We report an underlying event tune for the pythia8 Monte Carlo event generator that is applicable for hadron collisions primarily at s ranges available at the Relativistic Heavy-Ion Collider (RHIC). We compare our new pythia8 tuned predictions to midrapidity inclusive π± spectra, jet substructure, Drell-Yan production, and underlying event measurements from RHIC and the Tevatron, as well as underlying event data from the Large Hadron Collider. With respect to the default pythia8 Monash Tune, the new "Detroit"tune shows significant improvements in the description of the experimental data. Additionally, we explore the validity of pythia8 predictions for forward rapidity π in s=200 GeV collisions, where neither tune is able to sufficiently describe the data. We advocate for the new tune to be used for pythia8 studies at current and future RHIC experiments, and discuss future tuning exercises at lower center-of-mass energies, where forward/backward kinematics are essential at the upcoming Electron-Ion Collider.

Time and again, spin has been a key element in the exploration of fundamental physics. Spin-dependent observables have often revealed deficits in the assumed theoretical framework and have led to novel developments and concepts. Spin is exploited in many parity-violating experiments searching for physics beyond the Standard Model or studying the nature of nucleon-nucleon forces. The RHIC spin program plays a special role in this grand scheme: it uses spin to study how a complex many-body system such as the proton arises from the dynamics of QCD. Many exciting results from RHIC spin have emerged to date, most of them from RHIC running after the 2007 Long Range Plan. In this document we present highlights from the RHIC program to date and lay out the roadmap for the significant advances that are possible with future RHIC running.