For nearly a century, physicists have taught that a proton's identity — the quantum property called baryon number that makes it a proton rather than some other particle — is split evenly among its three "valence" quarks, each carrying a third. A new analysis of two decades of particle-collision data from Brookhaven National Laboratory's STAR experiment suggests that picture is incomplete, and that the gluons binding those quarks together may do most of the work instead.
The finding, published Sunday in the journal Science, comes from the STAR (Solenoidal Tracker at RHIC) collaboration's review of collisions recorded at the Relativistic Heavy Ion Collider (RHIC), which ran at Brookhaven from 2000 until it concluded operations earlier this year. Physicists counted where baryons — protons, neutrons and related particles — emerged after gold ions were smashed together at nearly the speed of light.
Twice as many baryons where they shouldn't be
If baryon number really traveled with the three valence quarks, the resulting baryons should mostly follow the quarks' paths, clustered near the direction of the original colliding beams. Instead, the STAR team found roughly twice as many baryons emerging perpendicular to the beamline as quark-only models predict — a pattern the researchers say is best explained by a separate carrier of baryon number: a Y-shaped tangle of gluons called a "baryon junction," which can lag behind as the quarks fly onward. The theoretical case for such a junction was first proposed in 1996 by Brookhaven and Stony Brook University physicist Dmitri Kharzeev.
"Traditionally, scientists have assumed that each of the three main 'valence' quarks inside a proton or neutron carries one-third of the baryon number," said Zhangbu Xu, a physicist with Kent State University and Brookhaven Lab, in the laboratory's announcement of the result. Colleague Prithwish Tribedy, a STAR physicist at Brookhaven, put the mechanism more simply: "In the collision, the baryon junction gets held behind and the quarks continue on."
Baryon number is one of physics' bedrock conserved quantities — the reason, as STAR physicist Nicole Lewis of Rice University noted, that "the number of protons and neutrons all together never changes as a function of time" since the Big Bang. Pinning down what physically carries that property inside ordinary matter has implications for one of cosmology's open puzzles: why the universe today is made almost entirely of matter rather than equal parts matter and antimatter, since the two should have been produced in equal amounts and annihilated each other.
In the STAR detector, we consistently see an excess of baryons coming out of the collisions perpendicular to the direction of the colliding beams.
Tommy Tsang, formerly of Kent State University, now at Argonne National Laboratory
The result builds on years of incremental STAR measurements rather than a single eureka moment, and outside theorists will now test whether the gluon-junction model holds up against competing explanations for the perpendicular excess. With RHIC retired, further direct tests may fall to Brookhaven's under-construction Electron-Ion Collider, designed specifically to map how quarks and gluons build up the mass, spin and other properties of protons and neutrons. The full study, "Tracking the baryon number with nuclear collisions," appears in Science, with additional detail on the collaboration's methods available through Brookhaven's STAR detector program page.
