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 \(\sqrt{s}=510\) GeV longitudinally polarized $p$$+$$p\( collisions at the Relativistic Heavy Ion Collider (RHIC) in the 2013 run using the PHENIX detector. At this collision energy, \)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<|y|<2.2\(. In this kinematic range, we measured the \)A_{LL}^{J/\psi}\( to be \)0.012 \pm 0.010\(~(stat)~\)\pm\(~\)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 \approx 0.05\( where recent RHIC data of jet and \)\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 \approx 2\times 10^{-3}\(. Thus our new results could be used to further constrain the gluon polarization for \)x< 0.05$.
Measurements of anisotropic flow Fourier coefficients (\(v_n\)) for inclusive charged particles and identified hadrons \(\pi^{\pm}\), \(K^{\pm}\), \(p\), and \(\bar{p}\) produced at midrapidity in ...Cu+Au collisions at \(\sqrt{s_{_{NN}}}=200\) GeV are presented. The data were collected in 2012 by the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC). The particle azimuthal distributions with respect to different order symmetry planes \(\Psi_n\), for \(n\)~=~1, 2, and 3 are studied as a function of transverse momentum \(p_T\) over a broad range of collisions centralities. Mass ordering, as expected from hydrodynamic flow, is observed for all three harmonics. The charged-particle results are compared to hydrodynamical and transport model calculations. We also compare these Cu\(+\)Au results with those in Cu\(+\)Cu and Au\(+\)Au collisions at the same \(\sqrt{s_{_{NN}}}\), and find that the \(v_2\) and \(v_3\), as a function of transverse momentum, follow a common scaling with \(1/(\varepsilon_n N_{\rm part}^{1/3})\).
The PHENIX Collaboration at the Relativistic Heavy Ion Collider has measured open heavy-flavor production in minimum bias Au\(+\)Au collisions at \(\sqrt{s_{_{NN}}}=200\) GeV via the yields of ...electrons from semileptonic decays of charm and bottom hadrons. Previous heavy-flavor electron measurements indicated substantial modification in the momentum distribution of the parent heavy quarks due to the quark-gluon plasma created in these collisions. For the first time, using the PHENIX silicon vertex detector to measure precision displaced tracking, the relative contributions from charm and bottom hadrons to these electrons as a function of transverse momentum are measured in Au\(+\)Au collisions. We compare the fraction of electrons from bottom hadrons to previously published results extracted from electron-hadron correlations in $p$$+$$p\( collisions at \)\sqrt{s_{_{NN}}}=200\( GeV and find the fractions to be similar within the large uncertainties on both measurements for \)p_T>4\( GeV/\)c\(. We use the bottom electron fractions in Au\)+\(Au and \)p$$+$$p\( along with the previously measured heavy flavor electron \)R_{AA}\( to calculate the \)R_{AA}\( for electrons from charm and bottom hadron decays separately. We find that electrons from bottom hadron decays are less suppressed than those from charm for the region \)3<p_T<4\( GeV/\)c$.
Measurements of the fractional momentum loss (\(S_{\rm loss}\equiv{\delta}p_T/p_T\)) of high-transverse-momentum-identified hadrons in heavy ion collisions are presented. Using \(\pi^0\) in Au\(+\)Au ...and Cu\(+\)Cu collisions at \(\sqrt{s_{_{NN}}}=62.4\) and 200 GeV measured by the PHENIX experiment at the Relativistic Heavy Ion Collider and and charged hadrons in Pb\(+\)Pb collisions measured by the ALICE experiment at the Large Hadron Collider, we studied the scaling properties of \(S_{\rm loss}\) as a function of a number of variables: the number of participants, \(N_{\rm part}\), the number of quark participants, \(N_{\rm qp}\), the charged-particle density, \(dN_{\rm ch}/d\eta\), and the Bjorken energy density times the equilibration time, \(\varepsilon_{\rm Bj}\tau_{0}\). We find that the \(p_T\) where \(S_{\rm loss}\) has its maximum, varies both with centrality and collision energy. Above the maximum, \(S_{\rm loss}\) tends to follow a power-law function with all four scaling variables. The data at \(\sqrt{s_{_{NN}}}\)=200 GeV and 2.76 TeV, for sufficiently high particle densities, have a common scaling of \(S_{\rm loss}\) with \(dN_{\rm ch}/d\eta\) and \(\varepsilon_{\rm Bj}\tau_{0}\), lending insight on the physics of parton energy loss.
The invariant yields for \(J/\psi\) production at forward rapidity \((1.2<|y|<2.2)\) in U\(+\)U collisions at \(\sqrt{s_{_{NN}}}\)=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 \(c\bar{c}\) coalescence. Two different parameterizations 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, \(N_{\rm coll}\), and these were found to give significantly different \(N_{\rm coll}\) values. Results using \(N_{\rm coll}\) 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 \(c\bar{c}\) coalescence becomes more important than the decrease in yield due to increased energy density. For midcentral collisions, the conclusions about the balance between \(c\bar{c}\) coalescence and suppression depend on which deformed Woods-Saxon distribution is used to determine \(N_{\rm coll}\).
Jet production rates are measured in $p$$+$$p\( and \)d$$+\(Au collisions at \)\sqrt{s_{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}\( algorithm from energy deposits in the electromagnetic calorimeter and charged tracks in multi-wire proportional chambers, and the jet transverse momentum (\)p_T\() spectra are corrected for the detector response. Spectra are reported for jets with \)12<p_T<50\( GeV/\)c\(, within a pseudorapidity acceptance of \)\left|\eta\right|<0.3\(. The nuclear-modification factor (\)R_{d{\rm Au}}\() values for 0\%--100\% \)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_{d{\rm Au}}\( values and central-to-peripheral ratios (\)R_{\rm CP}\() show large, \)p_T$-dependent deviations from unity, which challenge the conventional models that relate hard-process rates and soft-particle production in collisions involving nuclei.
Measurements of midrapidity charged particle multiplicity distributions, \(dN_{\rm ch}/d\eta\), and midrapidity transverse-energy distributions, \(dE_T/d\eta\), are presented for a variety of ...collision systems and energies. Included are distributions for Au\(+\)Au collisions at \(\sqrt{s_{_{NN}}}=200\), 130, 62.4, 39, 27, 19.6, 14.5, and 7.7 GeV, Cu\(+\)Cu collisions at \(\sqrt{s_{_{NN}}}=200\) and 62.4 GeV, Cu\(+\)Au collisions at \(\sqrt{s_{_{NN}}}=200\) GeV, U\(+\)U collisions at \(\sqrt{s_{_{NN}}}=193\) GeV, $d$$+\(Au collisions at \)\sqrt{s_{_{NN}}}=200\( GeV, \)^{3}\(He\)+\(Au collisions at \)\sqrt{s_{_{NN}}}=200\( GeV, and \)p$$+$$p\( collisions at \)\sqrt{s_{_{NN}}}=200\( GeV. Centrality-dependent distributions at midrapidity are presented in terms of the number of nucleon participants, \)N_{\rm part}\(, and the number of constituent quark participants, \)N_{q{\rm p}}\(. For all \)A$$+$$A\( collisions down to \)\sqrt{s_{_{NN}}}=7.7\( GeV, it is observed that the midrapidity data are better described by scaling with \)N_{q{\rm p}}\( than scaling with \)N_{\rm part}\(. Also presented are estimates of the Bjorken energy density, \)\varepsilon_{\rm BJ}\(, and the ratio of \)dE_T/d\eta\( to \)dN_{\rm ch}/d\eta$, the latter of which is seen to be constant as a function of centrality for all systems.
The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) has measured \(\phi\) meson production and its nuclear modification in asymmetric Cu\(+\)Au heavy-ion collisions at ...\(\sqrt{s_{NN}}=200\) GeV at both forward Cu-going direction (\(1.2<y<2.2\)) and backward Au-going direction (\(-2.2<y<-1.2\)), rapidities. The measurements are performed via the dimuon decay channel and reported as a function of the number of participating nucleons, rapidity, and transverse momentum. In the most central events, 0\%--20\% centrality, the \(\phi\) meson yield integrated over \(1<p_T<5\) GeV/\(c\) prefers a smaller value, which means a larger nuclear modification, in the Cu-going direction compared to the Au-going direction. Additionally, the nuclear-modification factor in Cu\(+\)Au collisions averaged over all centrality is measured to be similar to the previous PHENIX result in $d$$+$Au collisions for these rapidities.
We present measurements of \(e^+e^-\) production at midrapidity in Au\(+\)Au collisions at \(\sqrt{s_{_{NN}}}\) = 200 GeV. The invariant yield is studied within the PHENIX detector acceptance over a ...wide range of mass (\(m_{ee} <\) 5 GeV/\(c^2\)) and pair transverse momentum (\(p_T\) \(<\) 5 GeV/\(c\)), for minimum bias and for five centrality classes. The \ee yield is compared to the expectations from known sources. In the low-mass region (\(m_{ee}=0.30\)--0.76 GeV/\(c^2\)) 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 \(2.3\pm0.4({\rm stat})\pm0.4({\rm syst})\pm0.2^{\rm model}\) or to \(1.7\pm0.3({\rm stat})\pm0.3({\rm syst})\pm0.2^{\rm model}\) for minimum bias collisions when the open-heavy-flavor contribution is calculated with {\sc pythia} or {\sc mc@nlo}, respectively. The inclusive mass and \(p_T\) distributions as well as the centrality dependence are well reproduced by model calculations where the enhancement mainly originates from the melting of the \(\rho\) meson resonance as the system approaches chiral symmetry restoration. In the intermediate-mass region (\(m_{ee}\) = 1.2--2.8 GeV/\(c^2\)), the data hint at a significant contribution in addition to the yield from the semileptonic decays of heavy-flavor mesons.
PHENIX measurements are presented for the cross section and double-helicity asymmetry (\(A_{LL}\)) in inclusive \(\pi^0\) production at midrapidity from $p$$+$$p\( collisions at \)\sqrt{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\( and \)\sqrt{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>0.05\(. The data presented here extend to a currently unexplored region, down to \)x\sim0.01\(, and thus provide additional constraints on the value of \)\Delta G\(. The results confirm the evidence for nonzero \)\Delta G$ using a different production channel in a complementary kinematic region.
