/dη over all η in the PHENIX acceptance. At sNN=62.4, and 39 GeV, v2 scales with dNch/dη at midrapidity and forward rapidity, but falls off at backward rapidity. This departure from the dNch/dη scaling may be a further indication of nonflow effects dominating at backward rapidity
Measurements of pairs from open heavy flavor and Drell-Yan in collisions at GeV
International audiencePHENIX reports differential cross sections of ΌΌ pairs from semileptonic heavy-flavor decays and the Drell-Yan production mechanism measured in p+p collisions at s=200ââGeV at forward and backward rapidity (1.2<|η|<2.2). The ΌΌ pairs from ccÂŻ, bbÂŻ, and Drell-Yan are separated using a template fit to unlike- and like-sign muon pair spectra in mass and pT. The azimuthal opening angle correlation between the muons from ccÂŻ and bbÂŻ decays and the pair-pT distributions are compared to distributions generated using pythia and powheg models, which both include next-to-leading order processes. The measured distributions for pairs from ccÂŻ are consistent with pythia calculations. The ccÂŻ data present narrower azimuthal correlations and softer pT distributions compared to distributions generated from powheg. The bbÂŻ data are well described by both models. The extrapolated total cross section for bottom production is 3.75±0.24(stat)±0.500.35(syst)±0.45(global)ââ[ÎŒb], which is consistent with previous measurements at the Relativistic Heavy Ion Collider in the same system at the same collision energy and is approximately a factor of 2 higher than the central value calculated with theoretical models. The measured Drell-Yan cross section is in good agreement with next-to-leading-order quantum-chromodynamics calculations
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
Nonperturbative transverse-momentum-dependent effects in dihadron and direct photon-hadron angular correlations in collisions at GeV
International audienceDihadron and isolated direct photon-hadron angular correlations are measured in p+p collisions at s=200ââGeV. The correlations are sensitive to nonperturbative initial-state and final-state transverse momenta kT and jT in the azimuthal nearly back-to-back region ÎÏâŒÏ. To have sensitivity to small transverse momentum scales, nonperturbative momentum widths of pout, the out-of-plane transverse-momentum component perpendicular to the trigger particle, are measured. In this region, the evolution of pout can be studied when several different hard scales are measured. These widths are used to investigate possible effects from transverse-momentum-dependent factorization breaking. When accounting for the longitudinal-momentum fraction of the away-side hadron with respect to the near-side trigger particle, the widths are found to increase with the hard scale; this is qualitatively similar to the observed behavior in Drell-Yan and semi-inclusive deep-inelastic scattering interactions, where factorization is predicted to hold. The momentum widths are also studied as a function of center-of-mass energy by comparing to previous measurements at s=510ââGeV. The nonperturbative jet widths also appear to increase with s at a similar xT, which is qualitatively consistent to similar measurements in Drell-Yan interactions. Future detailed global comparisons between measurements of processes where transverse-momentum-dependent factorization is predicted to hold and be broken will provide further insight into the role of color in hadronic interactions
Correlations of , , and pairs in + collisions at GeV and implications for and production mechanisms
PHENIX has measured the azimuthal correlations of muon pairs from charm and bottom semi-leptonic decays in + collisions at GeV, using a novel analysis technique utilizing both unlike- and like-sign muon pairs to separate charm, bottom and Drell-Yan contributions. The dimuon measurements combined with the previous electron-muon and dielectron measurements span a wide range in rapidity, and are well described by PYTHIA Tune A. Through a Bayesian analysis based on PYTHIA Tune A, we show that leading order pair creation is the dominant contribution for production, whereas the data favor the scenario in which next-to-leading-order processes dominate production. The small contribution of next-to-leading-order processes in production at the collision energies of the Relativistic Heavy Ion Collider contrasts with the case at Large-Hadron-Collider energies, where next-to-leading-order processes are expected to dominate
Single-spin asymmetry of production in , Al, and Au collisions with transversely polarized proton beams at GeV
International audienceWe report the transverse single-spin asymmetries of J/Ï production at forward and backward rapidity, 1.2<|y|<2.2, as a function of J/Ï transverse momentum (pT) and Feynman-x (xF). The data analyzed were recorded by the PHENIX experiment at the Relativistic Heavy Ion Collider in 2015 from p+p, p+Al, and p+Au collisions with transversely polarized proton beams at sNN=200ââGeV. At this collision energy, single-spin asymmetries for heavy-flavor particle production of p+p collisions provide access to the spin-dependent gluon distribution and higher-twist correlation functions inside the nucleon, such as the gluon Qiu-Sterman and trigluon correlation functions. Proton+nucleus collisions offer an excellent opportunity to study nuclear effects on the correlation functions. The data indicate a positive asymmetry at the two-standard-deviation level in the p+p data for 2ââGeV/c<pT<10ââGeV/c at backward rapidity and negative asymmetries at the two-standard-deviation level in the p+Au data for pT<2ââGeV/c at both forward and backward rapidity, while in p+Al collisions the asymmetries are consistent with zero within the range of experimental uncertainties
Transverse-single-spin asymmetries of charged pions at midrapidity in transversely polarized collisions at ââGeV
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=200ââGeV. 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
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 midrapidity and mesons in polarized collisions at GeV
International audienceWe present a measurement of the transverse single-spin asymmetry for and mesons in collisions in the pseudorapidity range and at a center-of-mass energy of 200 GeV with the PHENIX detector at the Relativistic Heavy Ion Collider. In comparison with previous measurements in this kinematic region, these results have a factor of 3 smaller uncertainties. As hadrons, and mesons are sensitive to both initial- and final-state nonperturbative effects for a mix of parton flavors. Comparisons of the differences in their transverse single-spin asymmetries have the potential to disentangle the possible effects of strangeness, isospin, or mass. These results can constrain the twist-3 trigluon collinear correlation function as well as the gluon Sivers function
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