A new explanation of why the universe’s supply of nuclear matter is fixed?

A basic accounting rule in physics says whenever a particle collision creates a baryon—something like a proton or neutron that consists of three particles called quarks—it must simultaneously create a similar antibaryon. Such “baryon number conservation” implies the amount of nuclear matter in the universe is fixed. However, physicists have never determined where within a baryon the conserved number resides. For decades, the textbook answer has been simple. Each quark carries a baryon number of 1/3 and each antiquark carries a baryon number of –1/3. Because a collision can only ever make quark-antiquark pairs, the total baryon number cannot change. However, a study published today in Science suggests the baryon number is not carried by the quarks, but instead resides in an agglomeration of particles called gluons that bind the quarks together and is called a baryon junction. The data come from the STAR detector at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, which operated from 2000 until earlier this year. “If the baryon number flows with the quarks, I don’t think you will ever be able to explain this data,” says Zhangbu Xu, a nuclear physicist and STAR collaborator at Kent State University who led the analysis. “I’m delighted the STAR collaboration was able to do this,” says Dmitri Kharzeev, a theorist at Stony Brook University who 30 years ago predicted how the baryon junction might be detected but was not involved in the experiments. “It is very difficult and intricate measurement.” Because a proton or neutron has three quarks, it has a baryon number 1. An antiproton or antineutron has a baryon number of –1, and a particle made from a quark-antiquark pair—known generically as a meson—has a baryon number of 0. In a particle interaction, the total baryon number remains a fixed integer, no matter what kind of fleeting exotic baryons and mesons emerge. But no experiment has proved quarks carry the baryon number. Quarks bind to one another mightily through the strong nuclear force, which is conveyed by a fuzz of other particles called gluons. In the 1970s, theorists realized that to preserve a fundamental symmetry of the strong force, the gluons could not bind the quarks in a proton pairwise in a triangle, but instead had to form a Y-shaped wave that joins at a central point called the baryon junction. Computer simulations later confirmed the idea, says Zohar Komargodski, a theoretical physicist at the Simons Center for Geometry and Physics who is not involved with STAR. “From the theory standpoint, we already know the baryon junction exists.” And it could carry the baryon number. In 1996, Kharzeev suggested the baryon junction might be detected in high energy collisions of nuclei by separating it from its quarks, if only for one-quadrillionth of a nanosecond. If the junction didn’t exist, a collision would just knock a quark out of, say, an incoming proton. The strong force would rip a quark-antiquark pair from the vacuum to replace the scattered quark, leaving a baryon traveling most likely in the same direction as the incoming beam. However, were the junction present, the collision might instead scatter it sideways. The proton’s quarks could continue to fly in the direction of the beam and most likely form mesons. But the deflected junction would grab three new quarks from the vacuum to form a baryon emerging with higher probability perpendicular to the beam. That excess sideways “flow” of baryons would reveal the effect of the junction—although it would pale in comparison with the gushing of particles and antiparticles every which way. STAR researchers report three signs of excess sideways flow of baryons. First, in experiments with beams of gold nuclei, they looked for rare events in which a nucleus from one beam collided with a photon radiated by the other. Among the thousands of particles from each collision, researchers found an unexpectedly large number of baryons, relative to antibaryons, deflected outward, says Nicole Lewis, an experimentalist and STAR member at Rice University. “We just saw way more protons than antiprotons,” she says. “I thought it was a mistake.” Second, the team studied collisions of gold nuclei. Models without baryon junctions predict the sideways baryon flow should decline when two nuclei strike each other at an offset rather than collide head-on. Instead, the STAR researchers found the sideways flow persisted even in these peripheral collisions, an observation that jibes with scattering of the baryon junction. Finally, the researchers compared the net sideways flow of baryons with that of electric charge. If quarks carry a baryon number, the two quantities should roughly be equal because quarks also carry electric charge. Instead, the net sideways flow of the baryon number exceeded that of charge by a factor of two. To isolate the effect, the researchers compared collisions of zirconium nuclei with those of ruthenium nuclei, which have identical totals of protons and neutrons, but different charges. The comparison canceled out uncertainties that would have otherwise obscured the tiny signal, Xu says. Taken together, the observations support the existence of the baryon junction, says Zi-Wei Lin, a theorist at East Carolina University. “It’s true that current models [without the junction] do not describe the data,” Lin says. “Actually, they fail badly.” Because the baryons go where the junctions go, STAR researchers suggest the junction in fact carries the baryon number. That may be going too far, theorists say. The junction cannot exist without quarks attached to it, Komargodski says, so “whether you assign the baryon number to the junction or insist on counting only the quarks it’s connected to is purely a subjective choice, in my opinion.” Lin says baryon number may belong to both the quarks and the junction. “The paper talks about the two scenarios as two incompatible cases—either this or that—but maybe that’s too simplified.” Now that the baryon junction has emerged from the theoretical realm into the experimental one, it seems sure to receive more attention. Physicists already plan to study it with the RHIC’s successor, the $2.8 billion Electron-Ion Collider under development at Brookhaven, which should fire up in the mid-2030s, Lewis says. “These are measurements will be a big part of the Electron-Ion Collider program.”