New findings from the STAR detector on the Relativistic Heavy Ion Collider (RHIC) are difficult a well-recognized image of what offers protons one among their defining quantum properties. The outcomes recommend that gluons, the particles that act because the glue holding quarks collectively, might play a key function in carrying and conserving baryon quantity.
The proof comes from high-energy particle collisions at RHIC, a U.S. Division of Vitality (DOE) Workplace of Science consumer facility for nuclear physics analysis that operated at DOE’s Brookhaven Nationwide Laboratory from 2000 to early 2026. In keeping with the brand new research, printed in Science, baryon quantity could also be related to a Y-shaped “junction” of gluons connecting the proton’s three principal quarks. If confirmed, that will problem the long-standing assumption that baryon quantity belongs completely to these quarks.
“Historically, scientists have assumed that every of the three principal ‘valence’ quarks inside a proton or neutron carries one-third of the baryon quantity,” mentioned Zhangbu Xu, a professor at Kent State College with a joint appointment at Brookhaven Lab.
A A long time-Previous Concept About Gluons
Physicists first proposed the baryon junction, additionally referred to as a gluon junction, within the Nineteen Seventies as a option to describe how gluons join the valence quarks inside a proton. In 1996, 4 years earlier than RHIC started working, Dmitri Kharzeev, a theoretical physicist at Stony Brook College and Brookhaven Lab, proposed that this junction would possibly do one thing much more basic. Slightly than the valence quarks carrying baryon quantity, the junction itself could possibly be accountable.
The STAR collaboration has now developed a option to check that chance utilizing a number of kinds of collisions produced at RHIC.
“Utilizing knowledge collected from various kinds of particle collisions at RHIC, our outcomes recommend that the baryon quantity is just not merely carried by particular person quarks,” Xu added. “Our findings strongly assist the concept that baryon quantity is extra favorably carried and transported by gluons, the particles that maintain quarks collectively, when organized on this particular configuration.”
Why Baryon Quantity Issues
Figuring out what truly carries baryon quantity issues far past the inner construction of a proton. In RHIC collisions, conservation of baryon quantity signifies that the whole variety of baryons, three-quark particles equivalent to protons and neutrons, should stay unchanged earlier than and after the collision. The identical conservation precept additionally applies on the size of the universe.
“For the reason that Huge Bang, the variety of protons and neutrons all collectively by no means adjustments as a perform of time,” mentioned Nicole Lewis, a STAR physicist at Rice College who began this challenge as a postdoc at Brookhaven Lab in 2020. “The explanations for this conservation aren’t nicely understood. It is one of many mysteries of the universe, associated to why now we have extra matter than antimatter,” she mentioned.
Baryon quantity conservation additionally has a way more tangible consequence. It helps clarify the extraordinary stability of protons, which type a central a part of atomic nuclei and don’t seem to decay below unusual circumstances.
“It is believed that the lifetime of a proton is longer than the lifespan of the universe,” Lewis mentioned. “This permits atomic nuclei to type and be secure — which suggests matter, as we work together with it within the universe, can exist.”
A Extra Sophisticated Proton
The chance that gluons carry baryon quantity would overturn the usual simplified description discovered in lots of textbooks. In that image, a proton has a baryon variety of plus one, divided equally amongst its three principal valence quarks. Every quark subsequently carries plus one third of the baryon quantity, a lot because the proton’s electrical cost is distributed amongst its three valence quarks.
However actual protons are far more difficult than that simplified mannequin suggests.
“Within the naïve quark mannequin, there are three quarks inside a proton, however nothing else,” mentioned Tommy Tsang, previously a postdoc at Kent State College, now at DOE’s Argonne Nationwide Laboratory. “But when we have a look at particulars inside, there aren’t solely three quarks but additionally a number of gluons interacting, connecting between these quarks, and there are additionally quarks and antiquarks that pop up from the vacuum, so it is truly a extremely advanced object.”
Quantum chromodynamics (QCD), the speculation used to explain these interactions, has been extremely profitable in explaining the sturdy power that acts amongst quarks and gluons. Even so, fashions impressed by QCD usually want extra assumptions to breed a number of the particle patterns noticed when RHIC smashes nuclei collectively at practically the velocity of sunshine.
An Surprising Extra of Baryons
One remark specifically caught the STAR workforce’s consideration. The detector repeatedly information extra baryons than antibaryons rising sideways from the collisions, perpendicular to the course of the incoming beams.
“Within the STAR detector, we constantly see an extra of baryons popping out of the collisions perpendicular to the course of the colliding beams,” Tsang mentioned. “The truth that we find yourself with extra baryons than antibaryons — or extra matter than antimatter — isn’t a surprise since our collisions begin with matter,” he mentioned.
These extraordinarily energetic collisions convert super quantities of vitality into hundreds of newly created particles. What puzzled the researchers was not merely that extra baryons than antibaryons had been produced. It was the place the surplus baryons appeared.
If valence quarks alone carried the baryon quantity, explaining the surplus away from the beamline would require all three valence quarks from one colliding proton to cease close to the middle of the detector. They might then must endure a conversion from matter into vitality and again into matter, producing new baryons that transfer outward perpendicular to the beam.
The STAR researchers suspected there may be one other clarification.
Electrical Cost Supplies a Take a look at
The workforce discovered a option to examine the thriller by profiting from one other property of valence quarks: electrical cost. Scientists in contrast the online baryon quantity measured in numerous RHIC nuclear collisions with the best way electrical cost was redistributed in those self same occasions.
“Measuring the electrical cost popping out perpendicular to the collision offers you a definitive means of measuring what number of quarks are stopped and reworked into new particles,” mentioned Zebo Tang, a professor on the College of Science and Expertise of China who led a gaggle of scholars performing knowledge analyses and mannequin simulations.
The comparability revealed a placing mismatch. Researchers noticed roughly twice as many baryons as ought to have been produced primarily based on the electrical cost related to stopped quarks.
In keeping with fashions primarily based on QCD, which means too few quarks had been being stopped to account for all of the baryons showing within the detector.
That left an vital query: What was carrying the additional baryon quantity?
The STAR physicists argue that gluons provide a potential reply, particularly the three-pronged gluon junction that connects the proton’s valence quarks.
How the Gluon Junction May Carry Baryon Quantity
The proposed mechanism depends upon what occurs when protons inside colliding nuclei attain monumental energies. In keeping with the STAR workforce, the “gluon junction” or “baryon junction” that hyperlinks the quarks could also be a lot simpler to cease in a collision than the three quarks themselves.
If the junction is stopped, its vitality may be transformed into newly produced baryons that journey outward in instructions perpendicular to the beams. In the meantime, the valence quarks that had been beforehand related by the junction can proceed transferring ahead alongside the beampipe.
Understanding why requires trying on the altering inner construction of a proton as its vitality will increase.
“The baryon junction is all the time there at the same time as protons are accelerated to larger and better vitality,” Prithwish Tribedy, a STAR physicist at Brookhaven Lab. “However at excessive vitality, gluons inside the proton cut up and multiply.”
Because the variety of gluons will increase, the proton’s momentum turns into unfold amongst extra of them. Every particular person gluon, together with these forming the junction, subsequently carries a smaller portion of the proton’s complete momentum. The valence quarks, nevertheless, proceed to hold a lot of the proton’s ahead movement.
Because of this, when the collision happens, the comparatively slower three-pronged gluon junction needs to be simpler to cease and convert into new particles than the quickly transferring quarks.
Stopping one related construction can be easier than stopping three separate quarks, making such an interplay extra doubtless, in line with Tribedy.
“Within the collision, the baryon junction will get held behind, and the quarks proceed on,” he famous.
Constructing New Particles After the Collision
Quarks and gluons can’t stay remoted, so after the collision they rapidly mix with different particles.
In a simplified instance, a quark persevering with down the beampipe might be part of with an antiquark and type a two-quark particle referred to as a meson. On the identical time, the three-pronged gluon junction might behave considerably like a Y-shaped magnet, drawing in three newly created quarks from the vacuum and producing a brand new baryon.
Precise RHIC collisions are significantly extra violent and sophisticated.
“Regardless that we begin with nuclei that include roughly 100 protons and 100 neutrons, these collisions create hundreds of recent particles; 99% of the vitality is reworked into new particles,” mentioned Rongrong Ma, a Brookhaven Lab physicist.
The STAR workforce discovered that collisions producing bigger numbers of particles additionally confirmed a larger extra of “midrapidity” baryons in contrast with predictions primarily based on the easier image during which quarks alone carry baryon quantity.
The truth that so many of those baryons emerge perpendicular to the beamline offers sturdy proof, in line with the researchers, that the baryon junction exists and performs an vital function in transporting baryon quantity.
Rethinking a Basic Property of Matter
The outcomes recommend that one of many proton’s defining quantum properties might not reside solely in its three valence quarks. As an alternative, the gluon construction connecting these quarks could possibly be central to how baryon quantity is carried by energetic collisions.
“Our analysis challenges the long-held concept that baryon quantity is solely divided amongst and carried by the three quarks,” mentioned Ma. “This new understanding reshapes how we take into consideration the construction of matter and deepens our information of probably the most basic factor that’s chargeable for the universe in its present type.”
The analysis was supported by the DOE Workplace of Science, the U.S. Nationwide Science Basis (NSF), and quite a few worldwide businesses and organizations listed within the scientific paper. Researchers additionally used the Open Science Grid, which is supported straight by NSF, together with computing assets at Brookhaven Lab’s Scientific Knowledge and Computing Services and the Nationwide Vitality Analysis Scientific Computing Middle (NERSC), one other DOE Workplace of Science consumer facility situated at DOE’s Lawrence Berkeley Nationwide Laboratory.
