New findings from the STAR detector on the Relativistic Heavy Ion Collider (RHIC) are difficult a well-known image of what provides protons one among their defining quantum properties. The outcomes recommend that gluons, the particles that act because the glue holding quarks collectively, could play a key position in carrying and conserving baryon quantity.
The proof comes from high-energy particle collisions at RHIC, a U.S. Division of Power (DOE) Workplace of Science consumer facility for nuclear physics analysis that operated at DOE’s Brookhaven Nationwide Laboratory from 2000 to early 2026. In line with the brand new examine, printed in Science, baryon quantity could also be related to a Y-shaped “junction” of gluons connecting the proton’s three important quarks. If confirmed, that might problem the long-standing assumption that baryon quantity belongs completely to these quarks.
“Historically, scientists have assumed that every of the three important ‘valence’ quarks inside a proton or neutron carries one-third of the baryon quantity,” stated 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 Seventies as a strategy 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 may do one thing much more basic. Somewhat than the valence quarks carrying baryon quantity, the junction itself might be accountable.
The STAR collaboration has now developed a strategy to check that risk utilizing a number of forms of collisions produced at RHIC.
“Utilizing knowledge collected from several types of particle collisions at RHIC, our outcomes recommend that the baryon quantity will not be merely carried by particular person quarks,” Xu added. “Our findings strongly help 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 really carries baryon quantity issues far past the interior construction of a proton. In RHIC collisions, conservation of baryon quantity implies that the full variety of baryons, three-quark particles corresponding 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.
“Because the Large Bang, the variety of protons and neutrons all collectively by no means adjustments as a operate of time,” stated 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 properly understood. It is one of many mysteries of the universe, associated to why we’ve extra matter than antimatter,” she stated.
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 underneath extraordinary circumstances.
“It is believed that the lifetime of a proton is longer than the lifespan of the universe,” Lewis stated. “This enables atomic nuclei to type and be steady — which implies 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 important 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,” stated 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 in addition a variety of gluons interacting, connecting between these quarks, and there are additionally quarks and antiquarks that pop up from the vacuum, so it is really a very complicated object.”
Quantum chromodynamics (QCD), the idea used to explain these interactions, has been extremely profitable in explaining the sturdy pressure that acts amongst quarks and gluons. Even so, fashions impressed by QCD typically want further assumptions to breed a number of the particle patterns noticed when RHIC smashes nuclei collectively at almost the pace of sunshine.
An Surprising Extra of Baryons
One commentary particularly caught the STAR workforce’s consideration. The detector repeatedly information extra baryons than antibaryons rising sideways from the collisions, perpendicular to the path of the incoming beams.
“Within the STAR detector, we constantly see an extra of baryons popping out of the collisions perpendicular to the path of the colliding beams,” Tsang stated. “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 stated.
These extraordinarily energetic collisions convert great quantities of vitality into hundreds of newly created particles. What puzzled the researchers was not merely that extra baryons than antibaryons have 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’d then must bear 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 is perhaps one other clarification.
Electrical Cost Supplies a Take a look at
The workforce discovered a strategy to examine the thriller by making the most of one other property of valence quarks: electrical cost. Scientists in contrast the web baryon quantity measured in several 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 provides you a definitive method of measuring what number of quarks are stopped and remodeled into new particles,” stated 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 line with fashions primarily based on QCD, which means too few quarks have 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 doable reply, particularly the three-pronged gluon junction that connects the proton’s valence quarks.
How the Gluon Junction Might Carry Baryon Quantity
The proposed mechanism is dependent upon what occurs when protons inside colliding nuclei attain monumental energies. In line 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 might be transformed into newly produced baryons that journey outward in instructions perpendicular to the beams. In the meantime, the valence quarks that have been beforehand linked by the junction can proceed shifting ahead alongside the beampipe.
Understanding why requires wanting 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 greater 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 must be simpler to cease and convert into new particles than the quickly shifting quarks.
Stopping one linked 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 shortly 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 similar 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.
“Though we begin with nuclei that include roughly 100 protons and 100 neutrons, these collisions create hundreds of recent particles; 99% of the vitality is remodeled into new particles,” stated 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 wherein quarks alone carry baryon quantity.
The truth that so many of those baryons emerge perpendicular to the beamline gives sturdy proof, in line with the researchers, that the baryon junction exists and performs an vital position in transporting baryon quantity.
Rethinking a Elementary Property of Matter
The outcomes recommend that one of many proton’s defining quantum properties could not reside solely in its three valence quarks. As a substitute, the gluon construction connecting these quarks might be central to how baryon quantity is carried via energetic collisions.
“Our analysis challenges the long-held concept that baryon quantity is just divided amongst and carried by the three quarks,” stated Ma. “This new understanding reshapes how we take into consideration the construction of matter and deepens our data of probably the most basic ingredient that’s liable 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 immediately by NSF, together with computing sources at Brookhaven Lab’s Scientific Knowledge and Computing Services and the Nationwide Power Analysis Scientific Computing Middle (NERSC), one other DOE Workplace of Science consumer facility situated at DOE’s Lawrence Berkeley Nationwide Laboratory.
