There was a full-blown spin “ crisis” in the 1980s when an experiment at the European Center for Nuclear Research (CERN) revealed that the sum of quark and antiquark spins within a proton could account for, at best, a quarter of the overall spin. It’s not the first time that scientists’ view of proton spin has changed. There’s a more complicated picture and this result is giving us the first glimpse of what that picture looks like.” “It’s not a boring puzzle it’s not evenly divided. “This measurement shows that the quark piece of the proton spin puzzle is made of several pieces,” said James Drachenberg, a deputy spokesperson for STAR from Abilene Christian University. Understanding how proton spin arises from its inner building blocks may help scientists understand how the complex interactions within the proton give rise to its overall structure, and in turn to the nuclear structure of the atoms that make up nearly all visible matter in our universe-everything from stars to planets to people. But scientists are still striving to understand how the inner building blocks of protons-the quarks and gluons and sea of quark-antiquark pairs, as well as their motion within the proton-build up the overall particle’s spin. Aligning and flipping the polarity of proton spin is the basis for technologies like magnetic resonance imaging (MRI). Because particles have spin, they can act like tiny magnets with a particular polarity. Spin is a fundamental property of particles, as essential to a particle’s identity as its electric charge. The results, just published as a rapid communication in the journal Physical Review D, reveal definitively for the first time that different “flavors” of antiquarks contribute differently to the proton’s overall spin-and in a way that’s opposite to those flavors’ relative abundance. UPTON, NY-New data from the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) add detail-and complexity-to an intriguing puzzle that scientists have been seeking to solve: how the building blocks that make up a proton contribute to its spin.
The latest data from RHIC reveal that the antiquarks' contribution is more complex than previously thought.
Pieces of the puzzle include the orbital angular momentum of quarks and gluons (top left), gluon spin (top right) and quark and antiquark spin (bottom). The proton spin puzzle: Scientists want to know how different constituents of the proton contribute to its spin, a fundamental property that plays a role in how these building blocks give rise to nearly all visible matter in the universe.