PHENIX presents a simultaneous measurement of the production of direct \(\gamma\) and \(\pi^0\) in $d$$+\(Au collisions at \)\sqrt{s_{_{NN}}}=200\( GeV over a \)p_T\( range of 7.5 to 18 GeV/\)c\( for ...different event samples selected by event activity, i.e. charged-particle multiplicity detected at forward rapidity. Direct-photon yields are used to empirically estimate the contribution of hard-scattering processes in the different event samples. Using this estimate, the average nuclear-modification factor \)R_{d\rm Au,EXP}^{\gamma^{\rm dir}}\( is \)0.925{\pm}0.023({\rm stat}){\pm}0.15^{\rm (scale)}\(, consistent with unity for minimum-bias (MB) \)d$$+\(Au events. For event classes with moderate event activity, \)R_{d\rm Au,EXP}^{\gamma^{\rm dir}}\( is consistent with the MB value within 5\% uncertainty. These results confirm that the previously observed enhancement of high-\)p_T\( \)\pi^0\( production found in small-system collisions with low event activity is a result of a bias in interpreting event activity within the Glauber framework. In contrast, for the top 5\% of events with the highest event activity, \)R_{d\rm Au,EXP}^{\gamma^{\rm dir}}\( is suppressed by 20\% relative to the MB value with a significance of \)4.5\sigma$, which may be due to final-state effects.
Recently, the PHENIX Collaboration has published second- and third-harmonic Fourier coefficients \(v_2\) and \(v_3\) for midrapidity (\(|\eta|<0.35\)) charged hadrons in 0\%--5\% central $p$$+\(Au, ...\)d$$+\(Au, and \)^3\(He\)+\(Au collisions at \)\sqrt{s_{_{NN}}}=200\( GeV utilizing three sets of two-particle correlations for two detector combinations with different pseudorapidity acceptance Phys. Rev. C {\bf 105}, 024901 (2022). This paper extends these measurements of \)v_2\( to all centralities in \)p$$+\(Au, \)d$$+\(Au, and \)^3\(He\)+\(Au collisions, as well as \)p$$+$$p\( collisions, as a function of transverse momentum (\)p_T\() and event multiplicity. The kinematic dependence of \)v_2\( is quantified as the ratio \)R\( of \)v_2\( between the two detector combinations as a function of event multiplicity for \)0.5$$<$$p_T$$<$$1\( and \)2$$<$$p_T$$<$$2.5\( GeV/\)c\(. A multiphase-transport (AMPT) model can reproduce the observed \)v_2\( in most-central to midcentral \)d$$+\(Au and \)^3\(He\)+\(Au collisions. However, the AMPT model systematically overestimates the measurements in \)p$$+$$p\(, \)p$$+\(Au, and peripheral \)d$$+\(Au and \)^3\(He\)+\(Au collisions, indicating a higher nonflow contribution in AMPT than in the experimental data. The AMPT model fails to describe the observed \)R\( for \)0.5$$<$$p_T$$<$$1\( GeV/\)c\(, but there is qualitative agreement with the measurements for \)2$$<$$p_T$$<$$2.5\( GeV/\)c$.
The experimental study of the collisions of heavy nuclei at relativistic energies has established the properties of the quark-gluon plasma (QGP), a state of hot, dense nuclear matter in which quarks ...and gluons are not bound into hadrons. In this state, matter behaves as a nearly inviscid fluid that efficiently translates initial spatial anisotropies into correlated momentum anisotropies among the produced particles, producing a common velocity field pattern known as collective flow. In recent years, comparable momentum anisotropies have been measured in small-system proton-proton ($p$$+$$p\() and proton-nucleus (\)p$$+$$A\() collisions, despite expectations that the volume and lifetime of the medium produced would be too small to form a QGP. Here, we report on the observation of elliptic and triangular flow patterns of charged particles produced in proton-gold (\)p$$+\(Au), deuteron-gold (\)d$$+\(Au), and helium-gold (\)^3\(He\)+\(Au) collisions at a nucleon-nucleon center-of-mass energy \)\sqrt{s_{_{NN}}}$~=~200 GeV. The unique combination of three distinct initial geometries and two flow patterns provides unprecedented model discrimination. Hydrodynamical models, which include the formation of a short-lived QGP droplet, provide a simultaneous description of these measurements.
There is strong evidence for the formation of small droplets of quark-gluon plasma in \(p/d/^{3}\)He+Au collisions at the Relativistic Heavy Ion Collider (RHIC) and in \(p\)+\(p\)/Pb collisions at ...the Large Hadron Collider. In particular, the analysis of data at RHIC for different geometries obtained by varying the projectile size and shape has proven insightful. In the present analysis, we find excellent agreement with the previously published PHENIX at RHIC results on elliptical and triangular flow with an independent analysis via the two-particle correlation method, which has quite different systematic uncertainties and an independent code base. In addition, the results are extended to other detector combinations with different kinematic (pseudorapidity) coverage. These results provide additional constraints on contributions from nonflow and longitudinal decorrelations.
Asymmetric nuclear collisions of $p$$+\(Al, \)p$$+\(Au, \)d$$+\(Au, and \)^{3}\(He\)+\(Au at \)\sqrt{s_{_{NN}}}=200\( GeV provide an excellent laboratory for understanding particle production, as ...well as exploring interactions among these particles after their initial creation in the collision. We present measurements of charged hadron production \)dN_{\rm ch}/d\eta\( in all such collision systems over a broad pseudorapidity range and as a function of collision multiplicity. A simple wounded quark model is remarkably successful at describing the full data set. We also measure the elliptic flow \)v_{2}$ over a similarly broad pseudorapidity range. These measurements provide key constraints on models of particle emission and their translation into flow.
Recently, multiparticle-correlation measurements of relativistic \(p/d/^3\)He\(+\)Au, $p$$+\(Pb, and even \)p$$+$$p\( collisions have shown surprising collective signatures. Here we present ...beam-energy-scan measurements of 2-, 4-, and 6-particle angular correlations in \)d$$+\(Au collisions at \)\sqrt{s_{_{NN}}}\(=200, 62.4, 39, and 19.6 GeV. We also present measurements of 2- and 4-particle angular correlations in \)p$$+\(Au collisions at \)\sqrt{s_{_{NN}}}\(=200 GeV. We find the 4-particle cumulant to be real-valued for \)d$$+\(Au collisions at all four energies. We also find that the 4-particle cumulant in \)p$$+\(Au has the opposite sign as that in \)d$$+\(Au, indicating that the correlations are geometrical and therefore collective in origin. Further we find that the 6-particle cumulant agrees with the 4-particle cumulant in \)d$$+$Au collisions at 200 GeV, which indicates that nonflow effects are subdominant and provides strong evidence of collectivity.
We present measurements of the elliptic flow, \(v_2\), as a function of transverse momentum (\(p_T\)), pseudorapidity (\(\eta\)), and centrality in $d$$+\(Au collisions at \)\sqrt{s_{_{NN}}}=\(200, ...62.4, 39, and 19.6 GeV. The beam-energy scan of \)d$$+\(Au collisions provides a testing ground for the onset of flow signatures in small collision systems. We measure a nonzero \)v_2\( signal at all four collision energies, which, at midrapidity and low \)p_T\(, is consistent with predictions from viscous hydrodynamic models. Comparisons with calculations from parton transport models (based on the {\sc ampt} Monte Carlo generator) show good agreement with the data at midrapidity to forward (\)d\(-going) rapidities and low \)p_T\(. At backward (Au-going) rapidities and \)p_T>1.5\( GeV/\)c\(, the data diverges from {\sc ampt} calculations of \)v_2\( relative to the initial geometry, indicating the possible dominance of nongeometry related correlations, referred to as nonflow. We also present measurements of the charged-particle multiplicity (\)dN_{\rm ch}/d\eta\() as a function of \)\eta\( in central \)d$$+\(Au collisions at the same energies. We find that in \)d$$+\(Au collisions at \)\sqrt{s_{_{NN}}}=\(200 GeV the \)v_2\( scales with \)dN_{\rm ch}/d\eta\( over all \)\eta\( in the PHENIX acceptance. At \)\sqrt{s_{_{NN}}}=\(62.4, and 39 GeV, \)v_2\( scales with \)dN_{\rm ch}/d\eta\( at midrapidity and forward rapidity, but falls off at backward rapidity. This departure from the \)dN_{\rm ch}/d\eta$ scaling may be a further indication of nonflow effects dominating at backward rapidity.
During 2015 the Relativistic Heavy Ion Collider (RHIC) provided collisions of transversely polarized protons with Au and Al nuclei for the first time, enabling the exploration of ...transverse-single-spin asymmetries with heavy nuclei. Large single-spin asymmetries in very forward neutron production have been previously observed in transversely polarized $p$$+$$p\( collisions at RHIC, and the existing theoretical framework that was successful in describing the single-spin asymmetry in \)p$$+$$p\( collisions predicts only a moderate atomic-mass-number (\)A\() dependence. In contrast, the asymmetries observed at RHIC in \)p$$+$$A\( collisions showed a surprisingly strong \)A\( dependence in inclusive forward neutron production. The observed asymmetry in \)p$$+\(Al collisions is much smaller, while the asymmetry in \)p$$+\(Au collisions is a factor of three larger in absolute value and of opposite sign. The interplay of different neutron production mechanisms is discussed as a possible explanation of the observed \)A$ dependence.
The fraction of \(J/\psi\) mesons which come from B-meson decay, \(\textrm{F}_{B{\rightarrow}J/\psi}\), is measured for J/\(\psi\) rapidity \mbox{\(1.2<|y|<2.2\)} and \(p_T>0\) in \(p\)+\(p\) and ...Cu+Au collisions at \(\sqrt{s_{_{NN}}}\)=200 GeV with the PHENIX detector. The extracted fraction is \(\textrm{F}_{B{\rightarrow}J/\psi}\) = 0.025 \(\pm\) 0.006(stat) \(\pm\) 0.010(syst) for \(p\)+\(p\) collisions. For Cu+Au collisions, \(\textrm{F}_{B{\rightarrow}J/\psi}\) is 0.094 \(\pm\) 0.028(stat) \(\pm\) 0.037(syst) in the Au-going direction (\(-2.2<y<-1.2\)) and 0.089 \(\pm\) 0.026(stat) \(\pm\) 0.040(syst) in the Cu-going direction (\(1.2<y<2.2\)). The nuclear modification factor, \(R_{\rm CuAu}\), of B mesons in Cu+Au collisions is consistent with binary scaling of measured yields in \(p\)+\(p\) at both forward and backward rapidity.
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