Directed flow of particles and quarks from the RHIC Beam Energy Scan measured by STAR

The Beam Energy Scan (BES) program at the Relativistic Heavy Ion Collider aims to study the QCD phase diagram in a region where a critical point may exist. Possible signatures of a softening of the QCD equation of state have been reported at BES energies, and directed flow (rapidity-odd $v_1(y)$) is has been measured by STAR indicating an intriguing double sign change behavior. This talk will focus on $v_1$ and its slope, $dv_1/dy$, near midrapidity for ten identified particles in Au+Au collisions at eight beam energies measured by the STAR experiment. The coalescence assumption is used to interpret $v_1$ versus beam energy of the constituent quarks, which explores the separate contributions from quarks produced in the collision, and transported from the colliding nuclei. Tests of a coalescence sum rule are carried out for net particles, and implications for transported quark $v_1$ are also discussed. We will also discuss the detector upgrades and expected improvements in $v_1$ measurements from phase II of the of the Beam Energy Scan program.