⚛️ nuclear experiments

Tracking the baryon number with nuclear collisions

The STAR Collaboration's measurements of baryon-to-charge ratios and net-proton yields in isobar and photonuclear collisions disfavor the conventional valence quark model of baryon number conservation, providing evidence that supports the alternative baryon junction hypothesis.

Original authors: STAR Collaboration

Published 2026-08-17

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: STAR Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Great Cosmic Mystery: Who Carries the Heavy Luggage?

Imagine the universe as a giant, bustling train station. For over a century, physicists have been trying to figure out exactly how the "passengers" of the subatomic world move around. One of the most important passengers is something called baryon number. Think of this as a special ID card that says, "I am a piece of matter, not antimatter." Without this ID, the universe would be a chaotic mess of matter and antimatter canceling each other out, leaving nothing behind to build stars, planets, or you.

In the standard story of particle physics, this ID card is carried by tiny, fundamental particles called quarks. It's like saying every passenger carries their own backpack. Since a proton (a building block of atoms) is made of three quarks, each quark carries one-third of the ID card. But there's a rival theory, a bit more exotic, that suggests the ID card isn't carried by the passengers at all. Instead, it's carried by an invisible, Y-shaped "glue" structure called a baryon junction that connects the passengers. This glue is made of gluons, the particles that act like the super-strong tape holding quarks together.

Why does this matter? Because if we can figure out who is actually carrying the ID card, we might finally understand how the universe survived its own birth. If the backpack theory is right, the passengers should drag their heavy luggage with them. If the glue theory is right, the luggage might be able to hop onto a different train entirely. This is the question a massive team of scientists, the STAR Collaboration, set out to answer by smashing heavy atoms together at nearly the speed of light.

The Great Atomic Smackdown

To solve this mystery, the scientists at the Relativistic Heavy Ion Collider (RHIC) decided to play a high-speed game of tag with atoms. They didn't just smash atoms together; they set up three different "traps" to see how the baryon number (the ID card) behaves when things get chaotic.

The First Trap: The Ghost Photon
First, they looked at a very specific type of crash called a photon-gold interaction. Imagine a gold atom flying by, and a tiny, invisible "ghost" photon (a particle of light) bumps into it. The photon has no baryon number—it's a clean slate. When it hits the gold atom, it kicks things around. If the ID card is carried by the quarks (the backpacks), the ID should stay stuck to the heavy gold nucleus, barely moving. But if the ID is carried by the Y-shaped glue (the junction), that glue is light and fast. It should be able to zip right into the middle of the crash zone, leaving the heavy gold nucleus behind.

The scientists measured how many protons (which carry the ID) showed up in the middle of the crash. They found a lot of them! The data showed that the baryon number traveled much further and faster than the "backpack" theory predicted. In fact, the measurements were consistent with the "glue" theory, which predicts a specific range of behavior, suggesting the ID card can indeed hop onto a fast-moving glue train.

The Second Trap: The Isobar Rivalry
Next, the team set up a head-to-head race between two very similar atoms: Ruthenium (Ru) and Zirconium (Zr). These are "isobars," meaning they have the same total weight (96 units) but different electric charges (Ruthenium has 44 protons, Zirconium has 40). They smashed these atoms together in two different combinations: Ru+Ru and Zr+Zr.

If the ID card is tied to the electric charge (like a backpack that only fits on a specific person), then the amount of ID card moving to the middle of the crash should be exactly proportional to the difference in electric charge between the two atoms. But if the ID card is carried by the neutral glue, it doesn't care about the electric charge. It should move freely, regardless of whether the atom is Ruthenium or Zirconium.

The results were shocking. The scientists found that the amount of baryon number moving to the center was 1.84 times larger than what you would expect if it were tied to the electric charge. It was as if the glue was carrying the luggage so efficiently that it ignored the rules of the backpack theory entirely. The "backpack" idea simply couldn't explain why so much matter ended up in the middle of the crash.

The Third Trap: The Speed Test
Finally, they looked at how the baryon number moved at different speeds (rapidity). They found that the number of protons in the middle of the crash dropped off in a very specific way as you moved away from the center. This drop-off rate matched the predictions for the "glue" theory but was completely wrong for the "backpack" theory. The backpacks were supposed to be heavy and slow, but the data showed the baryon number was moving with the agility of the glue.

The Verdict

After running these massive experiments and crunching the numbers, the STAR Collaboration has a clear message: The old idea that quarks carry the baryon number is disfavored by the data.

The results make the "backpack" theory very hard to believe. Instead, the data shows that the baryon junction framework—the theory involving that mysterious, Y-shaped glue made of gluons—remains qualitatively consistent with all the observed phenomena. While the scientists note that the data doesn't definitively prove the junction is the carrier (ruling out other possibilities entirely), the evidence they have gathered makes the "backpack" theory highly unlikely.

It's a bit like realizing that in a crowded stadium, the people aren't carrying the heavy banners; instead, the banners are being flown by invisible drones that can zip through the crowd much faster than the people can walk. This discovery doesn't just change how we see a proton; it helps us understand the fundamental rules that kept our universe from disappearing in a flash of light and matter. The glue, it seems, is the most likely hero of the story, even if the mystery isn't fully solved yet.

Technical Summary: Tracking the Baryon Number with Nuclear Collisions

Problem Statement
The conservation of baryon number ( $B$ ) since the early Universe is a fundamental tenet of physics, yet the microscopic carrier of this quantum number remains experimentally unverified. The conventional Standard Model picture posits that valence quarks, each carrying $B=1/3$ , are the carriers of baryon number. An alternative hypothesis, proposed in the 1970s and supported by non-perturbative Lattice QCD studies, suggests that baryon number is instead carried by a topological, Y-shaped gluonic configuration known as a "baryon junction" ( $B=1, Q=0$ ). While both scenarios are consistent with existing conservation laws, they predict distinct mechanisms for "baryon transport"—the movement of baryon number from beam rapidity to mid-rapidity in high-energy nuclear collisions. In the valence quark picture, transporting baryon number to mid-rapidity requires the transport of high-momentum quarks, which is kinematically suppressed. Conversely, the junction picture allows low-momentum gluonic strings to be transported more easily to mid-rapidity, where they can couple with vacuum quark-antiquark pairs to form baryons.

Methodology
The STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC) employed three independent approaches to distinguish between these scenarios using data from $\sqrt{s_{NN}} = 54.4$ GeV and $200$ GeV collisions:

Photon-Gold ( $\gamma$ +Au) Interactions: The team analyzed inclusive $\gamma$ +Au events selected from Au+Au collisions. In these interactions, a quasi-real photon (carrying no baryon number) from one nucleus interacts with the other. The rapidity dependence of the net-proton yield ($dN/dy$) was measured. The slope parameter ( $\alpha_B$ ) of the exponential distribution $f(y) \propto \exp(-\alpha_B \Delta y)$ was extracted and compared to Regge theory predictions (which incorporate baryon junctions) and Monte Carlo models (PYTHIA 8.3, HERWIG 7.2) based on valence quarks.
Systematic Scaling in Au+Au Collisions: The study revisited previously measured net-proton yields across a wide range of beam energies ( $\sqrt{s_{NN}} = 7.7 - 200$ GeV) to extract the exponential slope parameter $\alpha_B$ as a function of collision centrality. This was compared against UrQMD simulations and PYTHIA tunes.
Isobar Collisions (Ru+Ru vs. Zr+Zr): Utilizing a dataset of $\sim 2$ billion events each for $^{96}_{44}$ Ru+ $^{96}_{44}$ Ru and $^{96}_{40}$ Zr+ $^{96}_{40}$ Zr collisions at $\sqrt{s_{NN}} = 200$ GeV, the collaboration measured the transport of net-baryon number ( $B$ ) and net-charge number ( $Q$ ). By calculating the ratio of the mean net-baryon number to the difference in net-charge between the two isobar systems ( $\langle B \rangle / \Delta Q$ ), scaled by the difference in atomic numbers ( $\Delta Z/A$ ), the study tested the prediction that $B/A \approx Q/Z$ for valence quarks versus $B/A > Q/Z$ for baryon junctions.

Key Results

Rapidity Slope in $\gamma$ +Au: The measured slope parameter for net-protons in $\gamma$ +Au interactions is $\alpha_B = 1.04 \pm 0.22$ . This value is consistent with the Regge theory prediction for baryon junctions ( $0.42 < \alpha_B < 1$ ) but significantly higher than predictions from PYTHIA 8.3 default and color reconnection tunes, which assume valence quark transport.
Centrality Independence in Au+Au: The slope parameter $\alpha_B$ in Au+Au collisions is found to be independent of collision centrality, with an average value of $0.64 \pm 0.05$ . This contradicts valence quark models (such as UrQMD) which predict a stronger dependence on centrality due to increased multiple scatterings in central collisions.
Baryon-to-Charge Ratio in Isobars: In the most central (0-10%) Ru+Ru and Zr+Zr collisions, the ratio $\langle B \rangle / \Delta Q \times \Delta Z/A$ is measured to be $1.84 \pm 0.02$ (stat) $\pm 0.09$ (syst) $\pm 0.16$ (feed-down). This value is significantly larger than the expectation of unity for valence quarks. The data shows a monotonic decrease from central to peripheral collisions, a trend not reproduced by UrQMD simulations (which predict values between 0.5 and 0.7) but qualitatively consistent with the baryon junction scenario where charge transport is suppressed relative to baryon transport.

Significance
The paper concludes that the experimental findings—specifically the enhanced baryon transport observed in $\gamma$ +Au interactions, the centrality-independent rapidity slope in Au+Au collisions, and the large baryon-to-charge ratio in isobar collisions—disfavor the conventional picture where valence quarks carry the baryon number. These results are qualitatively consistent with the baryon junction framework. The author states that while neither scenario has been unequivocally verified, the baryon junction framework remains the only model that simultaneously explains the observed phenomena across these different collision systems. The study underscores the need for further theoretical and experimental investigations, including future measurements at the Electron-Ion Collider, to directly probe the distribution function of baryon number carriers in nuclei.

The Great Cosmic Mystery: Who Carries the Heavy Luggage?

The Great Atomic Smackdown

The Verdict

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