Search

The cross section and transverse single-spin asymmetries of ${\mu }^{-}$ and ${\mu }^{+}$ from open heavy-flavor decays in polarized $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$ were measured by the PHENIX experiment during 2012 at the Relativistic Heavy Ion Collider. Because heavy-flavor production is dominated by gluon-gluon interactions at $\sqrt{s}=200\mathrm{GeV}$, these measurements offer a unique opportunity to obtain information on the trigluon correlation functions. The measurements are performed at forward and backward rapidity ($1.4<|y|<2.0$) over the transverse momentum range of $1.25<p_T<7\mathrm{GeV}/c$ for the cross section and $1.25<p_T<5\mathrm{GeV}/c$ for the asymmetry measurements. The obtained cross section is compared to a fixed-order-plus-next-to-leading-log perturbative-quantum-chromodynamics calculation. The asymmetry results are consistent with zero within uncertainties, and a model calculation based on twist-3 three-gluon correlations agrees with the data.

K-x-ray production cross sections have been measured in (nearly symmetric) ${}_{16}{}^{}{}_{}{}^{}$S${+\mathrm{}}_{18}$Ar and (very asymmetric) ${}_{16}{}^{}{}_{}{}^{}$S${+\mathrm{}}_2$He collisions as a function of the projectile charge state q (q=3–16) at low incident energies of 10 and 20 MeV. The charge-state dependence is very similar in the two cases, confirming results obtained at higher incident energies. The sensitivity of such studies to different K-vacancy-production mechanisms is discussed.

Energy and angular distributions of neutrons, protons, and alpha particles were measured in coincidence with the projectilelike fragment in deep inelastic collisions of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ on ${}_{}{}^{40,44,48}{}_{}{}^{}\mathrm{Ca}$ at 102 and 142 MeV. Using an event-by-event analysis which takes into account particle emission from both fragments and the recoil correction due to light particle emission, we show that our data are consistent with the assumption that the light particles are evaporated in the rest frame of the fully accelerated projectilelike and targetlike fragments. This conclusion is based on the results for neutrons and alpha particles. But because of kinematic limitations which affect the alpha particle data, the conclusions based on the latter are less definitive.

NUCLEAR REACTIONS ${}_{}{}^{40,44,48}{}_{}{}^{}\mathrm{Ca}({}_{}{}^{16}{}_{}{}^{}\mathrm{O}, X)$, $E=102, 142$ MeV, measured n, p, $\alpha$ in coincidence with projectilelike fragment; deduced c.m. energy spectra and multiplicities of the light particles. Results consistent with evaporation from the heavy fragments.

Neutrons emitted in the deep inelastic collisions of ${}_{}{}^{86}{}_{}{}^{}\mathrm{Kr}$ on ${}_{}{}^{166}{}_{}{}^{}\mathrm{Er}$ at 11.9 MeV/nucleon, were measured in coincidence with both fragments. The velocity and angular distributions of the neutrons cannot be accounted for by assuming only isotropic evaporation from the fully accelerated fragments. The experimental analysis shows the presence of a preequilibrium component of neutrons emitted mainly on the side of the light fragments in quasielastic events, and on the side of the heavy fragments in strongly damped events. The qualitative features of the data are reproduced by a simple model which assumes that ∼ 10% of the neutrons are knocked out at an early stage of the collision along the direction of the nuclear motion. Except for the preequilibrium component the neutron multiplicities are in good agreement with the predictions of evaporation model calculations.

NUCLEAR REACTIONS ${}_{}{}^{166}{}_{}{}^{}\mathrm{Er}({}_{}{}^{86}{}_{}{}^{}\mathrm{Kr}, A_L)A_H$, $E_{\mathrm{lab}}=1.02\mathrm{}$ GeV; $35\le A_L\le 126$, ${\theta }_{\mathrm{lab}}=6\mathrm{}°-28\mathrm{}°$; $E_n$, ${\theta }_n$, ${\phi }_n$ in coincidence with fragments; deduced c.m. neutron energy spectra, neutron multiplicities; evidence for preequilibrium neutron emission.

The PHENIX experiment at RHIC has measured the invariant cross section for ω-meson production at midrapidity in the transverse momentum range $2.5<p_T<9.25$ GeV/$c$ in $p+p$ and $d$+Au collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV. Measurements in two decay channels ($\omega \to {\pi }^0{\pi }^{+}{\pi }^{-}$ and $\omega \to {\pi }^0\gamma$) yield consistent results, and the reconstructed ω mass agrees with the accepted value within the $p_T$ range of the measurements. The $\omega /{\pi }^0$ ratio is found to be $0.85\pm 0.{05}^{\mathrm{stat}}\pm 0.{09}^{\mathrm{sys}}$ in $p+p$ and $0.94\pm 0.{08}^{\mathrm{stat}}\pm 0.{12}^{\mathrm{sys}}$ in $d$+Au collisions, independent of $p_T$. The nuclear modification factor $R_{d\mathrm{A}}^{\omega }$ is $1.03\pm 0.{12}^{\mathrm{stat}}\pm 0.{21}^{\mathrm{sys}}$ and $0.83\pm 0.{21}^{\mathrm{stat}}\pm 0.{17}^{\mathrm{sys}}$ in minimum bias and central (0–20%) $d$+Au collisions, respectively.

The impact-parameter dependence of the $2p\sigma -1s\sigma$ vacancy-sharing probability has been investigated in 35-MeV Cl on Ti and Cl on Ni collisions. A stronger impact-parameter depencence is observed with increasing asymmetry of the collision system. The impact-parameter dependence of the $K$-vacancy sharing is in good agreement with a simple analytical solution, derived by Briggs, of the two-state coupled-channel equations.

We have observed for the first time a pronounced increase of the molecular—$K$—x-ray production as a function of projectile charge state in 32-MeV S + Ar (gas target) collisions. The results are interpreted as evidence for strong excitation of $1s\sigma$ electrons into empty high-lying bound states. A comparison of this gas-target system and the solid-target systems Cl+Cl establishes unambiguously the predominance of one-collision processes for molecular—$K$—x-ray production.

Muon production at forward rapidity ($1.5\le |\eta |\le 1.8$) has been measured by the PHENIX experiment over the transverse momentum range $1\le p_T\le 3\mathrm{GeV}/c$ in $\sqrt{s}=200\mathrm{GeV}$ $p+p$ collisions at the Relativistic Heavy Ion Collider. After statistically subtracting contributions from light hadron decays an excess remains which is attributed to the semileptonic decays of hadrons carrying heavy flavor, i.e. charm quarks or, at high $p_T$, bottom quarks. The resulting muon spectrum from heavy flavor decays is compared to PYTHIA and a next-to-leading-order perturbative QCD calculation. PYTHIA is used to determine the charm quark spectrum that would produce the observed muon excess. The corresponding differential cross section for charm quark production at forward rapidity is determined to be $d{\sigma }_{c\overline{c}}/dy{|}_{y=1.6}=0.243\pm 0.013(\mathrm{stat}.)\pm 0.105(\mathrm{data}\mathrm{syst}.)\genfrac{}{}{0ex}{}{+0.049}{-0.087}(\mathrm{PYTHIA}\mathrm{syst}.)\mathrm{mb}$.

Data from $\mathrm{\text{Au}}+\mathrm{\text{Au}}$ interactions at $\sqrt{s_{NN}}=130\mathrm{\text{GeV}}$, obtained with the PHENIX detector at the Relativistic Heavy-Ion Collider, are used to investigate local net charge fluctuations among particles produced near midrapidity. According to recent suggestions, such fluctuations may carry information from the quark-gluon plasma. This analysis shows that the fluctuations are dominated by a stochastic distribution of particles, but are also sensitive to other effects, like global charge conservation and resonance decays.

Inclusive and coincidence measurements have been performed to study symmetric products from the reactions 74—186 MeV ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$+${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, 141 MeV ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$+${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, and 153 MeV ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$+${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$. The binary decay of the composite system has been verified. Energy spectra, angular distributions, and fragment correlations are presented. The total kinetic energies for the symmetric products from these very light composite systems are compared to liquid drop model calculations and fission systematics.

We present azimuthal angle correlations of intermediate transverse momentum () hadrons from dijets in and collisions at and 200 GeV. The away-side dijet induced azimuthal correlation is broadened, non-Gaussian, and peaked away from in central and semicentral collisions in all the systems. The broadening and peak location are found to depend upon the number of participants in the collision, but not on the collision energy or beam nuclei. These results are consistent with sound or shock wave models, but pose challenges to Cherenkov gluon radiation models.

The invariant yields, $dN/dy$, for $J/\psi$ production at forward rapidity $(1.2<|y|<2.2)$ in $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{{}_{NN}}}=193\mathrm{GeV}$ 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 $\mathrm{Au}+\mathrm{Au}$ collisions in the same rapidity range. Additionally, the direct ratio of the invariant yields from $\mathrm{U}+\mathrm{U}$ and $\mathrm{Au}+\mathrm{Au}$ collisions within the same centrality class is presented, and used to investigate the role of $c\overline{c}$ 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, $N_{\mathrm{coll}}$, and these were found to give significantly different $N_{\mathrm{coll}}$ values. Results using $N_{\mathrm{coll}}$ 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 $\mathrm{U}+\mathrm{U}$ and $\mathrm{Au}+\mathrm{Au}$ for peripheral and midcentral collisions, but that $J/\psi$ show less suppression for the most central $\mathrm{U}+\mathrm{U}$ collisions. The results are consistent with a picture in which, for central collisions, increase in the $J/\psi$ yield due to $c\overline{c}$ coalescence becomes more important than the decrease in yield due to increased energy density. For midcentral collisions, the conclusions about the balance between $c\overline{c}$ coalescence and suppression depend on which deformed Woods-Saxon distribution is used to determine $N_{\mathrm{coll}}$.

We present measurements of electrons and positrons from the semileptonic decays of heavy-flavor hadrons at midrapidity ($\left|y\right|<$ 0.35) in $\mathrm{Au}+\mathrm{Au}$ 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 $\mathrm{Au}+\mathrm{Au}$ 0%–20%, 20%–40%, and 40%–60% centralities at a comparable level. At this low beam energy this may be a result of the interplay between initial-state Cronin effects, final-state flow, and energy loss in medium. The $v_2$ of electrons from heavy-flavor decays is nonzero when averaged between $1.3<p_T^e<2.5$ GeV/$c$ for 0%–40% centrality 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.

$J/\psi$ production in $d+\mathrm{Au}$ and $p+p$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ has been measured by the PHENIX experiment at rapidities $-2.2<y<+2.4$. The cross sections and nuclear dependence of $J/\psi$ production versus rapidity, transverse momentum, and centrality are obtained and compared to lower energy $p+A$ results and to theoretical models. The observed nuclear dependence in $d+\mathrm{Au}$ collisions is found to be modest, suggesting that the absorption in the final state is weak and the shadowing of the gluon distributions is small and consistent with Dokshitzer-Gribov-Lipatov-Altarelli-Parisi-based parametrizations that fit deep-inelastic scattering and Drell-Yan data at lower energies.

Charged-pion-interferometry measurements were made with respect to the second- and third-order event plane for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. A strong azimuthal-angle dependence of the extracted Gaussian-source radii was observed with respect to both the second- and third-order event planes. The results for the second-order dependence indicate that the initial eccentricity is reduced during the medium evolution, which is consistent with previous results. In contrast, the results for the third-order dependence indicate that the initial triangular shape is significantly reduced and potentially reversed by the end of the medium evolution, and that the third-order oscillations are largely dominated by the dynamical effects from triangular flow.

Measurements of the fractional momentum loss $\left(S_{\mathrm{loss}}\equiv \delta p_T/p_T\right)$ of high-transverse-momentum-identified hadrons in heavy-ion collisions are presented. Using ${\pi }^0$ in $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ 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 $\mathrm{Pb}+\mathrm{Pb}$ collisions measured by the ALICE experiment at the Large Hadron Collider, we studied the scaling properties of $S_{\mathrm{loss}}$ as a function of a number of variables: the number of participants, $N_{\mathrm{part}}$, the number of quark participants, $N_{\mathrm{qp}}$, the charged-particle density, $d{N}_{\mathrm{ch}}/d\eta$, and the Bjorken energy density times the equilibration time, ${\varepsilon }_{\mathrm{Bj}}{\tau }_0$. We find that the $p_T$, where $S_{\mathrm{loss}}$ has its maximum, varies both with centrality and collision energy. Above the maximum, $S_{\mathrm{loss}}$ 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 $S_{\mathrm{loss}}$ with $d{N}_{\mathrm{ch}}/d\eta$ and ${\varepsilon }_{\mathrm{Bj}}{\tau }_0$, lending insight into the physics of parton energy loss.

In 2015 the PHENIX collaboration at the Relativistic Heavy Ion Collider recorded $p+p$, $p+\mathrm{Al}$, and $p+\mathrm{Au}$ collision data at center of mass energies of $\sqrt{s_{NN}}=200\mathrm{GeV}$ with the proton beam(s) transversely polarized. At very forward rapidities $\eta >6.8$ relative to the polarized proton beam, neutrons were detected either inclusively or in (anti)correlation with detector activity related to hard collisions. The resulting single spin asymmetries, that were previously reported, have now been extracted as a function of the transverse momentum of the neutron as well as its longitudinal momentum fraction $x_F$. The explicit kinematic dependence, combined with the correlation information allows for a closer look at the interplay of different mechanisms suggested to describe these asymmetries, such as hadronic interactions or electromagnetic interactions in ultraperipheral collisions, UPC. Events that are correlated with a hard collision indeed display a mostly negative asymmetry that increases in magnitude as a function of transverse momentum with only little dependence on $x_F$. In contrast, events that are not likely to have emerged from a hard collision display positive asymmetries for the nuclear collisions with a kinematic dependence that resembles that of a UPC based model. Because the UPC interaction depends strongly on the charge of the nucleus, those effects are very small for $p+p$ collisions, moderate for $p+\mathrm{Al}$ collisions, and large for $p+\mathrm{Au}$ collisions.

Two particle correlations between identified meson and baryon trigger particles with $2.5<p_T<4.0\mathrm{GeV}/c$ and lower $p_T$ charged hadrons have been measured at midrapidity by the PHENIX experiment at RHIC in $p+p,d+\mathrm{Au}$, and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathit{NN}}}=200\mathrm{GeV}$. In noncentral $\mathrm{Au}+\mathrm{Au}$ collisions, the probability of finding a hadron near in azimuthal angle to the trigger particles is almost identical for mesons and baryons and significantly higher than in $p+p$ collisions. The associated yields for trigger baryons decrease in the most central collisions, consistent with some baryon production by thermal recombination in addition to hard scattering.

We present measurements of $J/\psi$ yields in $d+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ 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 $J/\psi$ rapidity ($-2.2<y<2.4$) with high statistical precision and are compared with two theoretical models: one with nuclear shadowing combined with final state breakup and one with coherent gluon saturation effects. In order to remove model dependent systematic uncertainties we also compare the data to a simple geometric model. The forward rapidity data are inconsistent with nuclear modifications that are linear or exponential in the density weighted longitudinal thickness, such as those from the final state breakup of the bound state.

We present results for three charmonia states (${\psi }^{\prime }$, ${\chi }_c$, and $J/\psi$) in $d+\mathrm{Au}$ collisions at $|y|<0.35$ and $\sqrt{s_{NN}}=200\mathrm{GeV}$. We find that the modification of the ${\psi }^{\prime }$ yield relative to that of the $J/\psi$ scales approximately with charged particle multiplicity at midrapidity across $p+A$, $d+\mathrm{Au}$, and $A+A$ results from the Super Proton Synchrotron and the Relativistic Heavy Ion Collider. In large-impact-parameter collisions we observe a similar suppression for the ${\psi }^{\prime }$ and $J/\psi$, while in small-impact-parameter collisions the more weakly bound ${\psi }^{\prime }$ is more strongly suppressed. Owing to the short time spent traversing the Au nucleus, the larger ${\psi }^{\prime }$ suppression in central events is not explained by an increase of the nuclear absorption owing to meson formation time effects.