The Relativistic Heavy Ion Collider (RHIC), as the world's first and only polarized proton collider, offers a unique environment in which to study the spin structure of the proton. In order to study the proton's transverse spin structure, the PHENIX experiment at RHIC took data with transversely polarized beams in 2001-02 and 2005, and it has plans for further running with transverse polarization in 2006 and beyond. Results from early running as well as prospective measurements for the future will be discussed.

Data from Au + Au interactions at sqrt[s(NN)]=130 GeV, obtained with the PHENIX detector at the Relativistic Heavy-Ion Collider, are used to investigate local net charge fluctuations among particles produced near midrapidity. According to recent suggestions, such fluctuations may carry information from the quark-gluon plasma. This analysis shows that the fluctuations are dominated by a stochastic distribution of particles, but are also sensitive to other effects, like global charge conservation and resonance decays.

The PHENIX experiment has measured midrapidity ([FORMULA: SEE TEXT]) transverse momentum spectra ([FORMULA: SEE TEXT]) of electrons as a function of centrality in Au+Au collisions at [FORMULA: SEE TEXT]. Contributions from photon conversions and from light hadron decays, mainly Dalitz decays of pi0 and eta mesons, were removed. The resulting nonphotonic electron spectra are primarily due to the semileptonic decays of hadrons carrying heavy quarks. Nuclear modification factors were determined by comparison to nonphotonic electrons in p+p collisions. A significant suppression of electrons at high pT is observed in central Au+Au collisions, indicating substantial energy loss of heavy quarks.

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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