Search

Measurements in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ of jet correlations for a trigger hadron at intermediate transverse momentum ($p_{T,\mathrm{trig}}$) with associated mesons or baryons at lower $p_{T,\mathrm{assoc}}$ indicate strong modification of the away-side jet. The ratio of jet-associated baryons to mesons increases with centrality and $p_{T,\mathrm{assoc}}$. For the most central collisions, the ratio is similar to that for inclusive measurements. This trend is incompatible with in-vacuum fragmentation but could be due to jetlike contributions from correlated soft partons, which recombine upon hadronization.

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 ${\sigma }_{bb}^{NN}=3.4\pm 0.8\left(\mathrm{stat}\right)\pm 1.1\left(\mathrm{syst}\right)\mu b$.

We report a measurement of $e^{+}{e}^{-}$ pairs from semileptonic heavy-flavor decays in $p+p$ collisions at $\sqrt{s_{{}_{\mathit{\text{NN}}}}}=200$ GeV. The $e^{+}{e}^{-}$ pair yield from $b\overline{b}$ and $c\overline{c}$ is separated by exploiting a double differential fit done simultaneously in dielectron invariant mass and $p_T$. We used three different event generators, pythia, mc@nlo, and powheg, to simulate the $e^{+}{e}^{-}$ spectra from $c\overline{c}$ and $b\overline{b}$ production. The data can be well described by all three generators within the detector acceptance. However, when using the generators to extrapolate to $4\pi$, significant differences are observed for the total cross section. These difference are less pronounced for $b\overline{b}$ than for $c\overline{c}$. The same model dependence was observed in already published $d+A$ data. The $p+p$ data are also directly compared with $d+A$ data in mass and $p_T$, and within the statistical accuracy no nuclear modification is seen.

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.

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 $p_T$.

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.

Transverse momentum spectra of electrons ($p_T^e$) from semileptonic weak decays of heavy-flavor mesons in the range of $0.3<p_T^e<9.0$ GeV/$c$ have been measured at midrapidity ($\left|y\right|<0.35$) by the PHENIX experiment at the Relativistic Heavy Ion Collider in $p+p$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV. In addition, the azimuthal anisotropy parameter $v_2$ has been measured for $0.3<p_T^e<5.0$ GeV/$c$ in $\mathrm{Au}+\mathrm{Au}$ collisions. The substantial modification in the $p_T^e$ spectra in $\mathrm{Au}+\mathrm{Au}$ compared with $p+p$ collisions as well as the nonzero $v_2$ indicate substantial interactions and flow of heavy quarks in traversing the produced medium. Comparisons of these observables with detailed theoretical calculations can be used to identify the nature of these interactions and to quantify their extent.

Measurements of double-helicity asymmetries in inclusive hadron production in polarized $p+p$ collisions are sensitive to helicity-dependent parton distribution functions, in particular, to the gluon helicity distribution, $\Delta g$. This study focuses on the extraction of the double-helicity asymmetry in $\eta$ production ($\overrightarrow{p}+\overrightarrow{p}\to \eta +X$), the $\eta$ cross section, and the $\eta /{\pi }^0$ cross section ratio. The cross section and ratio measurements provide essential input for the extraction of fragmentation functions that are needed to access the helicity-dependent parton distribution functions.

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 $p_T$.

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.

We present azimuthal angular correlations between charged hadrons and energy deposited in calorimeter towers in central $d+\mathrm{Au}$ and minimum bias $p+p$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The charged hadron is measured at midrapidity $|\eta |<0.35$, and the energy is measured at large rapidity ($-3.7<\eta <-3.1$, Au-going direction). An enhanced near-side angular correlation across $|\mathrm{\Delta }\eta |>2.75$ is observed in $d+\mathrm{Au}$ collisions. Using the event plane method applied to the Au-going energy distribution, we extract the anisotropy strength $v_2$ for inclusive charged hadrons at midrapidity up to $p_T=4.5\mathrm{GeV}/c$. We also present the measurement of $v_2$ for identified ${\pi }^{\pm }$ and (anti)protons in central $d+\mathrm{Au}$ collisions, and observe a mass-ordering pattern similar to that seen in heavy-ion collisions. These results are compared with viscous hydrodynamic calculations and measurements from $p+\mathrm{Pb}$ at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$. The magnitude of the mass ordering in $d+\mathrm{Au}$ is found to be smaller than that in $p+\mathrm{Pb}$ collisions, which may indicate smaller radial flow in lower energy $d+\mathrm{Au}$ collisions.

The PHENIX experiment at RHIC has measured the centrality dependence of the direct photon yield from $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{{}_{\mathit{N}N}}}=200 \mathrm{GeV}$ down to $p_T=0.4 \mathrm{GeV}/c$. Photons are detected via photon conversions to $e^{+}{e}^{-}$ pairs and an improved technique is applied that minimizes the systematic uncertainties that usually limit direct photon measurements, in particular at low $p_T$. We find an excess of direct photons above the $N_{\mathrm{coll}}$-scaled yield measured in $p+p$ collisions. This excess yield is well described by an exponential distribution with an inverse slope of about $240\mathrm{MeV}/c$ in the $p_T$ range $0.6–2.0 \mathrm{GeV}/c$. While the shape of the $p_T$ distribution is independent of centrality within the experimental uncertainties, the yield increases rapidly with increasing centrality, scaling approximately with $N_{\mathrm{part}}^{\alpha }$, where $\alpha =1.38\pm 0.03\left(\mathrm{stat}\right)\pm 0.07\left(\mathrm{syst}\right)$.

We report on the event structure and double helicity asymmetry ($A_{LL}$) of jet production in longitudinally polarized $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$. Photons and charged particles were measured by the PHENIX experiment at midrapidity $|\eta |<0.35$ with the requirement of a high-momentum ($>2\mathrm{GeV}/c$) photon in the event. Event structure, such as multiplicity, $p_T$ density and thrust in the PHENIX acceptance, were measured and compared with the results from the pythia event generator and the geant detector simulation. The shape of jets and the underlying event were well reproduced at this collision energy. For the measurement of jet $A_{LL}$, photons and charged particles were clustered with a seed-cone algorithm to obtain the cluster $p_T$ sum ($p_T^{\mathrm{reco}}$). The effect of detector response and the underlying events on $p_T^{\mathrm{reco}}$ was evaluated with the simulation. The production rate of reconstructed jets is satisfactorily reproduced with the next-to-leading-order and perturbative quantum chromodynamics jet production cross section. For $4<p_T^{\mathrm{reco}}<12\mathrm{GeV}/c$ with an average beam polarization of $⟨P⟩=49\%$ we measured $A_{LL}=-0.0014\pm {0.0037}^{\mathrm{stat}}$ at the lowest $p_T^{\mathrm{reco}}$ bin ($4–5\mathrm{GeV}/c$) and $-0.0181\pm {0.0282}^{\mathrm{stat}}$ at the highest $p_T^{\mathrm{reco}}$ bin ($10–12\mathrm{GeV}/c$) with a beam polarization scale error of 9.4% and a $p_T$ scale error of 10%. Jets in the measured $p_T^{\mathrm{reco}}$ range arise primarily from hard-scattered gluons with momentum fraction $0.02<x<0.3$ according to pythia. The measured $A_{LL}$ is compared with predictions that assume various $\Delta G(x)$ distributions based on the Gluck-Reya-Stratmann-Vogelsang parameterization. The present result imposes the limit $-1.1<{\int }_{0.02}^{0.3}dx\Delta G(x,{\mu }^2=1{\mathrm{GeV}}^2)<0.4$ at 95% confidence level or ${\int }_{0.02}^{0.3}dx\Delta G(x,{\mu }^2=1{\mathrm{GeV}}^2)<0.5$ at 99% confidence level.

Flow coefficients $v_n$ for $n=2$, 3, 4, characterizing the anisotropic collective flow in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$, are measured relative to event planes ${\Psi }_n$, determined at large rapidity. We report $v_n$ as a function of transverse momentum and collision centrality, and study the correlations among the event planes of different order $n$. The $v_n$ are well described by hydrodynamic models which employ a Glauber Monte Carlo initial state geometry with fluctuations, providing additional constraining power on the interplay between initial conditions and the effects of viscosity as the system evolves. This new constraint can serve to improve the precision of the extracted shear viscosity to entropy density ratio $\eta /s$.

Back-to-back hadron pair yields in $d+\mathrm{Au}$ and $p+p$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$ were measured with the PHENIX detector at the Relativistic Heavy Ion Collider. Rapidity separated hadron pairs were detected with the trigger hadron at pseudorapidity $|\eta |<0.35$ and the associated hadron at forward rapidity (deuteron direction, $3.0<\eta <3.8$). Pairs were also detected with both hadrons measured at forward rapidity; in this case, the yield of back-to-back hadron pairs in $d+\mathrm{Au}$ collisions with small impact parameters is observed to be suppressed by a factor of 10 relative to $p+p$ collisions. The kinematics of these pairs is expected to probe partons in the Au nucleus with a low fraction $x$ of the nucleon momenta, where the gluon densities rise sharply. The observed suppression as a function of nuclear thickness, $p_T$, and $\eta$ points to cold nuclear matter effects arising at high parton densities.

We report the measurement of direct photons at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. The direct photon signal was extracted for the transverse momentum range of $4\mathrm{GeV}/c<p_T<22\mathrm{GeV}/c$, using a statistical method to subtract decay photons from the inclusive photon sample. The direct photon nuclear modification factor $R_{\mathrm{AA}}$ was calculated as a function of $p_T$ for different $\mathrm{Au}+\mathrm{Au}$ collision centralities using the measured $p+p$ direct photon spectrum and compared to theoretical predictions. $R_{\mathrm{AA}}$ was found to be consistent with unity for all centralities over the entire measured $p_T$ range. Theoretical models that account for modifications of initial direct photon production due to modified parton distribution functions in Au and the different isospin composition of the nuclei predict a modest change of $R_{\mathrm{AA}}$ from unity. They are consistent with the data. Models with compensating effects of the quark-gluon plasma on high-energy photons, such as suppression of jet-fragmentation photons and induced-photon bremsstrahlung from partons traversing the medium, are also consistent with this measurement.