Title: Directed Flow Splitting Between  and lambdabar^0 from Electromagnetic Effects in Heavy-Ion Collisions

Target Journal: Physical Review Letters

PA LIst: Xiatong Wu, Aditya Prasad Dash, Diyu Shen, Yufu Lin, Muhammad Farhan Taseer, Ze Qiu, Zhuo Wang, Hao Qiu, Shusu Shi, Gang Wang, Aihong Tang, Jinhui Chen, Subhash Singha and YuGang Ma

Ultra-strong electromagnetic fields generated in high-energy heavy-ion collisions can lift the degeneracy between the directed flows of oppositely charged particles, a phenomenon observed experimentally. If these electromagnetic effects act unequally on   and   quarks, and hadronization proceeds via quark coalescence, a similar splitting may occur between neutral strange hadrons and their antiparticles, in contrast to the naive expectation of no EM effects on charge neutral hadrons. In this Letter, we report STAR measurements of the directed flow slope difference    between   and   as a function of centrality in Au+Au collisions from the RHIC Beam Energy Scan Phase II. We find that   for   becomes negative in peripheral collisions, akin to the behavior observed for charged hadrons, which has been attributed to the combined Faraday and Coulomb effects. We also perform a first-order coalescence test using   with a second-order correction based on  .

These tests support the coalescence picture and suggest that   and   quarks are affected differently by electromagnetic fields in these collisions.

List of Figures

Figure 1: A demonstration of lambda^0 formation in the presence of electromagnetic field effects

Figure 3: delta(dv1/dy) for lambda, proton and proton-kaon as a function of beam energy in Au+Au collisions at the 0-10%, 10-40%, and 40-80% centrality ranges. The chisquare/ndf values in each panel compare lambda delta(dv1/dy) with those of protons and protons-kaons.

Conclusion

The directed flow slope difference delta(dv1/dy)  between charged particles and their antiparticles, such as K⁺-K^- and p-pbar , changes sign from positive in central collisions to negative in peripheral collisions at RHIC energies.

This sign reversal, observed across all available beam energies, and the increasingly negative values at lower energies, indicate that the Faraday and Coulomb effects dominate over the Hall effect and transported-quark contributions. We have observed a similar behavior for  lambda⁰ - lambdabar⁰ . 

A first-order quark coalescence test shows that its beam energy dependence roughly follows that of p-pbar , with improved agreement after applying a second-order correction based on K⁺-K^-.

These results suggest that the electromagnetic fields influence the collision system at the partonic level, with distinct effects on u/d  and s quarks.