In a groundbreaking discovery, physicists have uncovered a hidden layer of complexity within the humble proton, challenging our understanding of its fundamental properties. This revelation, made possible by the STAR detector at the Relativistic Heavy Ion Collider (RHIC), has the potential to rewrite the physics textbooks.
The focus of this research is on the baryon number, a quantum property that has long been assumed to be exclusively associated with the three main quarks within a proton. However, the new findings suggest that gluons, the particles that bind quarks together, may play a pivotal role in carrying and conserving this baryon number.
This idea of a gluon junction, or baryon junction, was first proposed in the 1970s as a way to describe how gluons connect the valence quarks inside a proton. Now, with the data collected from various particle collisions at RHIC, scientists are confirming this hypothesis.
"Our findings strongly support the idea that baryon number is more favorably carried and transported by gluons, arranged in a special configuration," says Zhangbu Xu, a professor at Kent State University.
The implications of this discovery extend far beyond the internal structure of a proton. The conservation of baryon number is a fundamental principle that applies not just to individual particles but to the entire universe. Since the Big Bang, the total number of protons and neutrons has remained constant, a mystery that is closely tied to the imbalance between matter and antimatter in our universe.
"It's one of the mysteries of the universe, related to why we have more matter than antimatter," explains Nicole Lewis, a STAR physicist at Rice University.
Furthermore, the stability of protons, which form the backbone of atomic nuclei, is attributed to this conservation principle. The lifetime of a proton is believed to be longer than the lifespan of the universe itself, allowing atomic nuclei to form and matter to exist as we know it.
However, the reality of a proton's structure is far more intricate than the simplified model presented in textbooks. In the words of Tommy Tsang, formerly a postdoc at Kent State University, "In the naïve quark model, there are three quarks inside a proton, but nothing else. But if we look at the details, there are not only three quarks but also a lot of gluons interacting, connecting between those quarks, and there are also quarks and antiquarks that pop up from the vacuum, so it's actually a really complex object."
The observation that caught the STAR team's attention was an excess of baryons emerging sideways from the collisions, perpendicular to the direction of the incoming beams. This anomaly could not be explained by the traditional assumption that baryon number is solely carried by valence quarks.
The team turned to electric charge as a test, comparing the net baryon number with the redistribution of electric charge in the same events. What they found was a significant mismatch, indicating that too few quarks were being stopped to account for all the baryons appearing in the detector.
"Measuring the electric charge coming out perpendicular to the collision gives you a definitive way of measuring how many quarks are stopped and transformed into new particles," says Zebo Tang, a professor at the University of Science and Technology of China.
The STAR physicists argue that the three-pronged gluon junction, which connects the proton's valence quarks, could be the carrier of this excess baryon number. When protons reach enormous energies, this junction may be more easily stopped in a collision than the quarks themselves, converting its energy into newly produced baryons.
"The baryon junction is always there, even as protons are accelerated to higher energies. But at high energy, gluons within the proton split and multiply," explains Prithwish Tribedy, a STAR physicist at Brookhaven Lab.
As the number of gluons increases, the proton's momentum becomes distributed among more particles, with each individual gluon carrying a smaller portion of the total momentum. This makes the comparatively slower gluon junction easier to stop and convert into new particles.
"In the collision, the baryon junction gets held behind, and the quarks continue on," Tribedy notes.
This discovery challenges the long-held belief that baryon number is simply divided among and carried by the three quarks. It reshapes our understanding of the structure of matter and deepens our knowledge of the fundamental elements that shape our universe.
"This new understanding reshapes how we think about the structure of matter and deepens our knowledge of the most fundamental element that is responsible for the universe in its current form," says Rongrong Ma, a Brookhaven Lab physicist.
