Measurements of midrapidity charged particle multiplicity distributions, dN ch /dη, and midrapidity transverse-energy distributions, dET /dη, are presented for a variety of collision systems and energies. Included are distributions for Au+Au collisions at For all A+A collisions down to √ s N N = 7.7 GeV, it is observed that the midrapidity data are better described by scaling withNqp than scaling with Npart. Also presented are estimates of the Bjorken energy density, εBJ, and the ratio of dET /dη to dN ch /dη, the latter of which is seen to be constant as a function of centrality for all systems.

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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.

163The standard model (SM) of particle physics is spectacularly successful, yet the measured value 164 of the muon anomalous magnetic moment (g − 2)µ deviates from SM calculations by 3.6σ. Several 165 theoretical models attribute this to the existence of a "dark photon," an additional U(1) gauge 166 boson, which is weakly coupled to ordinary photons. The PHENIX experiment at the Relativistic 167Heavy Ion Collider has searched for a dark photon, U , in π 0 , η → γe + e − decays and obtained 168upper limits of O(2 × 10 −6 ) on U -γ mixing at 90% CL for the mass range 30 < mU < 90 MeV/c 2 . 169Combined with other experimental limits, the remaining region in the U -γ mixing parameter space 170 that can explain the (g − 2)µ deviation from its SM value is nearly completely excluded at the 90% 184While a variety of mechanisms can be introduced to parameterize dark sector physics, a simple formulation pos- 185tulates a "dark photon" of mass m U which mixes with QED photons via a "kinetic coupling" term in the La-186 grangian [7, 8, 17, 18] 187where ε parametrizes the mixing strength. N 2γ is the invariant yield of 2γ decays of π 0 , η, α EM is the fine structure constant, and m e , m π 0 ,η are masses for From the peak height ratio,the dark photon mixing parameter can then be determined as:Note that in this approach the efficiencies for detection of e + e − pairs from Dalitz decays and from dark photons 209 cancel in the ratio R(m U ).The analysis presented here is based on a precise measurement of virtual photons from π 0 and η Dalitz decays [21] 211 across three PHENIX data sets at a collision energy of √ s N N = 200 GeV with an integrated luminosity of 4.8 pb correlations are evaluated using like-sign pairs. After scaling by the number of nucleon-nucleon collisions, the correlated 231 backgrounds in p+p and d+Au are very similar, indicating these background contributions are well understood. Pairs

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