New theoretical study explains spin alignment of vector mesons in heavy-ion collisions

Professor Wang Qun, leading a research team at the University of Science and Technology of China (USTC), part of the Chinese Academy of Sciences (CAS), has achieved a significant breakthrough in the field of vector meson spin physics. Specifically, their focus has been on understanding the behavior of ϕ mesons produced during collisions involving gold nuclei.

Their findings, published in the journal Physical Review Letters under the title “Spin Alignment of Vector Mesons in Heavy-Ion Collisions,” represent a remarkable milestone that challenges conventional theoretical models.

Vector fields are a powerful way to describe the strong interactions among exotic quarks. During the hadronization phase of high-energy heavy-ion collisions, where chiral symmetry spontaneously breaks down, the strongly interacting matter is described in terms of quarks and the SU(3) pseudo-Goldstone boson field surrounding them.

The vector field is associated with the gradient degree of the Goldstone boson field, particularly the vector field linked to strange quarks and anti-strange quarks, which is referred to as the ϕ-vector field.

In 2019, Professor Wang’s research group proposed that the ambient vector field influences strange quarks and anti-strange quarks, leading to a significant alteration in the spin alignment of ϕ mesons, by a factor of 1/3.

In their latest work, the researchers derived a relativistic spin Boltzmann equation for vector mesons from the Kadanoff-Baym equation. This connection allows them to establish a link between the spin alignment of ϕ mesons and the spin polarization of the exotic quarks and anti-exotic quarks comprising them during hadronization.

In this research, they introduced two parameters, the transverse rise and longitudinal fall of the vector field, to the model. These parameters account for the asymmetry of the quark-gluon plasma in both the transverse direction (perpendicular to the beam direction) and the longitudinal direction (along the beam direction).

The values of these two parameters, varying with different collision energies, explain the dependence of the spin alignment on the transverse momentum of the ϕ meson, as well as the spin alignment in the normal direction of the reaction surface and the direction of the collision parameters.

This study has the potential to advance the field of high-energy nuclear spin physics and open up new frontiers in the realm of heavy-ion collision physics.

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