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 https://www.star.bnl.gov/~fisyak/star/GMT/GMT.php

Title: Measurement of system size dependence of directed flow of protons (anti-protons) at RHIC

PAs: Jinhui Chen, Aditya Prasad Dash, Huan Huang, Hao Qiu, Diyu Shen, Subhash Singha, Aihong Tang, Muhammad Farhan Taseer and Gang Wang

Target journal: Phys. Rev. Lett.

Abstract:

We present the rapidity dependence of directed flow (v₁) and its slope (dv₁/dy) for π^±, K^± and p(p̅) as a function of centrality in Au+Au and Isobar (Ru+Ru and Zr+Zr) collisions at √s_NN = 200 GeV, and in U+U collisions at √s_NN = 193 GeV, as measured by the STAR experiment at RHIC. The slope dv₁/dy for p(p̅) and the difference Δ(dv₁/dy) exhibit a clear system size dependence, with an ordering of U+U > Au+Au > Isobar (Ru+Ru and Zr+Zr), while total baryons (p + p̅) remain independent of system size. This is the first observation of system size dependence of the v₁ and Δ(dv₁/dy) of baryons. In contrast, the mesons (π^± and K^±), show no dependence on system size, consistent with previous findings at RHIC for inclusive particles [1]. A hydrodynamic model incorporating an inhomogeneous baryonic profile and electromagnetic field effects can explain the observed patterns in the data. The system-size dependence of Δ(dv₁/dy) for protons in central collisions is likely dominated by enhanced baryon stopping in larger systems, where electromagnetic fields play a negligible role. In contrast, mid-central and peripheral collisions may arise from a combination of baryon transport and electromagnetic field effects. These measurements of v₁ across different centralities and system sizes offer valuable insights into the strength of electromagnetic fields, the medium's electrical conductivity, the baryon deposition and transport properties of the QCD medium [2, 3].

References:

1. STAR Collaboration, Phys. Rev. Lett. 101, 252301
2. STAR Collaboration, Phys. Rev. X 14, 011028
3. T Parida et al, arXiv: 2503.04660

Summary:

1. We observed a system size dependent v₁ and Δ(dv₁/dy) for protons (antiprotons) among three different collision systems at similar collision energy
2. In contrast, the mesons (pions and kaons) as well as total baryons (p + p̅) are found to be independent of system size 
3. These results will help understand baryon deposition (such as baryon stopping mechanism) in heavy-ion collisions and provide constraint on baryon transport (such as baryon diffusion)
4. These results will provide constraint on the strength and lifetime of the electromagnetic field as well as the electrical conductivity of the QGP 

Proposed Figures: