Scientists May Have Found Why Protons Never Fall Apart

Every physics textbook says the same thing about protons: their defining quantum property, baryon number, belongs to the three quarks locked inside them. New data from a particle collider on Long Island suggest that picture is incomplete, and that the glue holding those quarks together may be doing more work than anyone assumed.

The Key Number

KEY NUMBER
About twice as many baryons streamed out of RHIC's collisions, sideways from the beam, as the traditional three-quark model predicts based on the electric charge measured moving the same way.

Why This Matters

Baryon number sounds like an obscure bookkeeping rule, but it underwrites something people rarely think about: matter's refusal to fall apart. Protons, together with neutrons, make up almost every atomic nucleus in the universe, and their extraordinary stability, a lifetime longer than the universe's own age, is credited to baryon number conservation. Physicists have never fully explained why that conservation law exists or why matter so heavily outnumbers antimatter, and both puzzles trace back to the same question the STAR team just reopened: what, precisely, inside a proton is responsible for keeping the count? It joins a run of recent experiments testing textbook physics at extreme precision, including a magnetar observation that offered evidence for a decades-old quantum prediction about the vacuum.

What Happened

Since 2000, RHIC has smashed nuclei together at close to the speed of light, shattering them into thousands of new particles. Textbook quantum chromodynamics, the theory of the strong force, assumes each of a proton's three "valence" quarks carries one third of its baryon number, much like they share its electric charge. To test that assumption, the STAR team compared how baryon number and electric charge redistribute after a collision. If only quarks carried baryon number, the two should track each other closely, since quarks carry both properties. Instead, the researchers measured roughly double the baryon number they expected from the electric charge alone, a gap that widened as more particles were produced.

How It Works

The likely explanation, first proposed by Stony Brook University theorist Dmitri Kharzeev in 1996, is a structure called a baryon junction: a Y-shaped configuration of gluons that connects a proton's three quarks. As a proton accelerates toward RHIC's collision point, its gluons multiply and each one carries less of the proton's momentum, including the gluons forming the junction. That makes the junction easier to "stop" in a collision than the fast-moving quarks it connects, which mostly continue down the beampipe. When the junction stops, the STAR team argues, it pulls three new quarks out of the vacuum to form a fresh baryon, while the freed original quarks pair off with new partners to form lighter particles called mesons. One stopped junction, in other words, can account for an entire new baryon on its own, which would explain the excess the team measured.

What Happens Next

Brookhaven's next nuclear physics flagship, the Electron-Ion Collider, is being built partly on RHIC's old tunnel and is designed to probe exactly this kind of internal proton structure with a precision that heavy-ion smashups cannot match. Physicists on the STAR team say more targeted electron-proton scattering there could confirm, or rule out, the gluon junction directly, rather than through the indirect charge-versus-baryon comparison used in this study.

Takeaway

The finding does not overturn quantum chromodynamics, but it does suggest that the theory's simplest classroom version, three quarks carrying a proton's identity between them, leaves out the gluons doing much of the structural work. Whether that Y-shaped junction turns out to be the real anchor of baryon number will depend on collisions physicists have not yet been able to run.