Studies of directed flow with event plane method in the HIRFL-CSR external-target experiment

The Cooling-Storage-Ring External-target Experiment (CEE) at the Heavy Ion Research Facility in Lanzhou (HIRFL) is designed to study the properties of nuclear matter created in heavy-ion collisions at beam energies from a few hundred MeV/u up to 1 GeV/u. It aims to facilitate research on the quantum chromodynamics (QCD) phase structure in the high-baryon-density region. Collective flow is a fundamental observable in heavy-ion collision experiments, providing information on the bulk properties of the produced matter. Although the standard event plane method has been widely used to measure collective flow, it is still important to validate and optimize this method for the CEE spectrometer. In this paper, we study the experimental procedures for measuring directed flow in ²³⁸U + ²³⁸U collisions at 500 MeV/u, using event planes reconstructed by Multi-Wire Drift Chamber and Zero Degree Calorimeter, respectively. Jet AA Microscopic (JAM) transport generator is used to generate events, and the detector response is simulated by the CEE Fast Simulation (CFS) package. Finally, the optimal kinematic region for proton directed flow measurements is discussed for the future CEE experiment.