Jet production rates are measured in p+p and d+Au collisions at sqrts_{NN}=200 GeV recorded in 2008 with the PHENIX detector at the Relativistic Heavy Ion Collider. Jets are reconstructed using the ...R=0.3 anti-k_{t} algorithm from energy deposits in the electromagnetic calorimeter and charged tracks in multiwire proportional chambers, and the jet transverse momentum (p_{T}) spectra are corrected for the detector response. Spectra are reported for jets with 12<p_{T}<50 GeV/c, within a pseudorapidity acceptance of |η|<0.3. The nuclear-modification factor (R_{dAu}) 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 R_{dAu} values and central-to-peripheral ratios (R_{CP}) show large, p_{T}-dependent deviations from unity, challenging the conventional models that relate hard-process rates and soft-particle production in collisions involving nuclei.
We present the first measurement of elliptic (v(2)) and triangular (v(3)) flow in high-multiplicity (3)He+Au collisions at √(s(NN))=200 GeV. Two-particle correlations, where the particles have a ...large separation in pseudorapidity, are compared in (3)He+Au and in p+p collisions and indicate that collective effects dominate the second and third Fourier components for the correlations observed in the (3)He+Au system. The collective behavior is quantified in terms of elliptic v(2) and triangular v(3) anisotropy coefficients measured with respect to their corresponding event planes. The v(2) 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 (3)He nucleons on the Au nucleus expand hydrodynamically to generate the triangular flow. The agreement of these models with data may indicate the formation of low-viscosity quark-gluon plasma even in these small collision systems.
We report the $p+p$ and $p+d$ differential cross sections measured in the
SeaQuest experiment for $J/\psi$ and $\psi\left(2S\right)$ production at 120
GeV beam energy covering the forward $x$-Feynman ...($x_F$) range of $0.5 < x_F
<0.9$. The measured cross sections are in good agreement with theoretical
calculations based on the nonrelativistic QCD (NRQCD) using the long-distance
matrix elements deduced from a recent global analysis of proton- and
pion-induced charmonium production data. The $\sigma_{\psi\left(2S\right)} /
\sigma_{J/\psi}$ cross section ratios are found to increase as $x_F$ increases,
indicating that the $q \bar{q}$ annihilation process has larger contributions
in the $\psi\left(2S\right)$ production than the $J/\psi$ production. The
$\sigma_{pd}/2\sigma_{pp}$ cross section ratios are observed to be
significantly different for the Drell-Yan process and $J/\psi$ production,
reflecting their different production mechanisms. We find that the
$\sigma_{pd}/2\sigma_{pp}$ ratios for $J/\psi$ production at the forward $x_F$
region are sensitive to the $\bar{d}/ \bar{u}$ flavor asymmetry of the proton
sea, analogous to the Drell-Yan process. The transverse momentum ($p_T$)
distributions for $J/\psi$ and $\psi\left(2S\right)$ production are also
presented and compared with data collected at higher center-of-mass energies.
The jet fragmentation function is measured with direct photon-hadron correlations in p+p and Au+Au collisions at √s(NN)=200 GeV. The p(T) of the photon is an excellent approximation to the initial ...p(T) of the jet and the ratio z(T)=p(T)(h)/p(T)(γ) is used as a proxy for the jet fragmentation function. A statistical subtraction is used to extract the direct photon-hadron yields in Au+Au collisions while a photon isolation cut is applied in p+p. I(AA), the ratio of hadron yield opposite the photon in Au+Au to that in p+p, indicates modification of the jet fragmentation function. Suppression, most likely due to energy loss in the medium, is seen at high z(T). The associated hadron yield at low z(T) is enhanced at large angles. Such a trend is expected from redistribution of the lost energy into increased production of low-momentum particles.
The STAR collaboration at RHIC reports measurements of the inclusive yield of non-photonic electrons, which arise dominantly from semi-leptonic decays of heavy flavor mesons, over a broad range of ...transverse momenta ($1.2 < \pt < 10$ \gevc) in \pp, \dAu, and \AuAu collisions at \sqrtsNN = 200 GeV. The non-photonic electron yield exhibits unexpectedly large suppression in central \AuAu collisions at high \pt, suggesting substantial heavy quark energy loss in hot QCD matter. The centrality and \pt dependences of the suppression provide stringent constraints on theoretical models of suppression.
We present the scaling properties of Lambda, Xi, and Omega in midrapidity Au+Au collisions at the Brookhaven National Laboratory Relativistic Heavy Ion Collider at root s(NN)=200 GeV. The yield of ...multistrange baryons per participant nucleon increases from peripheral to central collisions more rapidly than that of Lambda, indicating an increase of the strange-quark density of the matter produced. The strange phase-space occupancy factor gamma(s) approaches unity for the most central collisions. Moreover, the nuclear modification factors of p, Lambda, and Xi are consistent with each other for 2 < p(T) < 5 GeV/c in agreement with a scenario of hadron formation from constituent quark degrees of freedom
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