Longitudinal Λ polarization as a competition between freeze-out geometry and elliptic flow in Pb-Pb collisions at √(sNN)=5.36 TeV
Abstract
The second sine harmonic Pz of longitudinal Λ polarization is sensitive to the balance between the kinematic-vorticity contribution Pzω and the kinematic-shear contribution PzΞ at decoupling. We quantify this competition in a scan of 48 Pb-Pb initial states evolved with the (3+1)-dimensional Particle-In-Cell Relativistic (PICR) ideal-hydrodynamic code. With the isothermal local-equilibrium (ILE) prescription, the total harmonic follows Pz≃ a efo−b v2, with a≃0.010 and b≃0.014, where efo is the freeze-out eccentricity and v2 the elliptic flow of Λ hyperons. With coefficients fixed on these 48 states, the relation describes 86\% of the variation of Pz in 12 states outside the fit. To identify the origin of the two terms, we decompose the ILE integrand exactly into an emission-gradient term and a curl term. On the PICR decoupling surfaces, the tangential emission gradient is positive on average, correlates most strongly with efo, and admits a surface-curvature representation, whereas the normal emission gradient and the curl term are negative on average and correlate most strongly with v2. A blast-wave calculation with the same integrand gives coefficients of comparable magnitude and shows how the cancellation point efo/v2=b/a changes with the flow and the surface geometry; both PICR and blast-wave calculations predict that b/a rises with hadron mass from the proton to the Ω when the decoupling surface is held fixed. Although the geometry inferred from ALICE data depends on the background, the measured polarization constrains the displacement of efo from the cancellation boundary efo=(b/a)v2. For the central ALICE measurement values at 5.36 TeV, the backgrounds examined give efo≃0.2--$0.4$ in 30--50\% Pb-Pb collisions.
Related
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- Elliptic flow of π0 mesons in Cu+Au collisions at √(sNN)=200 GeV and U+U at √(sNN)=193 GeV
- Exploring the origin of D0 meson elliptic flow in PbPb collisions at sNN=5.02TeV using event shape engineering
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BibTeX
@misc{xie2026longitudinal,
title = {{Longitudinal $Λ$ polarization as a competition between freeze-out geometry and elliptic flow in Pb-Pb collisions at $\sqrts_\mathrmNN=5.36\,\mathrmTeV$}},
author = {Yilong Xie and Simin Wu and Laszlo P. Csernai},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2609.26740},
eprint = {2609.26740},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.26740},
note = {CC BY-NC-ND 4.0},
}
RIS
TY - UNPB
TI - Longitudinal Λ polarization as a competition between freeze-out geometry and elliptic flow in Pb-Pb collisions at √(s_NN)=5.36 TeV
AU - Yilong Xie
AU - Simin Wu
AU - Laszlo P. Csernai
PY - 2026
DA - 2026/10/16
PB - arXiv
DO - 10.48550/arxiv.2609.26740
UR - https://arxiv.org/abs/2609.26740
AB - The second sine harmonic Pz of longitudinal Λ polarization is sensitive to the balance between the kinematic-vorticity contribution Pz^ω and the kinematic-shear contribution Pz^Ξ at decoupling. We quantify this competition in a scan of 48 Pb-Pb initial states evolved with the (3+1)-dimensional Particle-In-Cell Relativistic (PICR) ideal-hydrodynamic code. With the isothermal local-equilibrium (ILE) prescription, the total harmonic follows Pz≃ a efo−b v_2, with a≃0.010 and b≃0.014, where efo is the freeze-out eccentricity and v_2 the elliptic flow of Λ hyperons. With coefficients fixed on these 48 states, the relation describes 86\% of the variation of Pz in 12 states outside the fit. To identify the origin of the two terms, we decompose the ILE integrand exactly into an emission-gradient term and a curl term. On the PICR decoupling surfaces, the tangential emission gradient is positive on average, correlates most strongly with efo, and admits a surface-curvature representation, whereas the normal emission gradient and the curl term are negative on average and correlate most strongly with v_2. A blast-wave calculation with the same integrand gives coefficients of comparable magnitude and shows how the cancellation point efo/v_2=b/a changes with the flow and the surface geometry; both PICR and blast-wave calculations predict that b/a rises with hadron mass from the proton to the Ω when the decoupling surface is held fixed. Although the geometry inferred from ALICE data depends on the background, the measured polarization constrains the displacement of efo from the cancellation boundary efo=(b/a)v_2. For the central ALICE measurement values at 5.36 TeV, the backgrounds examined give efo≃0.2--$0.4$ in 30--50\% Pb-Pb collisions.
ER -