Physicists from the STAR Collaboration have reported the first observation of a global spin alignment signal in heavy-ion collisions. Published in Nature on Jan. 18, the study provides a potential new avenue for understanding the strong interaction at work at the sub-nucleon level. 

  The phi meson in the quark-gluon plasma. (Image from Brookhaven National Laboratory) 

The STAR (Solenoidal Tracker at RHIC) Experiment is based at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL). The STAR Collaboration comprises 717 collaborators from 71 institutions in 14 countries.  

This study was performed by researchers from the Institute of Modern Physics of the Chinese Academy of Sciences, Fudan University, BNL, Kent State University, and the University of Illinois Chicago. 

As its name implies, the strong force is the strongest of the four fundamental forces in nature. It’s what holds together the building blocks of atoms—the protons and neutrons that make up atomic nuclei, as well as their inner building blocks, quarks and gluons. 

At RHIC, heavy ions (e.g., gold nuclei) were accelerated to close to the speed of light and collided from opposite directions. The collisions “melted” the boundaries of individual protons and neutrons, setting free the quarks and gluons normally confined within to create a quark-gluon plasma (QGP). 

In collisions that are not exactly head-on, the colliding system generates a very large orbital angular momentum (OAM). Part of the OAM is transferred to the preferential alignment of the spin of particles along the OAM direction. Since the STAR detector couldn’t directly measure the spin direction, the physicists measured the spin alignment of these particles by tracking the distribution of their decay products relative to the direction perpendicular to the reaction plane of the colliding nuclei. 

In this study, the researchers measured the spin alignment of the phi and the K*0 mesons. For these particles, there are three possible orientations along the OAM. If no special physics mechanism presents, the probability of each of these three states should be equal to one-third. 

The researchers found that there was no preference for the K*0 mesons. However, the phi mesons showed a strong signal of global spin alignment, which increased with decreasing collision energy, clearly indicating that they prefer one state over the other two. It is the first time ever that such an alignment has been observed in heavy-ion collisions. 

The surprising spin-alignment pattern and magnitude for phi mesons cannot be explained by conventional mechanisms, such as the magnetic field strength, vorticity or fragmentation of polarized quarks. 

Theorists recently came up with the idea that local fluctuations in the strong force within the quark-gluon plasma could be driving the phi mesons’ apparent spin alignment preference. This explanation is still under debate and further experimental verification is needed. This connection, if fully established, will open a potential new avenue for studying the behavior of strong force fields.