How a proton gets its spin is surprisingly complicated

Like a quantum version of a whirling top, protons have angular momentum, known as spin. But the source of the subatomic particles’ spin has confounded physicists. Now scientists have confirmed that some of that spin comes from a frothing sea of particles known as quarks and their antimatter partners, antiquarks, found inside the proton.

Surprisingly, a less common type of antiquark contributes more to a proton’s spin than a more plentiful variety, scientists with the STAR experiment report March 14 in Physical Review D.

Quarks come in an assortment of types, the most common of which are called up quarks and down quarks. Protons are made up of three main quarks: two up quarks and one down quark. But protons also have a “sea,” or an entourage of transient quarks and antiquarks of different types, including up, down and other varieties (SN: 4/29/17, p. 22).

Previous measurements suggested that the spins of the quarks within this sea contribute to a proton’s overall spin. The new result — made by slamming protons together at a particle accelerator called the Relativistic Heavy Ion Collider, or RHIC — clinches that idea, says physicist Elke-Caroline Aschenauer of Brookhaven National Lab in Upton, N.Y., where the RHIC is located.

上海龙凤阁

A proton’s sea contains more down antiquarks than up antiquarks. But, counterintuitively, more of the proton’s spin comes from up than down antiquarks, the researchers found. In fact, the down antiquarks actually spin in the opposite direction, slightly subtracting from the proton’s total spin.

“Spin has surprises. Everybody thought it’s simple … and it turns out it’s much more complicated,” Aschenauer says.

Editor’s note: This story was updated April 3, 2019, to correct the subheadline to say that up antiquarks (not up quarks) add more angular momentum than do down antiquarks (not down quarks).

相关文章Chinas shipbuilding industry sees marked growth in orders in 2023, leading the worldChinas shipbuilding industry achieved marked growth in 2023, as shipbuilding completions exceeded half of the global total, or reaching 50.2 percent, further solidifying the countrys leading position in the shipbuilding sector, according to data released by the Ministry of Industry and Information Technology (MIIT) on Monday. In 2023, Chinas shipbuilding completions totaled 42.32 million deadweight […]

Large-scale public cemetery for Qin people discovered in ShaanxiA large-scale cemetery containing more than 200 tombs was recently discovered in Xianyang, Northwest Chinas Shaanxi Province. The tomb cluster was confirmed to be a burial place for the people of the Qin Dynasty (221BC-206BC).  Archaeologists found a total of 237 ancient tombs, excavated in Xianyangs Jiantan village, dated from the late Warring States Period […]

Neandertal kids were a lot like kids today — at least in how they grewA Neandertal child whose partial skeleton dates to around 49,000 years ago grew at the same pace as children do today, with a couple of exceptions. Growth of the child’s spine and brain lagged, a new study finds. It’s unclear, though, whether developmental slowing in those parts of the body applied only to Neandertals or […]

In the deep ocean, these bacteria play a key role in trapping carbonA mysterious group of microbes may be controlling the fate of carbon in the dark depths of the world’s oceans. Nitrospinae bacteria, which use the nitrogen compound nitrite to “fix” inorganic carbon dioxide into sugars and other compounds for food and reproduction, are responsible for 15 to 45 percent of such carbon fixation in the […]