Suppression of Back-to-Back Hadron Pairs at Forward Rapidity in d+ Au Collisions at sqrt [s_ {NN}]= 200 GeV

Back-to-back hadron pair yields in d+Au and p+p collisions at √sNN=200  GeV were measured with the PHENIX detector at the Relativistic Heavy Ion Collider. Rapidity separated hadron pairs were detected with the trigger hadron at pseudorapidity |η|\u3c0.35 and the associated hadron at forward rapidity (deuteron direction, 3.0\u3cη\u3c3.8). Pairs were also detected with both hadrons measured at forward rapidity; in this case, the yield of back-to-back hadron pairs in d+Au collisions with small impact parameters is observed to be suppressed by a factor of 10 relative to p+p collisions. The kinematics of these pairs is expected to probe partons in the Au nucleus with a low fraction x of the nucleon momenta, where the gluon densities rise sharply. The observed suppression as a function of nuclear thickness, pT, and η points to cold nuclear matter effects arising at high parton densities

Transverse-Momentum Dependence of the J/psi Nuclear Modification in d+ Au Collisions at sqrt (s_NN)= 200 GeV

We present measured J/ψ production rates in d+Au collisions at √sNN = 200 GeV over a broad range of transverse momentum (pT = 0–14 GeV/c) and rapidity (−2.2 \u3c y \u3c 2.2). We construct the nuclear-modification factor RdAu for these kinematics and as a function of collision centrality (related to impact parameter for the RdAu collision). We find that the modification is largest for collisions with small impact parameters, and observe a suppression (RdAu \u3c 1) for pT \u3c 4 GeV/c at positive rapidities. At negative rapidity we observe a suppression for pT \u3c 2 GeV/c then an enhancement (RdAu \u3e 1) for pT \u3e 2 GeV/c. The observed enhancement at negative rapidity has implications for the observed modification in heavy-ion collisions at high pT

J/ψ suppression at forward rapidity in Au+ Au collisions at sqrt [s_ {NN}]= 200 GeV

Heavy quarkonia are observed to be suppressed in relativistic heavy-ion collisions relative to their production in p + p collisions scaled by the number of binary collisions. In order to determine if this suppression is related to color screening of these states in the produced medium, one needs to account for other nuclear modifications including those in cold nuclear matter. In this paper, we present new measurements from the PHENIX 2007 data set of J/ψ yields at forward rapidity (1.2 \u3c |y| \u3c 2.2) in Au + Au collisions at √sNN = 200 GeV. The data confirm the earlier finding that the suppression of J/ψ at forward rapidity is stronger than at midrapidity, while also extending the measurement to finer bins in collision centrality and higher transverse momentum (pT ). We compare the experimental data to the most recent theoretical calculations that incorporate a variety of physics mechanisms including gluon saturation, gluon shadowing, initial-state parton energy loss, cold nuclear matter breakup, color screening, and charm recombination.We find J/ψ suppression beyond cold-nuclear-matter effects. However, the current level of disagreement between models and d + Au data precludes using these models to quantify the hot-nuclear-matter suppression

Cold Nuclear Matter Effects on J/ψ Yields as a Function of Rapidity and Nuclear Geometry in d+ A Collisions at sqrt [s_ {NN}]= 200 GeV

We present measurements of J/ψ yields in d+Au collisions at √sNN=200  GeV recorded by the PHENIX experiment and compare them with yields in p+p collisions at the same energy per nucleon-nucleon collision. The measurements cover a large kinematic range in J/ψ rapidity (-2.

Deviation from quark-number scaling of the anisotropy parameter v_2 of pions, kaons, and protons in Au+ Au collisions at sqrt (s_NN)= 200 GeV

Measurements of the anisotropy parameter v2 of identified hadrons (pions, kaons, and protons) as a function of centrality, transverse momentum pT , and transverse kinetic energy KET at midrapidity (|_| \u3c 0.35) in Au+Au collisions at psNN = 200 GeV are presented. Pions and protons are identified up to pT = 6 GeV/c, and kaons up to pT = 4 GeV/c, by combining information from time-of-flight and aerogel ˇ Cerenkov detectors in the PHENIX Experiment. The scaling of v2 with the number of valence quarks (nq) has been studied in different centrality bins as a function of transverse momentum and transverse kinetic energy. A deviation from previously observed quark-number scaling is observed at large values of KET /nq in noncentral Au+Au collisions (20–60%), but this scaling remains valid in central collisions (0–10%)

Measurements of Higher Order Flow Harmonics in Au+ Au Collisions at sqrt [s_ {NN}]= 200 GeV

Flow coefficients vn for n = 2, 3, 4, characterizing the anisotropic collective flow in Au + Au collisions at square root of S(NN) = 200 GeV, are measured relative to event planes psin, determined at large rapidity. We report vn as a function of transverse momentum and collision centrality, and study the correlations among the event planes of different order n. The vn are well described by hydrodynamic models which employ a Glauber Monte Carlo initial state geometry with fluctuations, providing additional constraining power on the interplay between initial conditions and the effects of viscosity as the system evolves. This new constraint can serve to improve the precision of the extracted shear viscosity to entropy density ratio n/s

Nuclear-Modification Factor for Open-Heavy-Flavor Production at Forward Rapidity in Cu+ Cu Collisions at sqrt (s_NN)= 200 GeV

Background: Heavy-flavor production in p+p collisions is a good test of perturbative-quantum- chromodynamics (pQCD) calculations. Modification of heavy-flavor production in heavy-ion collisions relative to binary-collision scaling from p+p results, quantified with the nuclear-modification factor (RAA), provides information on both cold- and hot-nuclear-matter effects. Midrapidity heavy-flavor RAA measurements at RHIC have challenged parton-energy-loss models and resulted in upper limits on the viscosity/entropy ratio that are near the quantum lower bound. Such measurements have not been made in the forward-rapidity region. Purpose: Determine transverse-momentum, pT spectra and the corresponding RAA for muons from heavy-flavor mesons decay in p+p and Cu+Cu collisions at √sNN = 200GeV and y = 1.65. Method: Results are obtained using the semi-leptonic decay of heavy-flavor mesons into negative muons. The PHENIX muon-arm spectrometers measure the pT spectra of inclusive muon candidates. Backgrounds, primarily due to light hadrons, are determined with a Monte-Carlo calculation using a set of input hadron distributions tuned to match measured-hadron distributions in the same detector and statistically subtracted. Results: The charm-production cross section in p+p collisions at √s = 200GeV, integrated over pT and in the rapidity range 1.4 \u3c y \u3c 1.9 is found to be dσc¯c/dy = 0.139 ± 0.029 (stat) +0.051−0.058 (syst) mb. This result is consistent with a perturbative fixed-order-plus-next-to-leading-log (FONLL) calculation within scale uncertainties and is also consistent with expectations based on the corresponding midrapidity charm-production cross section measured by PHENIX. The RAA for heavy-flavor muons in Cu+Cu collisions is measured in three centrality intervals for 1 \u3c pT \u3c 4 GeV/c. Suppression relative to binary-collision scaling (RAA \u3c 1) increases with centrality. Conclusions: Within experimental and theoretical uncertainties, the measured heavy-flavor yield in p+p collisions is consistent with state-of-the-art pQCD calculations. Suppression in central Cu+Cu collisions suggests the presence of significant cold-nuclear-matter effects and final-state energy loss

Event structure and double helicity asymmetry in jet production from polarized p+ p collisions at sqrt [s]= 200 GeV

We report on the event structure and double helicity asymmetry (ALL) of jet production in longitudinally polarized p+p collisions at √s=200  GeV. Photons and charged particles were measured by the PHENIX experiment at midrapidity |η|\u3c0.35 with the requirement of a high-momentum (\u3e2  GeV/c) photon in the event. Event structure, such as multiplicity, pT density and thrust in the PHENIX acceptance, were measured and compared with the results from the pythia event generator and the geant detector simulation. The shape of jets and the underlying event were well reproduced at this collision energy. For the measurement of jet ALL, photons and charged particles were clustered with a seed-cone algorithm to obtain the cluster pT sum (pTreco). The effect of detector response and the underlying events on pTreco was evaluated with the simulation. The production rate of reconstructed jets is satisfactorily reproduced with the next-to-leading-order and perturbative quantum chromodynamics jet production cross section. For

Cross sections and double-helicity asymmetries of midrapidity inclusive charged hadrons in p+ p collisions at sqrt (s)= 62.4 GeV

The comparison of cross-section predictions with data on single-inclusive hadron production in hadronic collisions, p+p → h + X, is important for understanding perturbative quantum chromodynamics (pQCD). For hadrons produced with transverse momenta pT 6b _QCD, the cross section factorizes into a convolution involving long-distance and short-distance components [1, 2]. Long-distance components include universal partondistribution functions (PDFs) describing the partonic structure of the initial hadrons and fragmentation functions (FFs) for the final-state hadron. The short-distance part describes the hard scattering of partons. The long distance components, PDFs and FFs, can be extracted from other processes, such as deep-inelastic scattering and hadron production in e+e− colliders. This allows for a test of the short-distance part of the convolution, which can be estimated using pQCD. In particular, differences between data and predictions can indicate the importance of neglected higher-order terms in the expansion or power-suppressed contributions [3]

Production of ω mesons in p+ p, d+ Au, Cu+ Cu, and Au+ Au collisions at sqrt [s_ {NN}]= 200 GeV

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured ω meson production via leptonic and hadronic decay channels in p + p, d + Au, Cu + Cu, and Au + Au collisions at √sNN = 200 GeV. The invariant transverse momentum spectra measured in different decay modes give consistent results. Measurements in the hadronic decay channel in Cu + Cu and Au + Au collisions show that ω production has a suppression pattern at high transverse momentum, similar to that of π0 and η in central collisions, but no suppression is observed in peripheral collisions. The nuclear modification factors, RAA, are consistent in Cu + Cu and Au + Au collisions at similar numbers of participant nucleons