The PHENIX experiment at RHIC has measured the centrality dependence of the direct photon yield from Au + Au collisions at root s(NN) = 200 GeV down to pT = 0.4 GeV/c. Photons are detected via photon ...conversions to e(+)e(-) pairs and an improved technique is applied that minimizes the systematic uncertainties that usually limit direct photon measurements, in particular at low pT. We find an excess of direct photons above the N-coll-scaled yield measured in p + p collisions. This excess yield is well described by an exponential distribution with an inverse slope of about 240 MeV/c in the pT range 0.6-2.0 GeV/c. While the shape of the pT distribution is independent of centrality within the experimental uncertainties, the yield increases rapidly with increasing centrality, scaling approximately with N-part(alpha), where alpha = 1.38 +/- 0.03(stat) +/- 0.07(syst).
In this paper, we present the first measurement of elliptic (v2) and triangular (v3) flow in high-multiplicity 3He+Aucollisions at √sNN=200 GeV. Two-particle correlations, where the particles have a ...large separation in pseudorapidity, are compared in 3He+Au and in p+p collisions and indicate that collective effects dominate the second and third Fourier components for the correlations observed in the 3He+Ausystem. The collective behavior is quantified in terms of elliptic v2 and triangular v3 anisotropy coefficients measured with respect to their corresponding event planes. The v2 values are comparable to those previously measured in d+Au collisions at the same nucleon-nucleon center-of-mass energy. Comparisons with various theoretical predictions are made, including to models where the hot spots created by the impact of the three 3He nucleons on the Au nucleus expand hydrodynamically to generate the triangular flow. Finally, the agreement of these models with data may indicate the formation of low-viscosity quark-gluon plasma even in these small collision systems.
Jet production rates are measured in p+p and d+Au collisions at $\sqrt{sNN}$=200 GeV recorded in 2008 with the PHENIX detector at the Relativistic Heavy Ion Collider. Jets are reconstructed using the ...R = 0.3 anti-kt algorithm from energy deposits in the electromagnetic calorimeter and charged tracks in multi-wire proportional chambers, and the jet transverse momentum (pT ) spectra are corrected for the detector response. Spectra are reported for jets with 12 < pT < 50 GeV/c, within a pseudorapidity acceptance of |η|< 0.3. The nuclear-modification factor (RdAu) values for 0%-100% d+Au events are found to be consistent with unity, constraining the role of initial state effects on jet production. However, the centrality-selected RdAu values and central-to-peripheral ratios (RCP) show large, pT -dependent deviations from unity, challenging the conventional models that relate hard-process rates and soft-particle production in collisions involving nuclei.
We 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 ow 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 > 1:5 GeV/c, the data diverges from ampt calculations of v2 relative to the initial geometry, indicating the possible dominance of nongeometry related corre- lations, referred to as nonflow. We also present measurements of the charged-particle multiplicity (dNch/d ) 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/d over all in the PHENIX acceptance. At √sNN = 62:4, and 39 GeV, v2 scales with dNch/d at midrapidity and forward rapidity, but falls o at backward rapidity. Furthermore, this departure from the dNch/dη scaling may be a further indication of non ow effects dominating at backward rapidity.
Here, we present measurements of e(+)e-production at midrapidity in Au + Au collisions at √sNN = 200 GeV. The invariant yield is studied within the PHENIX detector acceptance over a wide range of ...mass (mee < 5 GeV/c2) and pair transverse momentum (pT < 5 GeV/c) for minimum bias and for five centrality classes. We compare the e+e- yield to the expectations from known sources. In the low-mass region (mee = 0.30-0.76 GeV/c2) there is an enhancement that increases with centrality and is distributed over the entire pair pT range measured. It is significantly smaller than previously reported by the PHENIX experiment and amounts to 2.3 ± 0.4(stat) ± 0.4(syst) ± 0.2(model) or to 1.7 ± 0.3(stat) ± 0.3(syst) ± 0.2(model) for minimum bias collisions when the open heavy-flavor contribution is calculated with PYTHIA or MC@NLO, respectively. The inclusive mass and pT distributions, as well as the centrality dependence, are well reproduced by model calculations where the enhancement mainly originates from the melting of the ρ meson resonance as the system approaches chiral symmetry restoration. Finally, in the intermediate-mass region (mee = 1.2-2.8 GeV/c2), the data hint at a significant contribution in addition to the yield from the semileptonic decays of heavy-flavor mesons.
We present results for three charmonia states (ψ′, χc, and J/ψ) in d+Au collisions at |y|<0.35 and sNN−−−√=200 GeV. We find that the modification of the ψ′ yield relative to that of the J/ψ scales ...approximately with charged particle multiplicity at midrapidity across p+A, d+Au, and A+A results from the Super Proton Synchrotron and the Relativistic Heavy Ion Collider. In large-impact-parameter collisions we observe a similar suppression for the ψ′ and J/ψ, while in small-impact-parameter collisions the more weakly bound ψ′ is more strongly suppressed. Owing to the short time spent traversing the Au nucleus, the larger ψ′ suppression in central events is not explained by an increase of the nuclear absorption owing to meson formation time effects.
We present measurements of the cross section and double-helicity asymmetry ALL of direct-photon production in p→+p→ collisions at s=510 GeV. The measurements have been performed at midrapidity (|η|
Suppression of the J/ψ 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 ψ(2S) state in p+A and d+A collisions suggested the presence of final-state effects. Results of J/ψ and ψ(2S) measurements in the dimuon decay channel are presented here for p+p, p+Al, and p+Au collision systems at sNN=200GeV. The results are predominantly shown in the form of the nuclear-modification factor, RpA, the ratio of the ψ(2S) invariant yield per nucleon-nucleon collision in collisions of proton on target nucleus to that in p+p collisions. Measurements of the J/ψ and ψ(2S) nuclear-modification factor are compared with shadowing and transport-model predictions, as well as to complementary measurements at Large Hadron Collider energies.
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