Impact of spin polarization on the QCD equation of state

Spin polarization provides a novel probe of the rotational properties of the quark-gluon plasma formed in relativistic heavy-ion collisions. This work investigates the equation of state, particularly its transport and thermodynamic coefficients in noncentral O+O collisions, employing a parton distribution function that incorporates spin polarization induced by thermal vorticity. Within a kinetic theory framework, one finds that the magnitude of the squared speed of sound ($c_s^2$) is only weakly modified by spin polarization, whereas the specific shear viscosity ($η/s$), specific bulk viscosity ($ζ/s$), and mean free path ($λ$) show substantial changes. When spin polarization is included, both $c_s^2$ and $ζ/s$ develop a nonmonotonic dependence on the collision energy, with an inflection point near $\sqrt{s_{NN}}=27$ GeV, corresponding to an average parton chemical potential of $\langleμ_p\rangle=0.021$ GeV. These results suggest that spin polarization may serve as a useful probe for constraining the effective equation of state of QCD matter.

💡 Research Summary

This paper investigates how parton spin polarization, induced by thermal vorticity, influences the QCD equation of state (EoS) and associated transport coefficients in non‑central oxygen‑oxygen (O+O) collisions. The author adopts a kinetic‑theory framework in which the spin‑dependent phase‑space distribution is written as
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