Search

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 $\sqrt{s}=510\mathrm{GeV}$ longitudinally polarized $p+p$ collisions at the Relativistic Heavy Ion Collider 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 $x\approx 5\times {10}^{-2}$ where recent 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 $x<5\times {10}^{-2}$.

We report a measurement of $e^{+}{e}^{-}$ pairs from semileptonic heavy-flavor decays in $d+\text{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV. By exploring the mass and transverse-momentum dependence of the yield, the bottom decay contribution can be isolated from charm, and quantified by comparison to pythia and mc@nlo simulations. The resulting $b\overline{b}$-production cross section is ${\sigma }_{b\overline{b}}^{d\mathrm{Au}}=1.37\pm 0.28\left(\mathrm{stat}\right)\pm 0.46\left(\mathrm{syst}\right)$ mb, which is equivalent to a nucleon-nucleon cross section of ${\sigma }_{bb}^{NN}=3.4\pm 0.8\left(\mathrm{stat}\right)\pm 1.1\left(\mathrm{syst}\right)\mu b$.

The PHENIX Collaboration has measured $\varphi$ meson production in $d+\text{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV using the dimuon and dielectron decay channels. The $\varphi$ meson is measured in the forward (backward) $d$-going (Au-going) direction, $1.2<y<2.2$ ($-2.2<y<-1.2$) in the transverse-momentum ($p_T$) range from 1–7 $\mathrm{GeV}/c$ and at midrapidity $\left|y\right|<0.35$ in the $p_T$ range below 7 GeV/$c$. The $\varphi$ meson invariant yields and nuclear-modification factors as a function of $p_T$, rapidity, and centrality are reported. An enhancement of $\varphi$ meson production is observed in the Au-going direction, while suppression is seen in the $d$-going direction, and no modification is observed at midrapidity relative to the yield in $p+p$ collisions scaled by the number of binary collisions. Similar behavior was previously observed for inclusive charged hadrons and open heavy flavor, indicating similar cold-nuclear-matter effects.

LiFeAs is unique among the arsenic based Fe-pnictide superconductors because it is the only nearly stoichiometric compound which does not exhibit magnetic order. This is at odds with electronic structure calculations which find a very stable magnetic state and predict cylindrical hole- and electron-like Fermi surface sheets whose geometry suggests spin fluctuations and a possible instability toward long-range ordering at the nesting vector. In fact, a complex magnetic phase diagram is indeed observed in the isostructural NaFeAs compound. Previous angle-resolved photoemission (ARPES) experiments revealed the existence of both hole and electron-like surfaces, but with rather distinct cross-sectional areas and an absence of the nesting that is thought to underpin both magnetic order and superconductivity in the pnictide family of superconductors. These ARPES observations were challenged by subsequent de Haas–van Alphen (dHvA) measurements which detected a few, electron-like Fermi surface sheets in rough agreement with the original band calculations. Here, we show a detailed dHvA study unveiling additional, small and nearly isotropic Fermi surface sheets in LiFeAs single crystals, which ought to correspond to hole-like orbits, as previously observed by ARPES. Therefore, our results reconcile the apparent discrepancy between ARPES and the previous dHvA results. The small size of these Fermi surface pockets suggests a prominent role for the electronic correlations in LiFeAs. The absence of gap nodes, in combination with the coexistence of quasi-two-dimensional and three-dimensional Fermi surfaces, favor an $s$-wave pairing symmetry for LiFeAs. But similar electron-like Fermi surfaces combined with very different hole pockets between LiFeAs and LiFeP suggest that the nodes in the gap function of LiFeP might be located on the hole pockets. This would be difficult to reconcile with the current understanding of the $s\pm$ scenario.

In two-particle angular correlation measurements, jets give rise to a near-side peak, formed by particles associated to a higher-$p_{\mathrm{T}}$ trigger particle. Measurements of these correlations as a function of pseudorapidity ($\mathrm{\Delta }\eta$) and azimuthal ($\mathrm{\Delta }\phi$) differences are used to extract the centrality and $p_{\mathrm{T}}$ dependence of the shape of the near-side peak in the $p_{\mathrm{T}}$ range $1<p_{\mathrm{T}}<8$ $\mathrm{GeV}/c$ in Pb-Pb and $\mathit{pp}$ collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. A combined fit of the near-side peak and long-range correlations is applied to the data and the peak shape is quantified by the variance of the distributions. While the width of the peak in the $\mathrm{\Delta }\phi$ direction is almost independent of centrality, a significant broadening in the $\mathrm{\Delta }\eta$ direction is found from peripheral to central collisions. This feature is prominent for the low-$p_{\mathrm{T}}$ region and vanishes above $4 \mathrm{GeV}/c$. The widths measured in peripheral collisions are equal to those in $\mathit{pp}$ collisions in the $\mathrm{\Delta }\phi$ direction and above $3 \mathrm{GeV}/c$ in the $\mathrm{\Delta }\eta$ direction. Furthermore, for the 10% most central collisions and $1<p_{\mathrm{T},\mathrm{assoc}}<$ $2 \mathrm{GeV}/c, 1<p_{\mathrm{T},\mathrm{trig}}<$ $3 \mathrm{GeV}/c$, a departure from a Gaussian shape is found: a depletion develops around the center of the peak. The results are compared to A Multi-Phase Transport (AMPT) model simulation as well as other theoretical calculations indicating that the broadening and the development of the depletion are connected to the strength of radial and longitudinal flow.

The measurement of two-particle angular correlations is a powerful tool to study jet quenching in a $p_T$ region inaccessible by direct jet identification. In these measurements pseudorapidity ($\mathrm{\Delta }\eta$) and azimuthal ($\mathrm{\Delta }\phi$) differences are used to extract the shape of the near-side peak formed by particles associated with a higher $p_T$ trigger particle ($1<p_{T,\mathrm{trig}}<8\mathrm{GeV}/c$). A combined fit of the near-side peak and long-range correlations is applied to the data allowing the extraction of the centrality evolution of the peak shape in Pb-Pb collisions at $\sqrt{s_{NN}}=2.76\mathrm{TeV}$. A significant broadening of the peak in the $\mathrm{\Delta }\eta$ direction at low $p_T$ is found from peripheral to central collisions, which vanishes above $4\mathrm{GeV}/c$, while in the $\mathrm{\Delta }\phi$ direction the peak is almost independent of centrality. For the 10% most central collisions and $1<p_{T,\text{assoc}}<2\mathrm{GeV}/c$, $1<p_{T,\mathrm{trig}}<3\mathrm{GeV}/c$ a novel feature is observed: a depletion develops around the center of the peak. The results are compared to $pp$ collisions at the same center of mass energy and ampt model simulations. The comparison to the investigated models suggests that the broadening and the development of the depletion is connected to the strength of radial and longitudinal flow.

Measurements of transverse-single-spin asymmetries ($A_N$) in $p+p$ collisions at $\sqrt{s}=62.4$ and 200 GeV with the PHENIX detector at the Relativistic Heavy Ion Collider are presented. At midrapidity, $A_N$ is measured for neutral pion and eta mesons reconstructed from diphoton decay, and, at forward rapidities, neutral pions are measured using both diphotons and electromagnetic clusters. The neutral-pion measurement of $A_N$ at midrapidity is consistent with zero with uncertainties a factor of 20 smaller than previous publications, which will lead to improved constraints on the gluon Sivers function. At higher rapidities, where the valence quark distributions are probed, the data exhibit sizable asymmetries. In comparison with previous measurements in this kinematic region, the new data extend the kinematic coverage in $\sqrt{s}$ and $p_T$, and it is found that the asymmetries depend only weakly on $\sqrt{s}$. The origin of the forward $A_N$ is presently not understood quantitatively. The extended reach to higher $p_T$ probes the transition between transverse momentum dependent effects at low $p_T$ and multiparton dynamics at high $p_T$.

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 structure of the superconducting order parameter in the iron-pnictide superconductor BaFe${}_2$(As${}_{0.67}$P${}_{0.33}$)${}_2$ ($T_c=31$ K) with line nodes is studied by the angle-resolved thermal conductivity measurements in a magnetic field rotated within the basal plane. We find that the thermal conductivity displays distinct fourfold oscillations with minima when the field is directed at $\pm {45}^{\circ }$ with respect to the tetragonal $a$ axis. We discuss possible gap structures that can account for the data, and conclude that the observed results are most consistent with the closed nodal loops located at the flat parts of the electron Fermi surface with high Fermi velocity.

New measurements by the PHENIX experiment at the Relativistic Heavy Ion Collider for $\eta$ production at midrapidity as a function of transverse momentum ($p_T$) and collision centrality in $\sqrt{s_{\mathit{NN}}}=200$ GeV $\mathrm{Au}+\mathrm{Au}$ and $p+p$ collisions are presented. They indicate nuclear modification factors ($R_{\mathrm{AA}}$) which are similar in both magnitude and trend to those found in earlier ${\pi }^0$ measurements. Linear fits to $R_{\mathrm{AA}}$ as a function of $p_T$ in 5–20 GeV$/c$ show that the slope is consistent with zero within two standard deviations at all centralities, although a slow rise cannot be excluded. Having different statistical and systematic uncertainties, the ${\pi }^0$ and $\eta$ measurements are complementary at high $p_T$; thus, along with the extended $p_T$ range of these data they can provide additional constraints for theoretical modeling and the extraction of transport properties.

We report the observation of the de Haas–van Alphen effect in ${\mathrm{IrTe}}_2$ measured using torque magnetometry at low temperatures down to 0.4 K and in high magnetic fields up to 33 T. ${\mathrm{IrTe}}_2$ undergoes a major structural transition around $\sim 283\left(1\right)\mathrm{K}$ due to the formation of planes of Ir and Te dimers that cut diagonally through the lattice planes, with its electronic structure predicted to change significantly from a layered system with predominantly three-dimensional character to a tilted quasi-two-dimensional Fermi surface. Quantum oscillations provide direct confirmation of this unusual tilted Fermi surface and also reveal very light quasiparticle masses (less than $1m_e$), with no significant enhancement due to electronic correlations. We find good agreement between the angular dependence of the observed and calculated de Haas–van Alphen frequencies, taking into account the contribution of different structural domains that form while cooling ${\mathrm{IrTe}}_2$.

The $p_T$-differential production cross sections of the prompt charmed mesons $D^0$, $D^{+}$, $D^{*+}$, and $D_s^{+}$ and their charge conjugate in the rapidity interval $-0.96<y_{\text{cms}}<0.04$ were measured in $p\text{−}\mathrm{Pb}$ collisions at a center-of-mass energy $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ with the ALICE detector at the LHC. The nuclear modification factor $R_{p\mathrm{Pb}}$, quantifying the $D$-meson yield in $p\text{−}\mathrm{Pb}$ collisions relative to the yield in $pp$ collisions scaled by the number of binary nucleon-nucleon collisions, is compatible within the 15%–20% uncertainties with unity in the transverse momentum interval $1<p_T<24\mathrm{GeV}/c$. No significant difference among the $R_{p\mathrm{Pb}}$ of the four $D$-meson species is observed. The results are described within uncertainties by theoretical calculations that include initial-state effects. The measurement adds experimental evidence that the modification of the momentum spectrum of $D$ mesons observed in Pb-Pb collisions with respect to $pp$ collisions is due to strong final-state effects induced by hot partonic matter.

We present measured $J/\psi$ production rates in $d+\text{Au}$ collisions at $\sqrt{s_{{}_{NN}}}=200$ GeV over broad ranges of transverse momentum ($p_T=0$–14 GeV/$c$) and rapidity ($-2.2<y<2.2$). We construct the nuclear-modification factor $R_{d\mathrm{Au}}$ for these kinematics and as a function of collision centrality (related to impact parameter for the $d+\mathrm{Au}$ collision). We find that the modification is largest for collisions with small impact parameters and observe a suppression ($R_{d\mathrm{Au}}<1$) for $p_T<4$ GeV/$c$ at positive rapidities. At negative rapidity we observe a suppression for $p_T<2$ GeV/$c$ then an enhancement ($R_{d\mathrm{Au}}>1$) for $p_T>2$ GeV/$c$. The observed enhancement at negative rapidity has implications for the interpretation of the observed modification in heavy-ion collisions at high $p_T$.

We have measured the cross section and single-spin asymmetries from forward $W^{\pm }\to {\mu }^{\pm }\nu$ production in longitudinally polarized $p+p$ collisions at $\sqrt{s}=510\mathrm{GeV}$ using the PHENIX detector at the Relativistic Heavy Ion Collider. The cross sections are consistent with previous measurements at this collision energy, while the most forward and backward longitudinal single spin asymmetries provide new insights into the sea quark helicities in the proton. The charge of the $W$ bosons provides a natural flavor separation of the participating partons.

The PHENIX collaboration at the Relativistic Heavy Ion Collider (RHIC) reports measurements of azimuthal dihadron correlations near midrapidity in $d+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. These measurements complement recent analyses by experiments at the Large Hadron Collider (LHC) involving central $p+\mathrm{Pb}$ collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$, 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 $d+\mathrm{Au}$ collisions at RHIC compared to those seen in $p+\mathrm{Pb}$ collisions at the LHC. The larger extracted $v_2$ values in $d+\mathrm{Au}$ are consistent with expectations from hydrodynamic calculations owing to the larger expected initial-state eccentricity compared with that from $p+\mathrm{Pb}$ 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 experiment has measured the production of ${\pi }^0$s in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathit{NN}}}$ $=$ 200 GeV. The new data offer a fourfold increase in recorded luminosity, providing higher precision and a larger reach in transverse momentum, $p_T$, to 20 GeV/$c$. The production ratio of $\eta /{\pi }^0$ is $0.46\pm 0.01\left(\mathrm{stat}\right)\pm 0.05\left(\mathrm{syst}\right)$, constant with $p_T$ and collision centrality. The observed ratio is consistent with earlier measurements, as well as with the $p+p$ and $d+\mathrm{Au}$ values. ${\pi }^0$ are suppressed by a factor of 5, as in earlier findings. However, with the improved statistical precision a small but significant rise of the nuclear modification factor $R_{\mathrm{AA}}$ vs $p_T$, with a slope of 0.0106${\pm }_{0.0029}^{0.0034}$ (Gev/$c$)${}^{-1}$, is discernible in central collisions. A phenomenological extraction of the average fractional parton energy loss shows a decrease with increasing $p_T$. To study the path-length dependence of suppression, the ${\pi }^0$ yield is measured at different angles with respect to the event plane; a strong azimuthal dependence of the ${\pi }^0$ $R_{\mathrm{AA}}$ is observed. The data are compared to theoretical models of parton energy loss as a function of the path length $L$ in the medium. Models based on perturbative quantum chromodynamics are insufficient to describe the data, while a hybrid model utilizing pQCD for the hard interactions and anti-de-Sitter space/conformal field theory (AdS/CFT) for the soft interactions is consistent with the data.

The transverse momentum ($p_T$) spectra and ratios of identified charged hadrons (${\pi }^{\pm }$, $K^{\pm }$, $p$, $\overline{p}$) produced in $\sqrt{s_{{}_{NN}}}=200$ GeV $\mathrm{Au}+\mathrm{Au}$ and $d+\text{Au}$ collisions are reported in five different centrality classes for each collision species. The measurements of pions and protons are reported up to $p_T=6$ GeV/$c$ (5 GeV/$c$), and the measurements of kaons are reported up to $p_T=4$ GeV/$c$ (3.5 GeV/$c$) in $\mathrm{Au}+\mathrm{Au}$ ($d+\text{Au}$) collisions. In the intermediate $p_T$ region, between 2 and 5 GeV/$c$, a significant enhancement of baryon-to-meson ratios compared to those measured in $p+p$ collisions is observed. This enhancement is present in both $\mathrm{Au}+\mathrm{Au}$ and $d+\text{Au}$ collisions and increases as the collisions become more central. We compare a class of peripheral $\mathrm{Au}+\mathrm{Au}$ collisions with a class of central $d+\text{Au}$ collisions which have a comparable number of participating nucleons and binary nucleon-nucleon collisions. The $p_T$-dependent particle ratios for these classes display a remarkable similarity, which is then discussed.

The correlations between event-by-event fluctuations of anisotropic flow harmonic amplitudes have been measured in Pb-Pb collisions at $\sqrt{s_{\mathit{NN}}}=2.76$ TeV with the ALICE detector at the Large Hadron Collider. The results are reported in terms of multiparticle correlation observables dubbed symmetric cumulants. These observables are robust against biases originating from nonflow effects. The centrality dependence of correlations between the higher order harmonics (the quadrangular $v_4$ and pentagonal $v_5$ flow) and the lower order harmonics (the elliptic $v_2$ and triangular $v_3$ flow) is presented. The transverse momentum dependences of correlations between $v_3$ and $v_2$ and between $v_4$ and $v_2$ are also reported. The results are compared to calculations from viscous hydrodynamics and a multiphase transport (AMPT) model calculations. The comparisons to viscous hydrodynamic models demonstrate that the different order harmonic correlations respond differently to the initial conditions and the temperature dependence of the ratio of shear viscosity to entropy density $(\eta /s)$. A small average value of $\eta /s$ is favored independent of the specific choice of initial conditions in the models. The calculations with the AMPT initial conditions yield results closest to the measurements. Correlations among the magnitudes of $v_2, v_3$, and $v_4$ show moderate $p_{\mathrm{T}}$ dependence in midcentral collisions. This might be an indication of possible viscous corrections to the equilibrium distribution at hadronic freeze-out, which might help to understand the possible contribution of bulk viscosity in the hadronic phase of the system. Together with existing measurements of individual flow harmonics, the presented results provide further constraints on the initial conditions and the transport properties of the system produced in heavy-ion collisions.

Invariant differential yields of deuterons and antideuterons in $pp$ collisions at $\sqrt{s}$ = 0.9, 2.76 and 7 TeV and the yields of tritons, ${}^3\mathrm{He}$ nuclei, and their antinuclei at $\sqrt{s}$ = 7 TeV have been measured with the ALICE detector at the CERN Large Hadron Collider. The measurements cover a wide transverse momentum ($p_{\text{T}}$) range in the rapidity interval $\left|y\right|<0.5$, extending both the energy and the $p_{\text{T}}$ reach of previous measurements up to 3 GeV/$c$ for $A=2$ and 6 GeV/$c$ for $A=3$. The coalescence parameters of (anti)deuterons and ${}^3\overline{\text{He}}$ nuclei exhibit an increasing trend with $p_{\text{T}}$ and are found to be compatible with measurements in $pA$ collisions at low $p_{\text{T}}$ and lower energies. The integrated yields decrease by a factor of about 1000 for each increase of the mass number with one (anti)nucleon. Furthermore, the deuteron-to-proton ratio is reported as a function of the average charged particle multiplicity at different center-of-mass energies.