New Findings Challenge Understanding of Proton Structure at RHIC

New research from the STAR detector at the Relativistic Heavy Ion Collider (RHIC) reveals that gluons may be crucial in carrying and conserving baryon number, a fundamental property of protons. This study, published in the journal Science, indicates that baryon number could be associated with a Y-shaped “junction” of gluons linking the proton’s three main quarks, potentially overturning the long-held view that baryon number is solely attributed to quarks.

“Traditionally, scientists have assumed that each of the three main ‘valence’ quarks inside a proton or neutron carries one-third of the baryon number,” explained Zhangbu Xu, a Kent State University professor with a joint appointment at Brookhaven Lab.

Historical Context of Gluon Junctions

The concept of a baryon or gluon junction was introduced in the 1970s to explain how gluons connect valence quarks within a proton. In 1996, Dmitri Kharzeev of Stony Brook University suggested that this junction may fundamentally carry baryon number itself, rather than having it reside with valence quarks. The STAR collaboration has developed methods to investigate this idea using various collision types at RHIC.

Xu further noted, “Using data collected from different types of particle collisions at RHIC, our results suggest that the baryon number is not simply carried by individual quarks,” emphasizing that the findings support the notion that gluons are better at transporting baryon number when configured as a junction.

Significance of Baryon Number

Understanding the carriers of baryon number has broader implications beyond proton structure. In RHIC collisions, baryon number conservation indicates that the total count of baryons (composed of three quarks, like protons and neutrons) remains unchanged pre- and post-collision. Nicole Lewis, a STAR physicist at Rice University, noted, “Since the Big Bang, the number of protons and neutrons all together never changes as a function of time,” highlighting ongoing mysteries related to matter-antimatter asymmetry.

Baryon number conservation also underlies the stability of protons, which are essential components of atomic nuclei. “It’s believed that the lifetime of a proton is longer than the lifespan of the universe,” Lewis stated, ensuring the existence of stable matter as we know it.

Complexities of Proton Structure

The notion that gluons carry baryon number challenges the simplified textbook depiction of protons having a baryon number of one, equally divided among three quarks. “In the naïve quark model, there are three quarks inside a proton, but nothing else,” noted Tommy Tsang, formerly of Kent State University and now at Argonne National Laboratory. He emphasized that the true picture includes numerous interacting gluons and transient quark-antiquark pairs, leading to a more complex model of protons.

Observations of Baryon Excess

The STAR collaboration observed an unexpected surplus of baryons compared to antibaryons emerging in collisions directed perpendicular to the incoming beams. Tsang remarked, “In the STAR detector, we consistently see an excess of baryons coming out of the collisions perpendicular to the direction of the colliding beams,” reinforcing the hypothesis that gluons may play a pivotal role in baryon number distribution.

Investigating Electric Charge

To explore the baryon number excess, the STAR team utilized the electric charge properties of the valence quarks. Zebo Tang, a professor at the University of Science and Technology of China, highlighted that comparing electric charge distribution with baryon counts revealed a significant mismatch, with roughly double the expected baryons emerging. “That means too few quarks were being stopped to account for all the baryons appearing in the detector,” Tang said.

The Role of Gluon Junctions in Collisions

The mechanism proposed hinges on the behavior of protons during high-energy collisions, where the stable “gluon junction” may be easier to halt than the valence quarks. Prithwish Tribedy, a STAR physicist at Brookhaven Lab, explained that, “In the collision, the baryon junction gets held behind, and the quarks continue on.” The energy stored in the junction can then produce new baryons that travel outward perpendicular to the beamlines.

As the dynamics of these collisions unfold, quarks and gluons quickly combine to form new particles. Actual RHIC collisions yield countless new particles from immense energy conversion, and the STAR team found that collisions resulting in higher particle counts demonstrated greater baryon excess, particularly in the “midrapidity” region.

Implications for Understanding Matter

The findings suggest that gluon structures connecting quarks are fundamental to understanding how baryon number is conveyed in energetic collisions. “Our research challenges the long-held idea that baryon number is simply divided among and carried by the three quarks,” said Ma. This evolving understanding may reshape interpretations of matter’s fundamental nature, providing deeper insights into the building blocks of the universe.

The research was supported by the DOE Office of Science, the U.S. National Science Foundation, and various international organizations, utilizing resources from the Open Science Grid along with Brookhaven Lab’s Scientific Data and Computing Facilities.