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The ferroelectromagnet ${\mathrm{YMnO}}_3$ consists of weakly coupled triangular layers of $S=2$ spins. Below $T_N\approx 70\mathrm{K}$ muon-spin relaxation data show two oscillatory relaxing signals due to magnetic order, with no purely relaxing signals resolvable (which would require different coexisting spin distributions). The transition temperature $T_N$ increases with applied hydrostatic pressure, even though the ordered moment decreases. These results suggest that pressure increases both the exchange coupling between the layers and the frustration within the layers.

We report the results of a muon spin rotation study on electron-doped chromium sulfides, ${{\mathrm{Cr}}_2\mathrm{S}}_{3-x}$, with $0⩽x⩽0.08$. Two muon-spin precession frequencies are observed in each sample, corresponding to two distinct stopping sites. We show that even a small amount of electron doping, induced by sulfur deficiency, causes a drastic change in the magnetic properties.

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.

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.

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.

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.

The PHENIX experiment has measured midrapidity transverse momentum spectra ($0.4<p_T<4.0\mathrm{G}\mathrm{e}\mathrm{V}/c$) of single electrons as a function of centrality in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=200\mathrm{G}\mathrm{e}\mathrm{V}$. Contributions from photon conversions and Dalitz decays of light neutral mesons are measured by introducing a thin (1.7% $X_0$) converter into the PHENIX acceptance and are statistically removed. The subtracted nonphotonic electron spectra are primarily due to the semileptonic decays of hadrons containing heavy quarks, mainly charm at lower $p_T$. For all centralities, the charm production cross section is found to scale with the nuclear overlap function, $T_{AA}$. For minimum-bias collisions the charm cross section per binary collision is $N_{c\overline{c}}/T_{AA}=622\pm 57(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 160(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\mu \mathrm{b}$.

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.

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.

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.