The data reveal a surprising preference in particle spin alignment

The data reveal a surprising preference in particle spin alignment

The new data shows that local fluctuations in the strong nuclear force may affect the spin direction of particles called phi mesons (made of two quarks held together by exchanging gluons). Credit: Brookhaven National Laboratory

Given a choice of three different directions of “spin,” some particles emanating from collisions at the Relativistic Heavy Ion Collider (RHIC), an atom smasher at the US Department of Energy’s (DOE) Brookhaven National Laboratory, seem to have the edge. As shown in research just published in nature Through the STAR collaboration at RHIC, results revealed a preference for global spin alignment of particles called phi mesons. Conventional mechanisms – such as magnetic field strength or the vorticity of matter generated in particle collisions – cannot explain the data. But the new model that includes local fluctuations in the strong nuclear force could happen.

“That could be it Sheer force The twists are the missing factor. Previously we didn’t realize that the strong force could affect particle rotation in this way.”

STAR physicists say this explanation is still debated and more verification is needed. But if they prove correct, “these measurements give us a way to measure the extent of local fluctuations in the strong force. They provide a new avenue to study the strong force from different perspectiveTang said.

Unleash the powerful force

As its name suggests, the Strong Force is the most powerful of the four fundamental forces in nature. It’s what connects the building blocks of atoms — the protons and neutrons that make them up atomic nucleiBeside they interior building blocks, Quarks and gluons.

RHIC, a US Department of Energy Office of Science user facility for nuclear physics research, was built in large part so that scientists could study this force. They do this by smashing the nuclei of heavy atoms accelerating around RHIC’s twin accelerator rings Two opposite directions At nearly the speed of light. Head-on collisions “melt” the boundaries of individual protons and neutrons, freeing the quarks and gluons normally trapped inside to form a quark-gluon plasma (QGP). STAR takes snapshots and collects detailed data about the particles emerging from these collisions so scientists can learn about how quarks and gluons interact.

Deciphering rotational alignment codes

Previous measurements from STAR revealed that when gold nuclei collide in a somewhat off-center fashion, the sparkling effect whips up a hot soup of quarks and gluons. Rotation. Scientists have measured the “vortex” of the vortex quark-gluon plasma by tracking its effect on the spin of some of the particles emerging from the collisions.

You can think of the rotation as analogous to the rotation of a planet like Earth with north and south poles. For the particles in this previous study (lambda hyperons), the degree to which their spin axes align with the angular momentum generated in each off-center collision is a direct proxy for the QGP vortex measurement.

The data reveal a surprising preference in particle spin alignment

Colliding heavy ions “melt” the boundaries of individual protons and neutrons, freeing the quarks and gluons normally trapped inside to form a quark-gluon plasma (QGP). The scientists search for the spin alignment preferences among the particles originating from the QGP by tracing the distribution of the decay products with respect to an imaginary line drawn perpendicular to the plane of the interaction of the colliding nuclei. Credit: Brookhaven National Laboratory

Recent STAR analyzes have sought to measure the spin alignments of different types of particles, including phi and K*.0 I mentioned mesons in the present nature paper. For these particles, there are not only two directional directions of rotation (“north” and “south”), however three possible directions.

As in the previous study, the STAR physicists measured the spin alignment of these particles by tracking the distribution of the decay products with respect to the direction perpendicular to the plane of the interaction of the colliding nuclei. For phi and K*0 mesons, scientists translate these measurements into the probability that the original particle is in one of the three rotation cases.

“If the probability of each of these three states is equal to one-third, that means there is no preference for the particle in any of these three spin alignment states,” explained STAR physicist Xu Sun, a former postdoctoral fellow at the university. from Illinois in Chicago, who recently joined the Institute of Modern Physics, China, as a faculty scientist.

Here’s What Scientists Found Basically for K*0 Particles – without preference. But for phi mesons, there was a strong signal of a preference for one condition over the other two.

“Somehow nature decided that phi mesons had a preference for choosing one of those states,” Sun said.

Explanation of preference

Chensheng Zhou — who has worked with Tang on these measurements since 2016, starting when he was a graduate student at Fudan University in China — presented the initial results at a conference at Stony Brook University in 2017. That presentation has prompted theorists to make various attempts to explain Results with conventional mechanisms – including vortex, magnetic field, fragmentation, and others. Curiosity continued to grow as STAR collaborator Subhash Singh of the Institute of Modern Physics discussed the finding at conferences in 2019 (when he was a postdoc at Kent State University) and 2022.

Meanwhile, the STAR physicists checked their analyses, made new ones, and reduced the uncertainty in their results.

“Our results have withstood such scrutiny, and the numbers still don’t make sense,” Tang said. Describing the global rotational alignment of the phi meson using only conventional mechanisms would result in a value lower than what scientists measured in STAR.

The data reveal a surprising preference in particle spin alignment

Pairs of positively and negatively charged kaons (K.+K) – decay products of phi mesons (Φ) – revealed that these mesons appear to favor one of three possible x states. A new theoretical model suggests that local fluctuations in the strong nuclear force may explain this preference. Credit: Brookhaven National Laboratory

Recently theorists have come up with the idea that local fluctuations in the strong force within Quark-gluon plasma The apparent spin alignment preference could be driven by phi mesons. Understanding the different quark components of phi and K*0 Mesons may help explain how this happens — and provide a way to do further testing.

Every Mason is made of a quark and an antiquark from the same family of “flavor” (strange and anti-strange), explained Shen Nian Wang, a theoretical researcher at the Department of Energy’s Lawrence Berkeley National Laboratory. Strong force effects tend to build up and influence these same flavored particles in the same direction.

K*0 On the other hand, mesons consist of pairs of quarks and antiquarks of different flavors (down and anti-odd). “With this combination of flavors, the strong force is pointing in different directions, so its effect will not show as much as it does in phi meson,” Wang said.

To test this idea, STAR physicists plan to study the global spin alignment of another meson made of quarks of the same flavor family — a J/psi particle (made of charm and anti-charm quarks).

“This is on the STAR list of to-dos for RHIC racing in 2023 and 2025,” said Sun.

Finding a preference for the global spin alignment of J/psi particles will add support to the strong force interpretation. It would also validate the policy for using these grains‘ worldwide yarn Alignment as a method to study local strong force fluctuations in QGP.

“Even after more than 22 years of work, RHIC continues to refine our understanding of nature by surprising us with new discoveries,” Tang said.

Additional contributors to the analyzes that led to these findings include Jinhui Chen (Fudan University), Declan Keene (Kent State University), and Yugang Ma (Fudan University).

more information:
STAR collaboration, global rotation alignment pattern for and K*0 mesons in collisions of heavy ions, nature (2023). DOI: 10.1038/s41586-022-05557-5.

the quote: Data reveals surprising preference in particle rotation alignment (2023, January 18) Retrieved January 18, 2023 from

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