We present measurements of electrons and positrons from the semileptonic decays of heavy-flavor hadrons at midrapidity (\(|y|<\) 0.35) in Au\(+\)Au collisions at \(\sqrt{s_{_{NN}}}=62.4\) GeV. The ...data were collected in 2010 by the PHENIX experiment that included the new hadron-blind detector. The invariant yield of electrons from heavy-flavor decays is measured as a function of transverse momentum in the range \(1<p_T^e<5\) GeV/\(c\). The invariant yield per binary collision is slightly enhanced above the $p$$+$$p\( reference in Au\)+\(Au 0%--20%, 20%--40% and 40%--60% centralities at a comparable level. This may be a result of the interplay between initial-state Cronin effects, final-state flow, and energy loss for heavy-quark production at this low beam energy. The \)v_2\( of electrons from heavy-flavor decays is nonzero when averaged between \)1.3<p_T^e<2.5\( GeV/\)c\( from \)0<{\rm centrality}<40\(% collisions at \)\sqrt{s_{_{NN}}}=62.4\( GeV. For 20%--40% centrality collisions, the \)v_2\( at \)\sqrt{s_{_{NN}}}=62.4\( GeV is smaller than that for heavy flavor decays at \)\sqrt{s_{_{NN}}}=200\( GeV. The \)v_2\( of the electrons from heavy-flavor decay at the lower beam energy is also smaller than \)v_2\( for pions. Both results indicate that the heavy-quarks interact with the medium formed in these collisions, but they may not be at the same level of thermalization with the medium as observed at \)\sqrt{s_{_{NN}}}=200$ GeV.
We present the midrapidity charged pion invariant cross sections and the ratio of \(\pi^-\)-to-\(\pi^+\) production (\(5<p_T<13\) GeV/\(c\)), together with the double-helicity asymmetries ...(\(5<p_T<12\) GeV/\(c\)) in polarized $p$$+$$p\( collisions at \)\sqrt{s} = 200\( GeV. The cross section measurements are consistent with perturbative calculations in quantum chromodynamics within large uncertainties in the calculation due to the choice of factorization, renormalization, and fragmentation scales. However, the theoretical calculation of the ratio of \)\pi^-\(-to-\)\pi^+\( production when considering these scale uncertainties overestimates the measured value, suggesting further investigation of the uncertainties on the charge-separated pion fragmentation functions is needed. Due to cancellations of uncertainties in the charge ratio, direct inclusion of these ratio data in future parameterizations should improve constraints on the flavor dependence of quark fragmentation functions to pions. By measuring charge-separated pion asymmetries, one can gain sensitivity to the sign of \)\Delta G\( through the opposite sign of the up and down quark helicity distributions in conjunction with preferential fragmentation of positive pions from up quarks and negative pions from down quarks. The double-helicity asymmetries presented are sensitive to the gluon helicity distribution over an \)x\( range of \)\sim$0.03--0.16.
We report a measurement of \(e^+e^-\) pairs from semileptonic heavy-flavor decays in $d$$+\(Au collisions at \)\sqrt{s_{_{NN}}}=200\( GeV. Exploring the mass and transverse-momentum dependence of the ...yield, the bottom decay contribution can be isolated from charm, and quantified by comparison to {\sc pythia} and {\sc mc@nlo} simulations. The resulting \)b\bar{b}\(-production cross section is \)\sigma^{d{\rm Au}}_{b\bar{b}}=1.37{\pm}0.28({\rm stat}){\pm}0.46({\rm syst})\(~mb, which is equivalent to a nucleon-nucleon cross section of \)\sigma^{NN}_{bb}=3.4\pm0.8({\rm stat}){\pm}1.1({\rm syst})\ \mu$b.
We report on \(J/\psi\) production from asymmetric Cu+Au heavy-ion collisions at \(\sqrt{s_{_{NN}}}\)=200 GeV at the Relativistic Heavy Ion Collider at both forward (Cu-going direction) and backward ...(Au-going direction) rapidities. The nuclear modification of \(J/\psi\) yields in Cu\(+\)Au collisions in the Au-going direction is found to be comparable to that in Au\(+\)Au collisions when plotted as a function of the number of participating nucleons. In the Cu-going direction, \(J/\psi\) production shows a stronger suppression. This difference is comparable in magnitude and has the same sign as the difference expected from shadowing effects due to stronger low-\(x\) gluon suppression in the larger Au nucleus. The relative suppression is opposite to that expected from hot nuclear matter dissociation, since a higher energy density is expected in the Au-going direction.
The PHENIX collaboration at the Relativistic Heavy Ion Collider (RHIC) reports measurements of azimuthal dihadron correlations near midrapidity in $d$$+\(Au collisions at \)\sqrt{s_{_{NN}}}\(=200 ...GeV. These measurements complement recent analyses by experiments at the Large Hadron Collider (LHC) involving central \)p$$+\(Pb collisions at \)\sqrt{s_{_{NN}}}\(=5.02 TeV, which have indicated strong anisotropic long-range correlations in angular distributions of hadron pairs. The origin of these anisotropies is currently unknown. Various competing explanations include parton saturation and hydrodynamic flow. We observe qualitatively similar, but larger, anisotropies in \)d$$+\(Au collisions compared to those seen in \)p$$+\(Pb collisions at the LHC. The larger extracted \)v_2\( values in \)d$$+\(Au collisions at RHIC are consistent with expectations from hydrodynamic calculations owing to the larger expected initial-state eccentricity compared with that from \)p$$+$Pb collisions. When both are divided by an estimate of the initial-state eccentricity the scaled anisotropies follow a common trend with multiplicity that may extend to heavy ion data at RHIC and the LHC, where the anisotropies are widely thought to arise from hydrodynamic flow.
The PHENIX Collaboration at the Relativistic Heavy Ion Collider has measured open heavy flavor production in Cu\(+\)Cu collisions at \(\sqrt{s_{_{NN}}}\)=200 GeV through the measurement of electrons ...at midrapidity that originate from semileptonic decays of charm and bottom hadrons. In peripheral Cu\(+\)Cu collisions an enhanced production of electrons is observed relative to $p$$+$$p\( collisions scaled by the number of binary collisions. In the transverse momentum range from 1 to 5 GeV/\)c\( the nuclear modification factor is \)R_{AA}$$\sim\(1.4. As the system size increases to more central Cu\)+\(Cu collisions, the enhancement gradually disappears and turns into a suppression. For \)p_T>3\( GeV/\)c\(, the suppression reaches \)R_{AA}$$\sim\(0.8 in the most central collisions. The \)p_T\( and centrality dependence of \)R_{AA}\( in Cu\)+\(Cu collisions agree quantitatively with \)R_{AA}\( in \)d+\(Au and Au\)+\(Au collisions, if compared at similar number of participating nucleons \)\langle N_{\rm part} \rangle$.
