We present the first measurements of long-range angular correlations and the transverse momentum dependence of elliptic flow v2 in high-multiplicity p+Au collisions at √ s N N = 200 GeV. A comparison of these results with previous measurements in high-multiplicity d+Au and 3 He+Au collisions demonstrates a relation between v2 and the initial collision eccentricity ε2, suggesting that the observed momentum-space azimuthal anisotropies in these small systems have a collective origin 3 and reflect the initial geometry. Good agreement is observed between the measured v2 and hydrodynamic calculations for all systems, and an argument disfavoring theoretical explanations based on initial momentum-space domain correlations is presented. The set of measurements presented here allows us to leverage the distinct intrinsic geometry of each of these systems to distinguish between different theoretical descriptions of the long-range correlations observed in small collision systems.

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Recently, multiparticle-correlation measurements of relativistic p/d/^{3}He+Au, p+Pb, and even p+p collisions show surprising collective signatures. Here, we present beam-energy-scan measurements of two-, four-, and six-particle angular correlations in d+Au collisions at sqrt[s_{NN}]=200, 62.4, 39, and 19.6 GeV. We also present measurements of two- and four-particle angular correlations in p+Au collisions at sqrt[s_{NN}]=200  GeV. We find the four-particle cumulant to be real valued for d+Au collisions at all four energies. We also find that the four-particle cumulant in p+Au has the opposite sign as that in d+Au. Further, we find that the six-particle cumulant agrees with the four-particle cumulant in d+Au collisions at 200 GeV, indicating that nonflow effects are subdominant. These observations provide strong evidence that the correlations originate from the initial geometric configuration, which is then translated into the momentum distribution for all particles, commonly referred to as collectivity.

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We report a measurement of e + e − pairs from semileptonic heavy-flavor decays in p+p collisions at √ s N N = 200 GeV. The e + e − pair yield from bb and cc is separated by exploiting a double differential fit done simultaneously in dielectron invariant mass and pT. We used three different event generators, pythia, mc@nlo, and powheg, to simulate the e + e − spectra from cc and bb production. The data can be well described by all three generators within the detector acceptance. However, when using the generators to extrapolate to 4π, significant differences are observed for the total cross section. These difference are less pronounced for bb than for cc. 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 pT , and within the statistical accuracy no nuclear modification is seen.

1 nuclear collisions of p+Al, p+Au, d+Au, and 3 He+Au at √ s N N = 200 GeV 2 121 4Asymmetric nuclear collisions with a light projectile nucleus striking a heavier target nucleus have proven to be an 123 excellent testing ground for particle production models and the longitudinal dynamics following the initial collision -124 for an early review see Ref. [1]. Many calculations have successfully described the longitudinal (or rapidity) distribution 125 of produced particles in proton-nucleus (p+A) collisions via the fragmentation of color strings and with counting rules 126 based on the number of "wounded" or struck nucleons or quarks in the projectile and target. Recently, a proposal 127 for testing the wounded-quark model [2] was put forth that specifically called for the measurement of dN ch /dη over a 128 broad range of pseudorapidity in p+Au, d+Au, and 3 He+Au collisions [3]. Fully three-dimensional hydrodynamical 129 models also require input on the longitudinal distribution of initial deposited energy and gradients thereof [4]. Once 130 the initial partons or fluid elements are populated, the models evolve the system dynamically. Measurements of elliptic 131 flow as a function of pseudorapidity provide constraints on the longitudinal dynamics of the evolution. 132As the incoming hadrons or nuclei break up, the rapidity distribution of liberated partons may be determined by 133 the longitudinal parton distribution functions [5, 6] or via a universal color field breakup for each struck nucleon 134 or quark [7]. For that reason, calculations based on Monte Carlo Glauber models have been developed to calculate 135 the number of struck nucleons and struck quarks (see for example Refs. [8-10]). The PHOBOS collaboration has 136 previously published charged hadron dN ch /dη measurements over |η| < 5.4 in d+Au collisions at √ s N N = 200 GeV [11]. 137 PHENIX has also published dN ch /dη measurements in high-multiplicity d+Au collisions at √ s N N = 200, 62, 39, and 138 19.6 GeV [12]. The wounded-quark model has been constrained by the d+Au data and found to be in reasonable 139 agreement with the centrality dependence, while the wounded-nucleon model cannot describe the data [3]. A crucial 140 test of the wounded-quark model is to see if it is universal across different colliding systems. Additional measurements 141 in light and heavy systems at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) can 142 also be tested in this context-see for example different geometry tests in Refs. [13-15]. 143 157 section of 2.30, 2.26, 1.76, 0.54 barns for 3 He+Au, d+Au, p+Au, and p+Al respectively. The dN ch /dη analysis has 158 negligible statistical uncertainties and thus a subset of runs with the most stable detector configuration are utilized 159and the run-to-run variation is used in the determination of systematic uncertainties. For the elliptic flow v 2 analysis 160 in high-multiplicity events, also referred to as central events, an additional trigger was used that required the number 161 of fi...