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Measurement of directed flow in Au+Au collisions at √sNN =19.6 and 27 GeV with the STAR Event Plane Detector

PAs: Shinichi Esumi, Mike Lisa, Xiaoyu Liu, Prithwish Tribedy

Target Journal: Physical Review C

Paper Draft: https://drupal.star.bnl.gov/STAR/system/files/v1_paper_ver5_ack.pdf

Analysis Note: https://drupal.star.bnl.gov/STAR/starnotes/private/psn0812

Code: offline/paper/psn0812

PWGC Preview: https://drupal.star.bnl.gov/STAR/system/files/PWGC_preview_v1_27GeV_ver2_0.pdf

Abstract:
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, 2.1 < |η| < 5.1) was installed in STAR and used for the Beam Energy Scan phase-II (BES-II) data taking. The combination of EPD 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 √sNN = 19.6 and 27 GeV using the STAR EPD. The results of the analysis at √sNN = 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 analysis 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.

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Summary:
In this paper, we presented \veta\ measured at \snn\ 19.6 and 27 GeV over six units of $\eta$ using the STAR EPD. In order to use a scintillator detector as the particles-of-interest region, we developed an entirely-new method to ensure the accuracy of this measurement. The analysis results at \snn\ 19.6 GeV exhibited excellent consistency with the previous PHOBOS measurement in the same phase space (\vone\ integrated over all $p_{T}$), while elevating the precision to a new level. The increased precision of the measurement also revealed finer structures of the heavy-ion collision, including a potential evidence for the first-order event-plane decorrelation. A collision-energy scaling of $v_{1}(\eta-y_{\mathrm{beam}})$ was observed at $(\eta-y_{\mathrm{beam}})>0$ for \snn\ 19.6 and 27 GeV, which indicates the \vone\ at large $|\eta|$ might not only come from the deflection of nuclear fragments. Simulations from various models including transport, hydrodynamic, one-fluid hybrid and three-fluid hybrid models have been compared to this measurement. Only UrQMD (transport model) and MUFFIN (three-fluid hybrid model) were able to reproduce a significant \vone\ at the forward(backward) $\eta$ as observed in the experiment. This underscores the importance of incorporating all segments of the heavy-ion collision in model studies, especially at BES energies where nuclear fragments can substantially influence particle production across the entire pseudorapidity range. The comparison with the MUFFIN simulation indicates \vone\ at large $\eta$ might be sensitive to the QGP phase transition. Furthermore, the UrQMD study has shown significant discrepancy between $v_{1}\{\mathrm{EP}\}$ and $v_{1}\{\mathrm{RP}\}$, demonstrating the importance of employing the same reference when comparing experimental measurements and model calculations.