The PHENIX experiment at the Relativistic Heavy Ion Collider has measured the differential cross section of \(\phi\)(1020)-meson production at forward rapidity in $p$$+$$p\( collisions at ...\)\sqrt{s}=\(510 GeV via the dimuon decay channel. The partial cross section in the rapidity and \)p_T\( ranges \)1.2<|y|<2.2\( and \)2<p_T<7\( GeV/\)c\( is \)\sigma_\phi=2.28 \pm 0.09\,{\rm (stat)} \pm 0.14\,{\rm (syst)} \pm 0.27\, {\rm (norm)} \times 10^{-2}\(~mb. The energy dependence of \)\sigma_\phi\( (\)1.2<|y|<2.2, \; 2<p_T<5\( GeV/\)c\() is studied using the PHENIX measurements at \)\sqrt{s}=\(200 and 510 GeV and the Large-Hadron-Collider measurements at \)\sqrt{s}=$2.76 and 7 TeV. The experimental results are compared to various event generator predictions ({\sc pythia6, pythia8, phojet, ampt, epos3,} and {\sc epos-lhc}).
We present measurements of the transverse-momentum dependence of elliptic flow \(v_2\) for identified pions and (anti)protons at midrapidity (\(|\eta|<0.35\)), in 0%--5% central $p$$+\(Au and ...\)^3\(He\)+\(Au collisions at \)\sqrt{s_{_{NN}}}=200\( GeV. When taken together with previously published measurements in \)d$$+\(Au collisions at \)\sqrt{s_{_{NN}}}=200\( GeV, the results cover a broad range of small-collision-system multiplicities and intrinsic initial geometries. We observe a clear mass-dependent splitting of \)v_2(p_{T})\( in \)d$$+\(Au and \)^3\(He\)+\(Au collisions, just as in large nucleus-nucleus (\)A$$+$$A\() collisions, and a smaller splitting in \)p$$+\(Au collisions. Both hydrodynamic and transport model calculations successfully describe the data at low \)p_T\( (\)< 1.5\( GeV/\)c\(), but fail to describe various features at higher \)p_T\(. In all systems, the \)v_2\( values follow an approximate quark-number scaling as a function of the hadron transverse kinetic energy per constituent quark(\)KE_T/n_q\(), which was also seen previously in \)A$$+$$A$ collisions.
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.
