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arXiv · 2604.11847

Causal-Horizon Effects on Quarkonium Fragmentation in Jets

Yi Yang
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Abstract

We reassess a horizon-inspired description of quarkonium formation in jets, distinguishing momentum redistribution from loss of spin alignment. A normalized energy-sharing prescription, with benchmark inputs for the QCD string tension and quarkonium radii, softens specified $J/\psi$ and $\Upsilon(1S)$ fragmentation baselines without fitting their measured distributions. For the LO implementation displayed by LHCb, the mean momentum fraction changes from 0.629 to 0.476, compared with 0.452 in the prompt data. Applied to CMS-displayed PYTHIA bottomonium spectra, the same prescription produces shifts of the observed order, but neither comparison establishes agreement with the complete shape. An independent NLO bottomonium baseline already overestimates the low-fraction region, and an approximate acceptance-ordered application increases that excess. These results constrain the interpretation of an additional universal softening contribution. Momentum redistribution that preserves each event's spin matrix cannot change the fully integrated polarization. A separate $J/\psi$ formation-reweighting benchmark changes the polar anisotropy from 0.420 to 0.389; a larger reduction requires an additional spin-relaxation assumption. The earlier claim that the measured bottomonium fragmentation distribution establishes a resolution of the inclusive polarization puzzle is superseded. The remaining hypothesis is tested primarily through fragmentation in separately specified experimental phase spaces, with polarization as a conditional consequence.

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Yi Yang. 2026-04-12. Causal-Horizon Effects on Quarkonium Fragmentation in Jets. https://arxiv.org/abs/2604.11847

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