Sea quark surprise reveals deeper complexity in proton spin puzzle — ScienceDaily

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New Data from the STAR Experiment The Relativistic Heavy Ion Collider (RHIC) adds detail and complexity to an interesting puzzle that scientists have been seeking to solve: how the building blocks that make up protons contribute to their rotation. The results have just been published in the journal Physical Review D which for the first time clearly reveals that the different "tastes" of antiquarks have different effects on the overall rotation of protons – and to some extent the opposite of these tastes. rich.

"This measurement shows that the quark piece of the proton spin puzzle consists of several parts," said James Drachenberg, deputy spokesperson for STAR from Abilene Christian University. "This is not a boring puzzle; it is not evenly divided. There is a more complicated picture, and this result allows us to see the picture at first glance."

Scientists have not changed their views on proton spins for the first time. In the 1980s, an experiment by the European Center for Nuclear Research (CERN) showed that the sum of quarks and antiquark spins in protons can account for up to a quarter of the total, which is a comprehensive spin "crisis." Spin. RHIC is the US Department of Energy Science Office, part of the user facility for nuclear physics research at Brookhaven National Laboratory, so scientists can measure the contributions of other components, including antiquarks and gluons ("bonded" together) , or binding, quarks and antiquarks form particles such as protons and neutrons.

Anti-Quark has only a short existence. When the gluons split, they form a quark-antiquark pair.

"We call these pairs the quark sea," Drachenberg said. "In any instant, you have quarks, gluons and quarks – anti-quark pairs of oceans that contribute to the proton's description to some extent. We understand the role of these sea quarks in some respects, but not In terms of rotation. "

Exploring the taste of the sea

A key consideration is whether the different "flavors" of the sea quark will lead to different rotations.

Quarks have six flavors – the up and down variants of protons and neutrons that make up common visible matter, and four other alien species. Splitting gluons can produce quark/antiquark pairs, lower quark/antiquark pairs – sometimes even more exotic quark/antiquark pairs.

"There is no reason to believe that glues prefer to split into one of these flavors," said Ernst Sichtermann, STAR collaborator at the US Department of Energy's Lawrence Berkeley National Laboratory (LBNL), who plays a leading role in the Sea Quark. the study. "We look forward to the same number [of up and down pairs]but this is not what we see." The results of the European Nuclear Research Center and the US Department of Energy's Fermi National Accelerator Laboratory have consistently found that antiquarks are more than antiquarks.

"Because of this kind of surprise – these two flavors are asymmetrical – we think their role in rotation may be unexpected," Drachenberg said. In fact, the early results of RHIC suggest that there may be differences in the rotation of the two flavors, which prompted the STAR team to conduct more experiments.

Achieving a Rotational Target

This result represents the accumulation of data for the 20-year RHIC rotation program. This is the end result of one of the two initial pillars of the RHIC Twilight Initiative Rotation Plan.

For all of these experiments, STAR analyzed the results of polarized proton collisions in RHIC – the overall spin directions of the two protons of RHIC in a collision are arranged in a specific manner. Finding the difference in the number of particles produced when flipping the spin direction of a polarized proton beam can be used to track the spin alignment of the various components – and thus track their contribution to the overall proton spin.

For sea quark measurements, STAR physicists calculate electrons and positrons – the electrons of the inverse electronic version are identical in all respects except that they carry a positive charge rather than a negative charge. Electrons and positrons come from the decay of particles called W bosons, which also have negative and positive changes, depending on whether they contain anti-quarks up or down. The difference in the number of electrons produced when the spin direction of the colliding proton is flipped indicates the difference in W-production and serves as a pillar for measuring the spin alignment of the upward antiquark. Similarly, the difference in positrons comes from the difference in W+ generation and is used to measure the alternative role of the spin contribution of the downward antiquark.

New Detector, Increased Accuracy

The latest data includes the signals captured by STAR's end cap calorimeter, which picks up particles that travel forward and backward near the beamline from each collision. As these new data are added to particle data that appears perpendicular to the collision zone, scientists have narrowed the uncertainty of the results. The data shows that for the first time, the rotation of the inverse quark contributes more to the overall proton spin than to the downward antiquark rotation.

"This kind of ' taste asymmetry", as the scientists said, is surprising in itself, but more importantly, considering anti-quarks more than anti-quarks," Shandong University Xu Qinghua said that he is another major scientist who supervises one of them. The analysis of graduate students is crucial to the paper.

As Sichtermann said, "If you go back to the original proton spin puzzle, we know that the sum of the quark and antiquark rotations is only a small part of the proton spin. The next question is that the contribution of the gluon is What? What is the contribution of quark and gluon orbital motion? But why is the quark contribution so small? Is it because of the offset between quark and antiquark rotation contributions? Or is it because of the difference between different quark flavors?

"The previous RHIC results indicate that gluons play an important role in proton spins. This new analysis clearly shows that the ocean also plays an important role. It is more complicated than the gluon splitting into any flavor you like. Much more – – and very reason to understand the sea."

Physicist Bernd Surrow from Temple University helped develop the W boson method and supervised two graduate students who agreed with the analysis of the new publication. “After years of experimental work by RHIC, this exciting new achievement provides a deeper understanding of the quantum fluctuations of quarks and gluons within protons. These are the basic problems that appeal to young people – and will continue to expand our knowledge. "

Other STAR measurements may provide insight into the spin contributions of exotic quark/antiquark pairs. In addition, American scientists hope to delve into the mystery of the proposed future rotation of the electron ion collider. This particle accelerator will use electrons to directly detect the spin structure of the proton internal components – and finally solve the proton rotation puzzle.

Why study proton rotation?

Rotation is a fundamental property of particles and is essential to the properties and charge of particles. Because particles have rotation, they can act like tiny magnets with specific polarities. Aligning and flipping the polarity of proton spins is the basis of techniques such as magnetic resonance imaging (MRI). But scientists are still trying to understand the inner structural blocks of protons – quarks – gluons and quarks – the antiquarks of the oceans, and their movement within the protons – how to build the rotation of the entire particle. Understanding how proton rotation is generated from its internal building blocks can help scientists understand how complex interactions within protons produce their overall structure and, in turn, constitute the nuclear structure of the atoms that make up almost all of the visible matter in our universe – from stars Everything to the planet to humanity.


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