The PHENIX detector is designed to perform a broad study of A–A, p–A, and p–p collisions to investigate nuclear matter under extreme conditions. A wide variety of probes, sensitive to all timescales, are used to study systematic variations with species and energy as well as to measure the spin structure of the nucleon. Designing for the needs of the heavy-ion and polarized-proton programs has produced a detector with unparalleled capabilities. PHENIX measures electron and muon pairs, photons, and hadrons with excellent energy and momentum resolution. The detector consists of a large number of subsystems that are discussed in other papers in this volume. The overall design parameters of the detector are presented.

The production of $e^{+}{e}^{-}$ pairs for $m_{e^{+}{e}^{-}}<0.3\mathrm{GeV}/c^2$ and $1<p_T<5\mathrm{GeV}/c$ is measured in $p+p$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. An enhanced yield above hadronic sources is observed. Treating the excess as photon internal conversions, the invariant yield of direct photons is deduced. In central $\mathrm{Au}+\mathrm{Au}$ collisions, the excess of the direct photon yield over $p+p$ is exponential in transverse momentum, with an inverse slope $T=221\pm {19}^{\mathrm{stat}}\pm {19}^{\mathrm{syst}}\mathrm{MeV}$. Hydrodynamical models with initial temperatures ranging from $T_{\mathrm{init}}\sim 300–600\mathrm{MeV}$ at times of $\sim 0.6–0.15\mathrm{fm}/c$ after the collision are in qualitative agreement with the data. Lattice QCD predicts a phase transition to quark gluon plasma at $\sim 170\mathrm{MeV}$.

Differential measurements of elliptic flow ($v_2$) for $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{GeV}$ are used to test and validate predictions from perfect fluid hydrodynamics for scaling of $v_2$ with eccentricity, system size, and transverse kinetic energy (${\mathrm{KE}}_T$). For ${\mathrm{KE}}_T\equiv m_T-m$ up to $\sim 1\mathrm{GeV}$ the scaling is compatible with hydrodynamic expansion of a thermalized fluid. For large values of ${\mathrm{KE}}_T$ mesons and baryons scale separately. Quark number scaling reveals a universal scaling of $v_2$ for both mesons and baryons over the full ${\mathrm{KE}}_T$ range for $\mathrm{Au}+\mathrm{Au}$. For $\mathrm{Au}+\mathrm{Au}$ and $\mathrm{Cu}+\mathrm{Cu}$ the scaling is more pronounced in terms of ${\mathrm{KE}}_T$, rather than transverse momentum.

Transverse momentum distributions and yields for ${\pi }^{\pm }$, $K^{\pm }$, $p$, and $\mathrm{p̄}$ in $p+p$ collisions at $\sqrt{s}$ $=$ 200 and 62.4 GeV at midrapidity are measured by the PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC). These data provide important baseline spectra for comparisons with identified particle spectra in heavy ion collisions at RHIC. We present the inverse slope parameter $T_{\mathrm{inv}}$, mean transverse momentum $\langle p_T\rangle$, and yield per unit rapidity $\mathit{dN}/\mathit{dy}$ at each energy, and compare them to other measurements at different $\sqrt{s}$ in $p+p$ and $p+\mathrm{p̄}$ collisions. We also present the scaling properties such as $m_T$ scaling and $x_T$ scaling on the $p_T$ spectra between different energies. To discuss the mechanism of the particle production in $p+p$ collisions, the measured spectra are compared to next-to-leading-order or next-to-leading-logarithmic perturbative quantum chromodynamics calculations.

The PHENIX experiment presents results from the Relativistic Heavy Ion Collider 2005 run with polarized proton collisions at $\sqrt{s}=200\mathrm{GeV}$, for inclusive ${\pi }^0$ production at midrapidity. Unpolarized cross section results are given for transverse momenta $p_T=0.5$ to $20\mathrm{GeV}/c$, extending the range of published data to both lower and higher $p_T$. The cross section is described well for $p_T<1\mathrm{GeV}/c$ by an exponential in $p_T$, and, for $p_T>2\mathrm{GeV}/c$, by perturbative QCD. Double helicity asymmetries $A_{LL}$ are presented based on a factor of 5 improvement in uncertainties as compared to previously published results, due to both an improved beam polarization of 50%, and to higher integrated luminosity. These measurements are sensitive to the gluon polarization in the proton. Using one representative model of gluon polarization it is demonstrated that the gluon spin contribution to the proton spin is significantly constrained.