Spin-Momentum Decoupling in Quarkonium Hadronization: Polarization Quenching via Environment-Induced Decoherence in Jets

Abstract

The suppression of heavy quarkonium polarization at high transverse momentum ($p_T$) remains a persistent puzzle in quantum chromodynamics (QCD). We propose an effective open-quantum-system paradigm demonstrating that the heavy quark spin state and its macroscopic momentum effectively decouple during hadronization. By retaining the short-distance non-relativistic QCD (NRQCD) perturbative calculations as a kinematic baseline, we argue that the immense kinematic inertia at high $p_T$ parametrically preserves the power-law momentum spectrum. Concurrently, the intense, stochastic chromo-electric background within a fragmenting jet acts as a dynamic decoherence environment. Using a horizon-inspired picture as a physically motivated parametrization, we derive an effective temperature $T_{\text{eff}}(z) \propto \sqrt{\ln(1/z)}$ driven by the multiplicity of soft accompanying partons. By incorporating this effective temperature into a Lindblad dissipation framework, we predict a simultaneous quenching of the polar and azimuthal anisotropies towards a maximally mixed state. Crucially, the recently observed ``soft'' fragmentation of $\Upsilon(nS)$ by the CMS Collaboration provides a highly consistent phase-space weighting required in our framework to explain the historical inclusive unpolarized anomaly. Identifying the fragmentation fraction $z=p_T^{\mathcal{Q}}/p_T^{\text{jet}}$ as the critical control variable, we propose that a key testable prediction is the simultaneous $z$-dependent suppression of $\lambda_\theta$, $\lambda_\phi$, and $\tilde{\lambda}$ in fixed quarkonium and jet $p_T$ bins.