This description is still subject to debate and additional confirmation is needed, the STAR physicists say. If it shows to be true, “these measurements provide us a way to assess how big the local variations in the strong force are. They offer a brand-new avenue to study the strong force from a various perspective,” Tang stated.
Crashes of heavy ions “melt” the boundaries of individual protons and neutrons, releasing the quarks and gluons generally confined within to create a quark-gluon plasma (QGP). Researchers look for spin positioning choices amongst particles emerging from the QGP by tracking the circulation of their decay items relative to an imaginary line drawn perpendicular to the reaction aircraft of the clashing nuclei. Credit: Brookhaven National Laboratory
Unlocking the strong force
As its name implies, the strong force is the greatest of the 4 fundamental forces in nature. Its what holds together the building blocks of atoms– the protons and neutrons that make up atomic nuclei, in addition to their inner building blocks, gluons and quarks.
RHIC, a DOE Office of Science user center for nuclear physics research, was integrated in large part so researchers can study this force. They do this by smashing together the nuclei of heavy atoms speeding around RHICs twin accelerator rings in opposite directions at almost the speed of light. The head-on collisions “melt” the borders of specific protons and neutrons, releasing the quarks and gluons usually restricted within to produce a quark-gluon plasma (QGP). STAR takes pictures and collects detailed data about particles emerging from these smashups so scientists can learn about how the gluons and quarks interact.
Tracking pairs of favorably and negatively charged kaons (K+, K-)– the decay items of phi mesons (Φ)– exposed that these mesons appear to like one amongst three possible spin states. A brand-new theoretical design recommends that local fluctuations in the strong nuclear force may explain this choice. Credit: Brookhaven National Laboratory
Figuring out spin positionings
Earlier measurements from STAR revealed that when gold nuclei clash in a rather off-center way, the glancing impact sets the hot soup of gluons and quarks spinning. Researchers measured the “vorticity” of the swirling quark-gluon plasma by tracking its impact on the spins of certain particles emerging from the crashes.
You can consider spin as comparable to the rotation of a planet like Earth, with north and south poles. For the particles in this earlier research study (lambda hyperons), the degree to which their spin axes line up with the angular momentum produced in each off-center crash is a direct proxy for measuring the QGPs swirliness.
More current STAR analyses looked for to measure the spin positioning of various types of particles, including the k and the phi * 0 mesons reported on in the existing Nature paper. For these particles, there are not simply 2 directional orientations for spin (” north” and “south”), however three possible orientations.
As in the previous research study, the STAR physicists measured the spin positioning of these particles by tracking the distribution of their decay items relative to the direction perpendicular to the response aircraft of the clashing nuclei. For the phi and K * 0 mesons, the scientists translate those measurements into a likelihood that the moms and dad particle remained in among the 3 spin states.
” If the likelihood of each of these three states equates to one-third, then that indicates theres no preference for the particle to be in any one of these three spin positioning states,” explained STAR physicist Xu Sun, a former postdoctoral fellow at the University of Illinois at Chicago, who recently joined the Institute of Modern Physics, China, as a staff researcher.
Thats basically what the scientists found for the K * 0 particles– no preference For the phi mesons, there was a strong signal that one state was chosen over the other 2.
” Somehow nature decided the phi mesons have a preference in picking one of those states,” Sun said.
Describing the preference.
Chensheng Zhou– who has dealt with Tang on these measurements since 2016, starting when he was a graduate trainee at Fudan University in China– provided the preliminary findings at a conference at Stony Brook University in 2017. That presentation stimulated theorists to make numerous efforts to discuss the findings with conventional mechanisms– consisting of vorticity, the magnetic field, fragmentation, and others. Interest continued to grow when STAR collaborator Subhash Singha of the Institute of Modern Physics discussed the result at conferences in 2019 (when he was a postdoc at Kent State University) and 2022.
The STAR physicists examined their analyses, performed new analyses, and lowered the unpredictability of their results.
” Our results withstood this scrutiny, and still the numbers do not build up,” Tang stated. Describing the global spin positioning of the phi meson using only the traditional systems would result in a value lower than what the researchers determined at STAR.
Theorists just recently created the concept that regional variations in the strong force within the quark-gluon plasma could be driving the phi mesons obvious spin positioning choice. Understanding the different quark parts of the phi and K * 0 mesons might help to discuss how this occurs– and supply a method to perform additional tests.
Xin-Nian Wang, a theorist at DOEs Lawrence Berkeley National Laboratory, explained that each phi meson is made of a quark and antiquark of the exact same “flavor” family (odd and anti-strange). Strong-force impacts tend to add up and affect these same-flavor particles in the very same instructions.
K * 0 mesons, on the other hand, are made quark-antiquark pairs of different tastes (down and anti-strange). “With this mixture of tastes, the strong force is pointing in various directions, so its influence wouldnt appear as much as it does in the phi meson,” Wang said.
To check this idea, the STAR physicists plan to study the international spin positioning of another meson made from same-flavor-family quarks– the J/psi particle (made of appeal and anti-charm quarks).
” This is on STARs To-Do list for the RHIC runs of 2023 and 2025,” Sun said.
Discovering a global spin positioning choice for J/psi particles would add support to the strong-force description. It would likewise confirm the approach of utilizing these particles global spin positioning as a method to study regional strong-force fluctuations in the QGP.
” Even after 22+ years of operation, RHIC continues to hone our understanding of nature by surprising us with brand-new discoveries,” Tang stated.
Reference: “Pattern of Global Spin Alignment of f and K * 0 Mesons in Heavy-Ion Collisions” 18 January 2023, Nature.DOI: 10.1038/ s41586-022-05557-5.
Additional contributors to the analyses that led to these results include: Jinhui Chen (Fudan University), Declan Keane (Kent State University), and Yugang Ma (Fudan University).
This research was funded by the DOE Office of Science (NP), the U.S. National Science Foundation, and a range of global companies and agencies listed in the scientific paper. The STAR group used calculating resources at the Scientific Data and Computing Center at Brookhaven Lab, the National Energy Research Scientific Computing Center (NERSC) at DOEs Lawrence Berkeley National Laboratory, and the Open Science Grid consortium.
New information show that local fluctuations in the nuclear strong force may affect the spin orientation of particles called phi mesons (made of 2 quarks held together by the exchange of gluons). Credit: Brookhaven National Laboratory
Findings might indicate a previously unidentified impact of the strong force– and a method to determine its regional variations.
Provided the option of 3 various “spin” orientations, certain particles emerging from crashes at the Relativistic Heavy Ion Collider (RHIC), an atom smasher at the U.S. Department of Energys (DOE) Brookhaven National Laboratory, appear to have a choice. As explained in a paper simply released in Nature by RHICs STAR partnership, the outcomes reveal a choice in international spin positioning of particles called phi mesons. Conventional mechanisms– such as the magnetic field strength or the swirliness of the matter generated in the particle collisions– can not describe the information. But a new design that includes local variations in the nuclear strong force can.
” It could be that the strong force variations are the missing element. Previously we hadnt understood the strong force can influence particle spin in this way,” said Aihong Tang, a STAR physicist at Brookhaven who was associated with the analysis.
Provided the choice of three various “spin” orientations, particular particles emerging from collisions at the Relativistic Heavy Ion Collider (RHIC), an atom smasher at the U.S. Department of Energys (DOE) Brookhaven National Laboratory, appear to have a preference. As described in a paper just published in Nature by RHICs STAR partnership, the results expose a choice in global spin positioning of particles called phi mesons. Researchers look for spin alignment preferences amongst particles emerging from the QGP by tracking the distribution of their decay products relative to an imaginary line drawn perpendicular to the reaction airplane of the colliding nuclei. STAR takes snapshots and gathers in-depth data about particles emerging from these smashups so researchers can discover about how the gluons and quarks engage.
Tracking sets of favorably and adversely charged kaons (K+, K-)– the decay items of phi mesons (Φ)– revealed that these mesons appear to have a choice for one among three possible spin states.
