Abstract

In heavy-ion collision experiments, the global collectivity of final-state particles can be quantified by anisotropic flow coefficients (vn). The first-order flow coefficient, also referred to as the directed flow (v1 ), describes the collective sideward motion of produced particles and nuclear fragments in heavy-ion collisions. It carries information on the very early stage of the collision, especially at large pseudorapidity (η), where it is believed to be generated during the nuclear passage time. Directed flow therefore probes the onset of bulk collective dynamics during thermalization, providing valuable experimental guidance to models of the pre-equilibrium stage. In 2018, the Event Plane Detector (EPD) was installed in STAR and used for the Beam Energy Scan phase-II (BES-II) data taking. The combination of EPD (2.1 < |η| < 5.1) and high-statistics BES-II data enables us to extend the v1 measurement to the forward and backward η regions. In this paper, we present the measurement of v1 over a wide η range in Au+Au collisions at √s NN = 19.6 and 27 GeV using the STAR EPD. The results of the analysis at √s NN = 19.6 GeV exhibit excellent consistency with the previous PHOBOS measurement, while elevating the precision of the overall measurement. The increased precision of the measurement also revealed finer structures in heavy-ion collisions, including a potential observation of the first-order event-plane decorrelation. Multiple physics models were compared to the experimental results. Only a transport model and a three-fluid hybrid model can reproduce a sizable v1 at large η as was observed experimentally. The model comparison also indicates v1 at large η might be sensitive to the QGP phase transition.

Document Type

Article

Publication Date

2025

Notes/Citation Information

©2025 American Physical Society

Digital Object Identifier (DOI)

https://doi.org/10.1103/PhysRevC.111.014906

Funding Information

We thank the RHIC Operations Group and RCF at BNL, the NERSC Center at LBNL, and the Open Science Grid consortium for providing resources and support. This work was supported in part by the Office of Nuclear Physics within the U.S. DOE Office of Science; the U.S. National Science Foundation; National Natural Science Foundation of China; Chinese Academy of Science; the Ministry of Science and Technology of China and the Chinese Ministry of Education; the Higher Education Sprout Project by Ministry of Education at NCKU; the National Research Foundation of Korea; Czech Science Foundation and Ministry of Education, Youth and Sports of the Czech Republic; Hungarian National Research, Development and Innovation Office; New National Excellency Programme of the Hungarian Ministry of Human Capacities; Department of Atomic Energy and Department of Science and Technology of the Government of India; the National Science Centre and WUT ID-UB of Poland; the Ministry of Science, Education and Sports of the Republic of Croatia; German Bundesministerium für Bildung, Wissenschaft, Forschung and Technologie (BMBF); Helmholtz Association; Ministry of Education, Culture, Sports, Science, and Technology (MEXT); Japan Society for the Promotion of Science (JSPS); and Agencia Nacional de Investigación y Desarrollo (ANID) of Chile.

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