Particles produced by collisions at the Relativistic Heavy Ion Collider (RHIC) at the U.S. Department of Energy’s Brookhaven National Laboratory exhibit a surprising preference in “spin” orientations, as reported in a recent Nature paper by the RHIC’s STAR collaboration.
The study revealed a preference in global spin alignment of particles called phi mesons, which cannot be explained by conventional mechanisms such as magnetic field strength or the swirliness of the matter generated in the collisions.
But a new model that incorporates local fluctuations in the nuclear strong force can account for the data.
“It could be that the strong force fluctuations are the missing factor. Previously we hadn’t realized the strong force can influence particle spin in this way,” points out STAR physicist Aihong Tang.
The STAR scientists claim that this answer is still up for discussion and that further testing is required. If it turns out to be accurate, “these measurements give us a way to gauge how large the local fluctuations in the strong force are. They provide a new avenue to study the strong force from a different perspective,” Tang adds.
Decoding the Power of the Strong Force
The strong force is the most powerful of the four fundamental forces of nature, responsible for binding together the fundamental building blocks of atoms – protons, neutrons and the quarks and gluons within them.
The Relativistic Heavy Ion Collider (RHIC), a facility operated by the U.S Department of Energy Office of Science for nuclear physics research, was specifically designed to study the strong force. This is done by colliding the nuclei of heavy atoms at high speeds in opposite directions, creating a “melting” of the boundaries of protons and neutrons, freeing the quarks and gluons inside to create a quark-gluon plasma. The STAR collaboration at RHIC takes detailed measurements of the particles produced in these collisions to better understand the interactions between quarks and gluons.
“Unlocking the secrets of spin alignment
Previous measurements by STAR showed that when gold nuclei collide in an off-center way, the glancing impact causes the hot soup of quarks and gluons to spin. Scientists were able to measure the swirling motion of the quark-gluon plasma by observing the effect it had on the spins of particles produced in the collisions.
Spin can be thought of as similar to the rotation of a planet, such as Earth, with north and south poles. In the earlier study, scientists used the degree of alignment of the particle’s spin axis with the angular momentum generated in each off-center collision as a way to measure the vorticity of the quark-gluon plasma.
Recent analyses by STAR aimed at measuring the spin alignment of various particles, such as the phi and the K*0 mesons, as reported in the recent Nature paper. Unlike the earlier study, these particles have not just two but three possible orientations.
Similar to the previous study, STAR physicists measured the spin alignment by tracking the distribution of the decay products of these particles relative to the direction perpendicular to the reaction plane of the colliding nuclei. For the phi and K*0 mesons, the scientists used these measurements to calculate the probability that the parent particle was in one of the three spin states.
“If the probability of each of these three states equals one-third, then that means there’s no preference for the particle to be in any one of these three spin alignment states,” explains STAR physicist Xu Sun.
That is exactly what the scientists discovered for the K*0 particles—no preference. However, there was a clear indication that one state was preferred over the other two for the phi mesons.
“Somehow nature decided the phi mesons have a preference in choosing one of those states,” Sun adds.
Uncovering the reason behind the preference
Chensheng Zhou, who has been working with Aihong Tang on these measurements since 2016, first presented the preliminary findings of this research at a conference at Stony Brook University in 2017. This sparked interest among theorists, who attempted to explain the observations using conventional mechanisms such as vorticity, magnetic fields, fragmentation and others. The curiosity continued to grow as STAR collaborator Subhash Singha discussed the results at conferences in 2019 and 2022.
While this was happening, the STAR physicists have been continuing to verify their analyses, conduct new ones and reduce the uncertainty of their results.
“Our results stood up to this scrutiny, and still the numbers do not add up,” Tang adds.
Using solely conventional techniques to describe the global spin alignment of the phi meson would result in a number lower than what scientists recorded at STAR.
Each phi meson, according to theorist Xin-Nian Wang of the DOE’s Lawrence Berkeley National Laboratory, is composed of a quark and an antiquark from the same “flavor” family (strange and anti-strange). These particles with the same flavor have a tendency to combine and be influenced in the same way by strong forces.
On the other hand, K*0 mesons are created using quark-antiquark pairings of various flavors (down and anti-strange).
“With this mixture of flavors, the strong force is pointing in different directions, so its influence wouldn’t show up as much as it does in the phi meson,” Wang adds.
To validate this theory, the STAR physicists intend to investigate the global spin alignment of another meson that is composed of quarks from the same flavor family: the J/psi particle (composed of charm and anti-charm quarks).
“This is on STAR’s To-Do list for the RHIC runs of 2023 and 2025,” Sun adds.
Discovering a global spin alignment preference for J/psi particles would provide evidence for the strong force explanation and confirm the use of global spin alignment as a method for analyzing local fluctuations in the strong force within the quark-gluon plasma.
Source: 10.1038/s41586-022-05557-5
Image Credit: Brookhaven National Laboratory