Despite decades of progress since Yukawa originally described the force between nucleons in terms of meson exchange, a comprehensive understanding of the strong interaction is still a major impediment to contemporary research. A remaining difficulty arises from the non-perturbative nature of the strong force, which leads to quark confinement at distances on the order of the proton.

In a new study, physicists at the STAR Collaboration have reported the first observation of a global spin alignment signal in heavy ion collisions. The study offers a new perspective on the robust interaction operating at the subnucleon level.

As the name suggests, the strong force is the strongest of the four fundamental forces of nature. It holds together the atomic building components, quarks and gluons, that make up the protons and neutrons that make up the atomic nuclei.

Heavy ions (like gold nuclei) collided at RHIC after accelerating almost as fast as light. The boundaries of individual protons and neutrons were “melted” by the collisions, releasing the quarks and gluons typically trapped inside to form a quark-gluon plasma (QGP).

The impacting system creates very high orbital angular momentum in non-exact head-on (OAM) collisions. The preferential alignment of a particle’s spin along the OAM direction receives a portion of the OAM. The physicists assessed the spin alignment of these particles by observing the distribution of their decay products relative to the direction perpendicular to the reaction plane of the colliding nuclei, since the STAR detector could not measure the spin orientation directly.

This study measured the spin alignment of the K*0 and phi mesons. There are three possible orientations along the OAM for these particles. The probability of each of these three states should be equal to one third if no special physical mechanism is present.

The scientists found that there was no preference for the K*0 mesons. However, the phi mesons showed a strong global spin alignment signal that grew with decreasing collision energy. It blatantly shows that they prefer one state over the other two. It is the first time an alignment of this nature has ever been observed in heavy ion collisions.

Conventional explanations, such as the strength of the magnetic field, the vorticity or the fragmentation of polarized quarks, cannot explain the surprising spin alignment pattern and size of phi mesons.

Theorists recently proposed that local oscillations could cause the apparent spin alignment bias of the phi mesons in the strong force in the quark-gluon plasma. Additional experimental confirmation is required for this theory, which is still debatable. If fully established, this connection will open up a potential new field of research into the behavior of powerful force fields.

Magazine reference:

  1. STAR collaboration. Pattern of global spin alignment of ϕ and K*0 mesons in heavy ion collisions. Nature (2023). DOI: 10.1038/s41586-022-05557-5