The “Glue” Inside Protons May Hold the Secret to Matter’s Identity

New findings from the STAR detector at the Relativistic Heavy Ion Collider (RHIC) recommend that gluons, the particles that imitate glue in between quarks inside protons, might play a much larger function in protecting baryon number than researchers as soon as believed. Baryon number is an essential part of a particle’s quantum identity.

The outcomes originate from high-energy particle crashes at RHIC, a U.S. Department of Energy (DOE) Office of Science user center for nuclear physics research study that ran at DOE’s Brookhaven National Laboratory from 2000 to early 2026. The brand-new research study, released in Science, recommends that baryon number might be brought by a Y-shaped “junction” of gluons connecting the proton’s 3 primary quarks.

If validated, the outcome would challenge the enduring presumption that baryon number is brought just by those 3 quarks.

“Traditionally, researchers have actually presumed that each of the 3 primary ‘valence’ quarks inside a proton or neutron brings one-third of the baryon number,” stated Zhangbu Xu, a teacher at Kent State University with a joint consultation at Brookhaven Lab.

A Decades-Old Idea About Gluon Junctions

Physicists initially proposed the baryon junction, likewise called a gluon junction, in the 1970s as a method to explain how gluons link the valence quarks inside protons.

In 1996, 4 years before RHIC started running, theoretical physicist Dmitri Kharzeev of Stony Brook University and Brookhaven Lab proposed that this junction may do more than hold the quarks together. He recommended that it might in fact bring baryon number rather of the valence quarks themselves.

The STAR partnership has actually now established a method to evaluate that possibility utilizing numerous kinds of particle crashes at RHIC.

“Using information gathered from various kinds of particle crashes at RHIC, our outcomes recommend that the baryon number is not merely brought by specific quarks,” Xu included. “Our findings highly support the concept that baryon number is more positively brought and transferred by gluons, the particles that hold quarks together, when set up in this unique setup.”

Why Baryon Number Matters

Identifying what in fact brings baryon number might have effects far beyond the crashes produced at RHIC.

In those experiments, baryon number preservation suggests that the overall variety of baryons, particles made from 3 quarks such as protons and neutrons, need to stay constant before and after an accident. This preservation law likewise uses on a much bigger scale throughout the universe.

“Since the Big Bang, the variety of protons and neutrons completely never ever modifications as a function of time,” stated Nicole Lewis, a STAR physicist at Rice University who began this task as a postdoc at Brookhaven Lab in 2020. “The factors for this preservation are not well comprehended. It’s one of the secrets of deep space, associated to why we have more matter than antimatter,” she stated.

Baryon number preservation is likewise connected to the amazing stability of protons, which form a main part of atomic nuclei and do not appear to decay under regular conditions.

“It’s thought that the life time of a proton is longer than the life expectancy of deep space,” Lewis stated. “This enables atomic nuclei to form and be steady– which indicates matter, as we communicate with it in deep space, can exist.”

A Textbook Picture Comes Under Pressure

The possibility that gluons assist bring baryon number disputes with the standard description discovered in lots of physics books.

A proton has a baryon variety of plus one. In the conventional image, that worth is divided uniformly amongst its 3 main valence quarks, providing each quark a baryon variety of plus one 3rd. The concept resembles how a proton’s electrical charge is dispersed amongst those very same valence quarks.

A genuine proton is far more complex than an easy collection of 3 quarks.

“In the ignorant quark design, there are 3 quarks inside a proton, however absolutely nothing else,” stated Tommy Tsang, previously a postdoc at Kent State University, now at DOE’s Argonne National Laboratory. “But if we take a look at information within, there are not just 3 quarks however likewise a great deal of gluons connecting, linking in between those quarks, and there are likewise quarks and antiquarks that appear from the vacuum, so it’s in fact an actually complicated things.”

That intricate environment is explained by quantum chromodynamics (QCD), the theory of the strong force that governs interactions amongst quarks and gluons. QCD has actually been very effective, however designs influenced by it in some cases need additional presumptions to describe what researchers observe when nuclei moving close to the speed of light clash at RHIC.

An Unexpected Excess of Baryons

One observation has actually been particularly perplexing.

“In the STAR detector, we regularly see an excess of baryons coming out of the accidents perpendicular to the instructions of the clashing beams,” Tsang stated. “The truth that we wind up with more baryons than antibaryons– or more matter than antimatter– is not unexpected given that our accidents begin with matter,” he stated.

These very energetic accidents transform huge quantities of energy into countless recently developed particles.

What captured scientists’ attention was where the additional baryons appeared. STAR identified a net excess of baryons emerging far from the beamline.

If valence quarks alone was accountable for bring baryon number, all 3 quarks from an inbound proton would require to stop throughout the accident. They would then need to transform from matter into energy and ultimately back into matter near the center of the detector, producing the excess baryons seen moving far from the beamline.

The STAR group presumed that another system may be included.

Electric Charge Provides a Critical Test

The scientists discovered a method to evaluate the concept by benefiting from a residential or commercial property valence quarks certainly bring: electrical charge.

They compared the net baryon number produced in various kinds of RHIC crashes with the method electrical charge was rearranged in those exact same occasions.

“Measuring the electrical charge coming out perpendicular to the crash offers you a conclusive method of determining the number of quarks are stopped and changed into brand-new particles,” stated Zebo Tang, a teacher at the University of Science and Technology of China who led a group of trainees carrying out information analyses and design simulations.

The contrast exposed a striking inequality.

Scientist observed about two times as lots of baryons as need to have been produced based upon the electrical charge related to stopped quarks. According to QCD-based designs, the variety of quarks being stopped just was not big enough to represent the baryons appearing in the detector.

That raised an apparent concern: if the quarks were not producing all of the excess baryons, what was?

The STAR group argues that the response might lie with gluons, specifically the three-pronged gluon junction linking a proton’s valence quarks.

How a Gluon Junction Could Carry Baryon Number

The scientists propose that when nuclei clash at RHIC, the quarks and the gluon junction inside their protons do not always act the exact same method.

The “gluon junction” or “baryon junction” might be a lot easier to stop throughout the accident than the 3 fast-moving valence quarks. If the junction decreases while the quarks keep taking a trip along the beam instructions, the junction’s energy can rather be transformed into brand-new baryons that emerge in other instructions.

Comprehending how this might take place needs taking a look at what happens inside a proton as it is sped up to exceptionally high energies.

“The baryon junction is constantly there even as protons are sped up to greater and greater energy,” stated Prithwish Tribedy, a STAR physicist at Brookhaven Lab. “But at high energy, gluons within the proton split and increase.”

As the variety of gluons boosts, each specific gluon brings a smaller sized share of the proton’s total momentum. That consists of the gluons that comprise the junction. The valence quarks, on the other hand, continue to bring much of the proton’s forward momentum.

As an outcome, when the crash occurs, the fairly slower three-pronged gluon structure might be much easier to stop and transform into brand-new particles than the quickly moving quarks.

The truth that just one structure requires to be stopped likewise makes the procedure most likely than a situation needing all 3 valence quarks to stop separately, Tribedy stated.

“In the crash, the baryon junction gets held behind, and the quarks continue,” he kept in mind.

A Y-Shaped Junction Rebuilds Into New Matter

Quarks and gluons can not exist easily by themselves, so after the accident they quickly integrate with other particles.

In a streamlined example, each quark continuing down the beampipe might couple with an antiquark to form a two-quark particle called a meson. The three-pronged gluon junction, on the other hand, might act rather like a Y-shaped magnet, attracting 3 brand-new quarks from the vacuum and forming a brand-new baryon.

Genuine RHIC crashes are even more intricate.

“Even though we begin with nuclei which contain approximately 100 protons and 100 neutrons, these accidents develop countless brand-new particles; 99% of the energy is changed into brand-new particles,” stated Rongrong Ma, a Brookhaven Lab physicist.

The scientists discovered that accidents producing bigger varieties of particles likewise revealed a higher excess of “midrapidity” baryons compared to what would be anticipated if quarks alone brought baryon number.

The a great deal of baryons appearing perpendicular to the beamline for that reason offers strong assistance, the scientists state, for the presence and value of the baryon junction.

“Our research study challenges the long-held concept that baryon number is merely divided amongst and brought by the 3 quarks,” stated Ma. “This brand-new understanding improves how we consider the structure of matter and deepens our understanding of the most essential aspect that is accountable for deep space in its present type.”

Recommendation: “Tracking the baryon number with nuclear accidents” by STAR Collaboration *, B. E. Aboona, J. Adam, L. Adamczyk, I. Aggarwal, M. M. Aggarwal, Z. Ahammed, A. K. Alshammri, E. C. Aschenauer, S. Aslam, J. Atchison, V. Bairathi, X. Bao, P. Barik, K. Barish, S. Behera, R. Bellwied, P. Bhagat, A. Bhasin, S. Bhatta, S. R. Bhosale, J. Bielcik, J. Bielcikova, J. D. Brandenburg, C. Broodo, X. Z. Cai, H. Caines, M. Calderon de la Barca Sánchez, D. Cebra, J. Ceska, I. Chakaberia, P. Chaloupka, Y. S. Chang, Z. Chang, A. Chatterjee, D. Chen, J. Chen, J. H. Chen, L. Chen, Q. Chen, W. Chen, Z. Chen, J. Cheng, Y. Cheng, W. Christie, X. Chu, S. Corey, H. J. Crawford, M. Csanad, G. Dale-Gau, A. Das, D. De Souza Lemos, I. M. Deppner, A. Deshpande, A. Dhamija, A. Dimri, P. Dixit, X. Dong, J. L. Drachenberg, E. Duckworth, J. C. Dunlop, …, H. Zbroszczyk, W. Zha, C. Zhang, D. Zhang, J. Zhang, K. Zhang, L. Zhang, S. Zhang, W. Zhang, X. Zhang, Y. Zhang, Y. Zhang, Y. Zhang, Y. Zhang, Z. Zhang, Z. Zhang, F. Zhao, J. Zhao, S. Zhou, Y. Zhou, C. Zhu, X. Zhu, M. Zurek and M. Zyzak, 13 August 2026, STAR Collaboration
DOI: 10.1126/ science.ads5962

The research study was supported by the DOE Office of Science, the U.S. National Science Foundation (NSF), and numerous global firms and companies noted in the clinical paper. The group likewise utilized the Open Science Grid, supported straight by NSF, together with computing resources from the Scientific Data and Computing Facilities at Brookhaven Lab and the National Energy Research Scientific Computing Center (NERSC), another DOE Office of Science user center situated at DOE’s Lawrence Berkeley National Laboratory.

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