Speaker
Description
We study the spin alignment of precursor ccbar or quarkonium states in the quark-gluon plasma and its possible connection to the experimentally observed J/psi spin alignment in heavy-ion collisions. Since the final soft E1 transition to the physical J/psi is spin-independent at leading order, the observed spin alignment is expected to be inherited from the precursor state, up to corrections associated with the soft emitted gluon. Within a spin-density-matrix framework based on the chromomagnetic-spin interaction, we show that anisotropic chromomagnetic-field fluctuations can generate a nonzero rho_00 - 1/3. We consider two sources of anisotropy: the relative motion between the quarkonium and the medium, and the shear of the expanding QGP. Both mechanisms lead to a negative spin alignment, rho_00 - 1/3 < 0, consistent with the sign observed for J/psi. In our estimate, the relative-motion effect provides the dominant contribution, with a typical magnitude of 10^-2--10^-1, while the shear-induced contribution is much smaller, around 10^-5 for typical hydrodynamic gradients.
| 请选择分会 | 高能重离子物理 |
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