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.

A numerically self-consistent solution for the recently proposed field theoretical $\pi N$ $p$-wave separable model was found by the method of singular integral equations. The method of solution and form factors will be presented.

NUCLEAR REACTIONS Solution by singular integral equation, self-consistent form factor.

The influence of the conventional nuclear structure on the European Muon Collaboration effect is critically reviewed. The ratio of deep inelastic lepton nucleon to lepton nucleus cross sections is shown to be sensitively dependent on the proper kinematics of the Fermi motion and the careful treatment of the binding effect of nucleons in nuclei. We systematically consider the example ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ at -$Q^2$=20 (GeV/c${)}^2$.

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.

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.

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.

We present a new analysis of $J/\psi$ production yields in deuteron-gold collisions at $\sqrt{s_{\mathit{NN}}}=200$ GeV using data taken from the PHENIX experiment in 2003 and previously published in S. S. Adler et al. [Phys. Rev. Lett 96, 012304 (2006)]. The high statistics proton-proton $J/\psi$ data taken in 2005 are used to improve the baseline measurement and thus construct updated cold nuclear matter modification factors ($R_{d\mathrm{Au}}$). A suppression of $J/\psi$ in cold nuclear matter is observed as one goes forward in rapidity (in the deuteron-going direction), corresponding to a region more sensitive to initial-state low-$x$ gluons in the gold nucleus. The measured nuclear modification factors are compared to theoretical calculations of nuclear shadowing to which a $J/\psi$ (or precursor) breakup cross section is added. Breakup cross sections of ${\sigma }_{\mathrm{breakup}}=2.8_{-1.4}^{+1.7}$ $(2.2_{-1.5}^{+1.6})$ mb are obtained by fitting these calculations to the data using two different models of nuclear shadowing. These breakup cross-section values are consistent within large uncertainties with the $4.2\pm 0.5$ mb determined at lower collision energies. Projecting this range of cold nuclear matter effects to copper-copper and gold-gold collisions reveals that the current constraints are not sufficient to firmly quantify the additional hot nuclear matter effect.

Measurements of midrapidity charged-particle multiplicity distributions, $d{N}_{\mathrm{ch}}/d\eta$, and midrapidity transverse-energy distributions, $d{E}_T/d\eta$, are presented for a variety of collision systems and energies. Included are distributions for $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$, 130, 62.4, 39, 27, 19.6, 14.5, and 7.7 GeV, $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ and 62.4 GeV, $\mathrm{Cu}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV, $\mathrm{U}+\mathrm{U}$ collisions at $\sqrt{s_{{}_{NN}}}=193$ GeV, $d+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV, ${}^3\mathrm{He}+\mathrm{Au}$ 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, $N_{\mathrm{part}}$, and the number of constituent quark participants, $N_{\mathrm{qp}}$. 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 $N_{\mathrm{qp}}$ than scaling with $N_{\mathrm{part}}$. Also presented are estimates of the Bjorken energy density, ${\varepsilon }_{\mathrm{BJ}}$, and the ratio of $d{E}_T/d\eta$ to $d{N}_{\mathrm{ch}}/d\eta$, the latter of which is seen to be constant as a function of centrality for all systems.

We present a systematic study of charged-pion and kaon interferometry in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=200$ GeV. The kaon mean source radii are found to be larger than pion radii in the outward and longitudinal directions for the same transverse mass; this difference increases for more central collisions. The azimuthal-angle dependence of the radii was measured with respect to the second-order event plane and similar oscillations of the source radii were found for pions and kaons. Hydrodynamic models qualitatively describe the similar oscillations of the mean source radii for pions and kaons, but they do not fully describe the transverse-mass dependence of the oscillations.

We have studied the dependence of azimuthal anisotropy $v_2$ for inclusive and identified charged hadrons in $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions on collision energy, species, and centrality. The values of $v_2$ as a function of transverse momentum $p_T$ and centrality in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=200$ and 62.4 GeV are the same within uncertainties. However, in $\mathrm{Cu}+\mathrm{Cu}$ collisions we observe a decrease in $v_2$ values as the collision energy is reduced from 200 to 62.4 GeV. The decrease is larger in the more peripheral collisions. By examining both $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions we find that $v_2$ depends both on eccentricity and the number of participants, $N_{\mathrm{part}}$. We observe that $v_2$ divided by eccentricity ($\varepsilon$) monotonically increases with $N_{\mathrm{part}}$ and scales as $N_{\mathrm{part}}^{1/3}$. The $\mathrm{Cu}+\mathrm{Cu}$ data at 62.4 GeV falls below the other scaled $v_2$ data. For identified hadrons, $v_2$ divided by the number of constituent quarks $n_q$ is independent of hadron species as a function of transverse kinetic energy $K{E}_T=m_T-m$ between $0.1<K{E}_T/n_q<1$ GeV. Combining all of the above scaling and normalizations, we observe a near-universal scaling, with the exception of the $\mathrm{Cu}+\mathrm{Cu}$ data at 62.4 GeV, of $v_2/(n_q·\varepsilon ·N_{\mathrm{part}}^{1/3})$ vs $K{E}_T/n_q$ for all measured particles.

Transverse momentum distributions and yields for ${\pi }^{\pm }$, $K^{\pm }$, $p$, and $\mathrm{p̄}$ in $p+p$ collisions at $\sqrt{s}$ $=$ 200 and 62.4 GeV at midrapidity are measured by the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC). These data provide important baseline spectra for comparisons with identified particle spectra in heavy ion collisions at RHIC. We present the inverse slope parameter $T_{\mathrm{inv}}$, mean transverse momentum $\langle p_T\rangle$, and yield per unit rapidity $\mathit{dN}/\mathit{dy}$ at each energy, and compare them to other measurements at different $\sqrt{s}$ in $p+p$ and $p+\mathrm{p̄}$ collisions. We also present the scaling properties such as $m_T$ scaling and $x_T$ scaling on the $p_T$ spectra between different energies. To discuss the mechanism of the particle production in $p+p$ collisions, the measured spectra are compared to next-to-leading-order or next-to-leading-logarithmic perturbative quantum chromodynamics calculations.

Recently, multiparticle-correlation measurements of relativistic $p/d/{}^3\mathrm{He}+\mathrm{Au}$, $p+\mathrm{Pb}$, and even $p+p$ collisions show surprising collective signatures. Here, we present beam-energy-scan measurements of two-, four-, and six-particle angular correlations in $d+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200$, 62.4, 39, and 19.6 GeV. We also present measurements of two- and four-particle angular correlations in $p+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. We find the four-particle cumulant to be real valued for $d+\mathrm{Au}$ collisions at all four energies. We also find that the four-particle cumulant in $p+\mathrm{Au}$ has the opposite sign as that in $d+\mathrm{Au}$. Further, we find that the six-particle cumulant agrees with the four-particle cumulant in $d+\mathrm{Au}$ collisions at 200 GeV, indicating that nonflow effects are subdominant. These observations provide strong evidence that the correlations originate from the initial geometric configuration, which is then translated into the momentum distribution for all particles, commonly referred to as collectivity.

We present a measurement of the cross section and transverse single-spin asymmetry ($A_N$) for $\eta$ mesons at large pseudorapidity from $\sqrt{s}=200\mathrm{GeV}$ $p^{\uparrow }+p$ collisions. The measured cross section for $0.5<p_T<5.0\mathrm{GeV}/c$ and $3.0<|\eta |<3.8$ is well described by a next-to-leading-order perturbative-quantum-chromodynamics calculation. The asymmetries $A_N$ have been measured as a function of Feynman-$x$ ($x_F$) from $0.2<|x_F|<0.7$, as well as transverse momentum ($p_T$) from $1.0<p_T<4.5\mathrm{GeV}/c$. The asymmetry averaged over positive $x_F$ is $⟨A_N⟩=0.061\pm 0.014$. The results are consistent with prior transverse single-spin measurements of forward $\eta$ and ${\pi }^0$ mesons at various energies in overlapping $x_F$ ranges. Comparison of different particle species can help to determine the origin of the large observed asymmetries in $p^{\uparrow }+p$ collisions.

The PHENIX experiment at the Relativistic Heavy Ion Collider has performed a systematic study of $K_S^0$ and $K^{*0}$ meson production at midrapidity in $p+p,d+\mathrm{Au}$, and $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{{}_{\mathit{NN}}}}=200$ GeV. The $K_S^0$ and $K^{*0}$ mesons are reconstructed via their $K_S^0\to {\pi }^0(\to \gamma \gamma ){\pi }^0(\to \gamma \gamma )$ and $K^{*0}\to K^{\pm }{\pi }^{\mp }$ decay modes, respectively. The measured transverse-momentum spectra are used to determine the nuclear modification factor of $K_S^0$ and $K^{*0}$ mesons in $d+\text{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions at different centralities. In the $d+\mathrm{Au}$ collisions, the nuclear modification factor of $K_S^0$ and $K^{*0}$ mesons is almost constant as a function of transverse momentum and is consistent with unity, showing that cold-nuclear-matter effects do not play a significant role in the measured kinematic range. In $\mathrm{Cu}+\mathrm{Cu}$ collisions, within the uncertainties no nuclear modification is registered in peripheral collisions. In central collisions, both mesons show suppression relative to the expectations from the $p+p$ yield scaled by the number of binary nucleon-nucleon collisions in the $\mathrm{Cu}+\mathrm{Cu}$ system. In the $p_T$ range $2–5 \mathrm{GeV}/c$, the strange mesons $(K_S^0,K^{*0})$ similarly to the $\varphi$ meson with hidden strangeness, show an intermediate suppression between the more suppressed light quark mesons $\left({\pi }^0\right)$ and the nonsuppressed baryons $(p,\overline{p})$. At higher transverse momentum, $p_T>5 \mathrm{GeV}/c$, production of all particles is similarly suppressed by a factor of $\approx 2$.

The PHENIX experiment has measured the spin alignment for inclusive $J/\psi \to e^{+}{e}^{-}$ decays in proton-proton collisions at $\sqrt{s}=510\mathrm{GeV}$ at midrapidity. The angular distributions have been measured in three different polarization frames, and the three decay angular coefficients have been extracted in a full two-dimensional analysis. Previously, PHENIX saw large longitudinal net polarization at forward rapidity at the same collision energy. This analysis at midrapidity, complementary to the previous PHENIX results, sees no sizable polarization in the measured transverse momentum range of $0.0<p_T<10.0\mathrm{GeV}/c$. The results are consistent with a previous one-dimensional analysis at midrapidity at $\sqrt{s}=200\mathrm{GeV}$. The transverse-momentum-dependent cross section for midrapidity $J/\psi$ production has additionally been measured, and after comparison to world data, a simple logarithmic dependence of the cross section on $\sqrt{s}$ was found.