Measurements of azimuthal anisotropy and charged-particle multiplicity in d + Au collisions at √sNN = 200, 62.4, 39, and 19.6 GeV

We present measurements of the elliptic flow (v(2)) as a function of transverse momentum (p(T)), pseudorapidity (eta), and centrality in d + Au collisions at root s(NN) = 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 v(2) signal at all four collision energies, which, at midrapidity and low p(T), 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 p(T). At backward (Au-going) rapidities and p(T) \u3e 1.5GeV/c, the data diverges from AMPT calculations of v(2) 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 (d N-ch/d eta) as a function of eta in central d + Au collisions at the same energies. We find that in d + Au collisions at root s(NN) = 200 GeV the v(2) scales with d N-ch/d eta over all eta in the PHENIX acceptance. At root s(NN) = 62.4, and 39 GeV, v(2) scales with d N-ch/d eta at midrapidity and forward rapidity, but falls off at backward rapidity. This departure from the d N-ch/d eta scaling may be a further indication of nonflow effects dominating at backward rapidity

Measurements of double-helicity asymmetries in inclusive J/Psi production in longitudinally polarized p plus p collisions at root s=510 GeV

We report the double-helicity asymmetry, A(LL)(J/Psi), in inclusive J/Psi production at forward rapidity as a function of transverse momentum p(T) and rapidity |y|. The data analyzed were taken during root s = 510 GeV longitudinally polarized p + p collisions at the Relativistic Heavy Ion Collider in the 2013 run using the PHENIX detector. At this collision energy, J/Psi particles are predominantly produced through gluon-gluon scatterings, thus A(LL)(J/Psi) is sensitive to the gluon polarization inside the proton. We measured A(LL)(J/Psi) by detecting the decay daughter muon pairs mu(+)mu(-) within the PHENIX muon spectrometers in the rapidity range 1.2 \u3c |y| \u3c 2.2. In this kinematic range, we measured the A(LL)(J/Psi) to be 0.012 +/- 0.010 (stat) +/- 0.003 (syst). The A(LL)(J.Psi) can be expressed to be proportional to the product of the gluon polarization distributions at two distinct ranges of Bjorken x: one at moderate range x approximate to 5 x 10(-2) where recent data of jet and pi(0) double helicity spin asymmetries have shown evidence for significant gluon polarization, and the other one covering the poorly known small-x region x approximate to 2 x 10(-3). Thus our new results could be used to further constrain the gluon polarization for x \u3c 5 x 10(-2)

Measurement of long-range angular correlations and azimuthal anisotropies in high-multiplicity p plus Au collisions at root S-NN=200 GeV

We present measurements of long-range angular correlations and the transverse momentum dependence of elliptic flow v(2) in high-multiplicity p + Au collisions at root S-NN = 200 GeV. A comparison of these results to previous measurements in high-multiplicity d + Au and He-3+Au collisions demonstrates a relation between v(2) and the initial collision eccentricity epsilon(2), suggesting that the observed momentum-space azimuthal anisotropies in these small systems have a collective origin and reflect the initial geometry. Good agreement is observed between the measured v(2) and hydrodynamic calculations for all systems, and an argument disfavoring theoretical explanations based on initial momentum-space domain correlations is presented. The set of measurements presented here allows us to leverage the distinct intrinsic geometry of each of these systems to distinguish between different theoretical descriptions of the long-range correlations observed in small collision systems

Nonperturbative-transverse-momentum broadening in dihadron angular correlations in √sNN = 200 GeV proton-nucleus collisions

The PHENIX collaboration has measured high-pT dihadron correlations in p+p, p+Al, and p+Au collisions at √sNN=200 GeV. The correlations arise from inter- and intrajet correlations and thus have sensitivity to nonperturbative effects in both the initial and final states. The distributions of pout, the transverse-momentum component of the associated hadron perpendicular to the trigger hadron, are sensitive to initial- and final-state transverse momenta. These distributions are measured multidifferentially as a function of xE, the longitudinal momentum fraction of the associated hadron with respect to the trigger hadron. The near-side pout widths, sensitive to fragmentation transverse momentum, show no significant broadening between p+Au, p+Al, and p+p. The away-side nonperturbative pout widths are found to be broadened in p+Au when compared to p+p; however, there is no significant broadening in p+Al compared to p+p collisions. The data also suggest that the away-side pout broadening is a function of Ncoll, the number of binary nucleon-nucleon collisions, in the interaction. The potential implications of these results with regard to initial- and final-state transverse-momentum broadening and energy loss of partons in a nucleus, among other nuclear effects, are discussed

Inclusive cross section and double-helicity asymmetry for pi(0) production at midrapidity in p plus p collisions at root s=510 GeV

PHENIX measurements are presented for the cross section and double-helicity asymmetry (A(LL)) in inclusive pi(0) production at midrapidity from p + p collisions at root s = 510 GeV from data taken in 2012 and 2013 at the Relativistic Heavy Ion Collider. The next-to-leading-order perturbative-quantum-chromodynamics theory calculation is in excellent agreement with the presented cross section results. The calculation utilized parton-to-pion fragmentation functions from the recent DSS14 global analysis, which prefer a smaller gluon-to-pion fragmentation function. The pi(0)A(LL) results follow an increasingly positive asymmetry trend with p(T) and root s with respect to the predictions and are in excellent agreement with the latest global analysis results. This analysis incorporated earlier results on pi(0) and jet A(LL) and suggested a positive contribution of gluon polarization to the spin of the proton Delta G for the gluon momentum fraction range x \u3e 0.05. The data presented here extend to a currently unexplored region, down to x similar to 0.01, and thus provide additional constraints on the value of Delta G

Measurement of the relative yields of psi(2S) to psi(1S) mesons produced at forward and backward rapidity in p plus p, p plus Al, p + Au, and He-3+Au collisions at root S-NN=200 GeV

The PHENIX Collaboration has measured the ratio of the yields of psi(2S) to psi(1S) mesons produced in p + p, p + Al, p + Au, and He-3+Au collisions at root S-NN = 200 GeV over the forward and backward rapidity intervals 1.2 \u3c | y | \u3c 2.2. We find that the ratio in p + p collisions is consistent with measurements at other collision energies. In collisions with nuclei, we find that in the forward (p-going or He-3-going) direction, the relative yield of psi(2S) mesons to psi(1S) mesons is consistent with the value measured in p + p collisions. However, in the backward (nucleus-going) direction, the psi(2S) meson is preferentially suppressed by a factor of similar to 2. This suppression is attributed in some models to the breakup of the weakly bound psi(2S) meson through final-state interactions with comoving particles, which have a higher density in the nucleus-going direction. These breakup effects may compete with color screening in a deconfined quark-gluon plasma to produce sequential suppression of excited quarkonia states

Correlations of μμ, eμ, and ee pairs in p+p collisions at s√=200 GeV and implications for cc¯ and bb¯ production mechanisms

PHENIX has measured the azimuthal correlations of muon pairs from charm and bottom semi-leptonic decays in p+p collisions at s√=200 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 (76%±1419%) contribution for bb¯ production, whereas the data favor the scenario in which next-to-leading-order processes dominate cc¯ production. The small contribution of next-to-leading-order processes in bb¯ 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

Measurements of Multiparticle Correlations in d + Au Collisions at 200, 62.4, 39, and 19.6 GeV and p + Au Collisions at 200 GeV and Implications for Collective Behavior

Recently, multiparticle-correlation measurements of relativistic p/d(3)He + Au, p + Pb, and even p + p collisions show surprising collective signatures. Here, we present beam-energy-scan measurements of two, four-, and six-particle angular correlations in d + Au collisions at root s(NN) = 200, 62.4, 39, and 19.6 GeV. We also present measurements of two-and four-particle angular correlations in p + Au collisions at root s(NN) = 200 GeV. We find the four-particle cumulant to be real valued for d + Au collisions at all four energies. We also find that the four-particle cumulant in p + Au has the opposite sign as that in d + Au. Further, we find that the six-particle cumulant agrees with the four-particle cumulant in d + Au collisions at 200 GeV, indicating that nonflow effects are subdominant. These observations provide strong evidence that the correlations originate from the initial geometric configuration, which is then translated into the momentum distribution for all particles, commonly referred to as collectivity

Forward J/psi production in U plus U collisions at root S-NN=193 GeV

The invariant yields, dN/dy, for J/psi production at forward rapidity (1.2 \u3c vertical bar y vertical bar \u3c 2.2) in U + U collisions at root S-NN = 193 GeV have been measured as a function of collision centrality. The invariant yields and nuclear-modification factor R-AA are presented and compared with those from Au + Au collisions in the same rapidity range. Additionally, the direct ratio of the invariant yields from U + U and Au + Au collisions within the same centrality class is presented, and used to investigate the role of c (c) over bar coalescence. Two different parametrizations of the deformed Woods-Saxon distribution were used in Glauber calculations to determine the values of the number of nucleon-nucleon collisions in each centrality class, N-coll, and these were found to give significantly different Ncoll values. Results using N-coll values from both deformed Woods-Saxon distributions are presented. The measured ratios show that the J/psi suppression, relative to binary collision scaling, is similar in U + U and Au + Au for peripheral and midcentral collisions, but that J/psi show less suppression for the most central U + U collisions. The results are consistent with a picture in which, for central collisions, increase in the J/psi yield due to c (c) over bar coalescence becomes more important than the decrease in yield due to increased energy density. For midcentral collisions, the conclusions about the balance between c (c) over bar coalescence and suppression depend on which deformed Woods-Saxon distribution is used to determine N-coll

Measurements of μμ pairs from open heavy flavor and Drell-Yan in p + p collisions at √s = 200 GeV

PHENIX reports differential cross sections of mu mu pairs from semileptonic heavy-flavor decays and the Drell-Yan production mechanism measured in p + p collisions at root s = 200 GeV at forward and backward rapidity (1.2 \u3c vertical bar eta vertical bar \u3c 2.2). The mu mu pairs from c (c) over bar, b (b) over bar, and Drell-Yan are separated using a template fit to unlike- and like-sign muon pair spectra in mass and p(T). The azimuthal opening angle correlation between the muons from c (c ) over bar and b (b) over bar decays and the pair-p(T) 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 a are consistent with PYTHIA calculations. The c (c) over bar data present narrower azimuthal correlations and softer p(T) distributions compared to distributions generated from POWHEG. The b (b ) over bar data are well described by both models. The extrapolated total cross section for bottom production is 3.75 +/- 0.24(stat) +/-(0.35)(0.50) (syst) +/- 0.45(global) [mu b], 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