Physicists from the STAR Collaboration at the Relativistic Heavy Ion Collider, an atom smasher at Brookhaven National Laboratory, have tracked pairs of positively and negatively charged kaons — the decay products of phi mesons, made of two quarks held together by the exchange of gluons — and found that these mesons have a preference for one among three possible spin states.
The phi meson is made of a strange quark and strange antiquark held together by the exchange of gluons. Image credit: Brookhaven National Laboratory.
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.
The Relativistic Heavy Ion Collider (RHIC) was built in large part so physicists can study this force.
They do this by smashing together the nuclei of heavy atoms speeding around RHIC’s twin accelerator rings in opposite directions at nearly the speed of light.
The head-on collisions ‘melt’ the boundaries of individual protons and neutrons, setting free the quarks and gluons normally confined within to create a quark-gluon plasma.
Earlier measurements from the STAR Collaboration revealed that when gold nuclei collide in a somewhat off-center way, the glancing impact sets the hot soup of quarks and gluons spinning.
The physicists measured the vorticity of the swirling quark-gluon plasma by tracking its influence on the spins of certain particles emerging from the collisions.
For the particles in an earlier study, the degree to which their spin axes align with the angular momentum generated in each off-center collision is a direct proxy for measuring the swirliness of the quark-gluon plasma.
More recent analyses sought to measure the spin alignment of different types of particles, including the phi and the K*0 mesons. For these particles, there are not just two directional orientations for spin, but three possible orientations.
As in the previous study, the STAR 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.
For the phi and K*0 mesons, they translate those measurements into a 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,” said Dr. Xu Sun, a researcher at the China’s Institute of Modern Physics and a member of the STAR Collaboration.
“That’s essentially what we found for the K*0 particles — no preference. But for the phi mesons, there was a strong signal that one state was preferred over the other two.”
“Somehow nature decided the phi mesons have a preference in choosing one of those states.”
Theoretical physicicts 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.
Understanding the different quark components of the phi and K*0 mesons might help to explain how this happens — and provide a way to conduct further tests.
“Each phi meson is made of a quark and antiquark of the same flavor family (strange and anti-strange),” said Dr. Xin-Nian Wang, a theoretical physicict at Lawrence Berkeley National Laboratory.
“Strong-force effects tend to add up and influence these same-flavor particles in the same direction.”
“K*0 mesons, on the other hand, are made quark-antiquark pairs of different 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.”
To test this idea, the STAR physicists plan to study the global spin alignment of another meson made of same-flavor-family quarks — the J/psi particle (made of charm and anti-charm quarks).
“Even after over 22 years of operation, RHIC continues to sharpen our understanding of nature by surprising us with new discoveries,” said Dr. Aihong Tang, a physicist at Brookhaven Lab.
The results appear in the journal Nature.
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STAR Collaboration. 2023. Nature, in press; doi: 10.1038/s41586-022-05557-5
