Initial-state control of vorticity--shear competition in longitudinal Λ polarization in Pb--Pb collisions at √(sNN)=5.36 TeV
Abstract
We investigate how the transverse initial energy-density distribution controls the second sine harmonic of longitudinal Λ polarization in Pb--Pb collisions at √(sNN)=5.36 TeV. We use (3+1)-dimensional Particle-In-Cell Relativistic ideal hydrodynamics and the Becattini--Buzzegoli--Palermo isothermal local-equilibrium prescription. Across the twelve selected initial states, the kinematic-vorticity contribution Pz,s2ω is negative, whereas the kinematic-shear contribution Pz,s2Ξ is positive. Changing the initial energy-density normalization epeak, transverse smoothing width σ⊥, or reduced impact parameter b0 modifies the relative magnitudes of the two contributions rather than merely rescaling their sum. Reducing the transverse smoothing width σ⊥ generally strengthens both contributions, whereas increasing the initial energy-density normalization epeak can preferentially weaken Pz,s2Ξ and reverse the total harmonic. At fixed epeak, the impact-parameter dependence is mainly shear driven for narrower transverse smoothing and mainly vorticity driven for broader transverse smoothing. Applying the Liu--Yin prescription to the two tested hydrodynamic backgrounds reverses the total sign, showing that the sign depends on the polarization formula as well as on the fluid fields. Our results obtained with ideal hydrodynamics and the ILE prescription have the same sign and order of magnitude as the ALICE measurements for peripheral Pb--Pb collisions in the $50−−70\%$ centrality interval, showing that such a positive harmonic can arise from an initial-state-controlled, shear-dominated balance at decoupling.
Related
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BibTeX
@misc{wu2026initialstate,
title = {{Initial-state control of vorticity--shear competition in longitudinal $Λ$ polarization in Pb--Pb collisions at $\sqrts_\mathrmNN=5.36\,\mathrmTeV$}},
author = {Simin Wu and Leyao Lin and Yilong Xie},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.23266},
eprint = {2609.23266},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.23266},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - UNPB
TI - Initial-state control of vorticity--shear competition in longitudinal Λ polarization in Pb--Pb collisions at √(s_NN)=5.36 TeV
AU - Simin Wu
AU - Leyao Lin
AU - Yilong Xie
PY - 2026
DA - 2026/09/20
PB - arXiv
DO - 10.48550/arxiv.2609.23266
UR - https://arxiv.org/abs/2609.23266
AB - We investigate how the transverse initial energy-density distribution controls the second sine harmonic of longitudinal Λ polarization in Pb--Pb collisions at √(s_NN)=5.36 TeV. We use (3+1)-dimensional Particle-In-Cell Relativistic ideal hydrodynamics and the Becattini--Buzzegoli--Palermo isothermal local-equilibrium prescription. Across the twelve selected initial states, the kinematic-vorticity contribution P_z,s2^ω is negative, whereas the kinematic-shear contribution P_z,s2^Ξ is positive. Changing the initial energy-density normalization e_peak, transverse smoothing width σ_⊥, or reduced impact parameter b_0 modifies the relative magnitudes of the two contributions rather than merely rescaling their sum. Reducing the transverse smoothing width σ_⊥ generally strengthens both contributions, whereas increasing the initial energy-density normalization e_peak can preferentially weaken P_z,s2^Ξ and reverse the total harmonic. At fixed e_peak, the impact-parameter dependence is mainly shear driven for narrower transverse smoothing and mainly vorticity driven for broader transverse smoothing. Applying the Liu--Yin prescription to the two tested hydrodynamic backgrounds reverses the total sign, showing that the sign depends on the polarization formula as well as on the fluid fields. Our results obtained with ideal hydrodynamics and the ILE prescription have the same sign and order of magnitude as the ALICE measurements for peripheral Pb--Pb collisions in the $50−−70\%$ centrality interval, showing that such a positive harmonic can arise from an initial-state-controlled, shear-dominated balance at decoupling.
ER -