A proton’s identity as a particle called a baryon may reside with a Y-shaped gluon structure (yellow) at the center the particle, rather than its two up quarks (u) and one down quark (d). If that’s true, a collision can separate the quarks from the junction, and the baryon number. New quarks and antiquarks will appear out of the vacuum to accompany the freed quarks and gluons.
Valerie A. Lentz/Brookhaven National Laboratory
A key facet of a proton’s identity can be traced to the glue that holds it together.
A new study suggests that a property called the baryon number — a defining characteristic of protons and related particles — resides with the gluey particles inside. These particles, called gluons, carry the forces that bind the proton’s smaller matter particles together.
Scientists assumed that these matter particles, called quarks, carry the baryon number. But some physicists have argued that gluons could be the seat of baryon number, specifically a Y-shaped gluon structure that forms in the center of a proton. In collisions of atomic nuclei slamming together at nearly the speed of light, the types and angles of particles that spewed outward matched the gluon picture better than the quark one, researchers from the STAR collaboration report in the Aug. 13 Science.
The lives of quarks and gluons are deeply intertwined, so it’s challenging to distinguish the two ideas. “For a long time, it did not seem like we could tell the difference,” says physicist Spencer Klein of Lawrence Berkeley National Lab in California, who was not involved with the research. “This work, for the first time, really shows that we can.”
Protons and neutrons belong to a class of particles called baryons that are made up of three main quarks, plus gluons. (Additionally, short-lived pairs of quarks and antiquarks constantly flit throughout the particles.) Baryons have a baryon number of 1. Antibaryons, such as antiprotons, have a baryon number of -1. The sum total of this quantum mechanical property is conserved in all reactions scientists have observed. That means you can’t destroy a baryon without creating another (or destroying an antibaryon). And that explains why protons don’t decay, and atoms can persist indefinitely.
To get a baryon number of 1, the longstanding view was that each of a proton’s three quarks has a baryon number of 1/3. But confusingly, if you separate one of the three quarks, you don’t get a third of a baryon number. Instead, that quark combines with an antiquark to create a particle called a meson, with baryon number 0, and a new quark takes the place of the old quark in the original baryon. An alternative idea is that the baryon number of 1 resides in a Y-shaped structure called a baryon junction, which connects the three quarks.
The distinction may seem subtle. “But there are tangible consequences,” says Chun Yuen Tsang, who worked on the study at Kent State University in Ohio, and is now at Argonne National Laboratory in Lemont, Ill. “The way in which particles are emitted in a collision is different depending on which model you use. So it’s not just semantics.”
In smashups of atomic nuclei created by the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, N.Y., some particles keep going in the original direction of the beam, but others get stopped in their tracks and flung out perpendicular to the beam. Compared with the junction of gluons, the quarks carry a larger fraction of a proton’s momentum, making them harder to stop. If the quark picture is correct, the number of baryons that are emitted (minus the number of antibaryons emitted) will be greater in the forward direction. But if the junction carries the baryon number, that number should be increased in the direction perpendicular to the beam. In several different kinds of collisions, this is what the researchers found.
The evidence “strongly suggests that the junction picture is favored,” says theoretical physicist Chun Shen of Wayne State University in Detroit, who was not involved with the research. But “it’s not the smoking gun yet.” To further solidify the case, he says, theoretical physicists need to improve their predictions to allow a better comparison with the experimental data. New insights could also come from the future Electron-Ion Collider, planned to replace the Relativistic Heavy Ion Collider, which shut down in February.
But not everyone believes the distinction is that meaningful. “The two pictures can be compatible with each other and not contradictory to each other,” says theoretical physicist Zi-Wei Lin of East Carolina University in Greenville, N.C. That’s because the junction doesn’t exist independently of quarks.
Still, when it comes to the proton, gluons have a history of punching above their weight. They’ve been shown to provide a hefty portion of the mass of a proton, as well as the spin, another quantum property of the particle. Baryon number may follow that trend.
