STAR experiment results published in Science

Baryon number is one of the quantum numbers used to describe the properties of particles of matter. Baryons, such as protons and neutrons, are assigned a positive baryon number, while their antiparticles have a negative one. This makes it possible to distinguish baryonic matter from antimatter: a proton and an antiproton have opposite baryon numbers, as do a neutron and an antineutron. Baryon number does not simply mean the "amount of matter", however. It is an intrinsic quantum property of a particle that is conserved in the vast majority of physical processes.

Intriguing imbalance between matter and antimatter

The concept of baryon number has implications far beyond the description of individual particles. Almost all the visible matter surrounding us is made up of baryons, primarily protons and neutrons. At the same time, the Universe exhibits a clear predominance of matter over antimatter. Why did this happen if, in the early Universe, both should have been produced in roughly equal quantities? This remains one of the major unanswered questions in modern physics. Understanding how baryon number is connected to the structure of matter may help scientists investigate this problem further.

"The results obtained by the STAR detector at the RHIC accelerator suggest that baryon number does not necessarily have to be associated exclusively with the three quarks that make up a baryon. It may also be 'carried' by the gluon field, through a specific topological configuration of gluons known as a baryon junction. In other words, although gluons themselves have no baryon number, they may play an important role in transporting it," says Professor Hanna Zbroszczyk from the Faculty of Physics at the Warsaw University of Technology, who is also affiliated with Brookhaven National Laboratory. "This discovery concerns the very way matter is structured. A proton or neutron is not simply a stationary 'ball' made up of three quarks. Its interior is the site of a continuous and extraordinarily complex interplay between quarks and gluons. Gluons are responsible for the strong interaction that binds quarks together. Research such as the work conducted by STAR allows us to investigate how the fundamental properties of particles are distributed within this dynamic system."

Baryon number is conserved in the vast majority of known physical processes and is associated with the matter that makes up the observable Universe. The traditional model assumes that each of the three valence quarks forming a baryon carries part of its baryon number. In this picture, baryon number is therefore directly associated with quarks. An alternative hypothesis, formulated as early as the 1970s, assigns this role to a non-local, Y-shaped configuration of the gluon field connecting the quarks. Until now, neither of these concepts had been experimentally confirmed.

Baryon number carrier and transport mechanism, figure: Science / STAR Collaboration

Surprising "transfer" of baryon number

The STAR team investigated the question by comparing the behaviour of baryon number with that of electric charge in different types of collisions. At RHIC, scientists collide atomic nuclei at extremely high energies. These collisions create conditions that allow matter to be studied at the level of its most fundamental constituents. The STAR detector records the particles produced and uses their properties to reconstruct information about processes occurring in the extremely brief period following a collision.

"The starting point was straightforward. If quarks are primarily responsible for transporting baryon number, this process should also involve the transfer of electric charge – after all, quarks carry electric charge. We can therefore use electric charge as a kind of reference point and investigate whether it behaves in a similar way to baryon number," explains Professor Zbroszczyk. "However, STAR measurements revealed something surprising: baryon number may be transported in a way that is much less closely linked to electric charge than would be expected from a simple model based exclusively on quarks. This finding is also consistent with earlier STAR observations from gold nucleus collisions, which indicated an unusual mechanism of baryon number transport."

The result is consistent with a picture in which gluons, the particles responsible for interactions between quarks, play an important role. According to predictions from quantum chromodynamics, gluons can form a special branching structure known as a baryon junction. Such a structure could carry information about baryon number without transporting electric charge in the same way as quarks.

This does not mean that the mystery has been solved. The STAR results provide an important clue, but they do not yet constitute definitive proof that the baryon junction is the mechanism responsible for carrying baryon number. Further measurements are needed to determine whether this mechanism is indeed responsible for the observed effect. It will also be important to establish whether a single theoretical framework can explain all the existing observations simultaneously, rather than just one experimental result.

Future experiments and measurements at facilities including CEBAF (Continuous Electron Beam Accelerator Facility), the LHC (Large Hadron Collider) and the EIC (Electron-Ion Collider), currently under construction, may provide answers. The EIC will be particularly well suited to detailed investigations of atomic nuclear structure and the distribution of quarks and gluons within nuclei. Further experiments will therefore help determine whether the effect observed by STAR is genuinely evidence of the long-sought mechanism or points towards the need for a different theoretical description.

As Professor Zbroszczyk emphasises, the STAR team's findings illustrate an important feature of modern particle physics: even well-established concepts such as baryon number can conceal questions to which we still do not have complete answers. Experiments conducted at large particle accelerators gradually reveal this structure by colliding particles at enormous energies and observing what happens at the most fundamental scale. These findings could help scientists understand how strong interactions between quarks and gluons organise stable matter and contribute to explaining the asymmetry between matter and antimatter in the Universe.

The full research article is available here: Tracking the baryon number with nuclear collisions. The publication's co-authors from the Faculty of Physics at the Warsaw University of Technology are Daniel Kikoła, Jędrzej Kołaś, Diana Pawłowska, Ashutosh Kumar Pandey, Jan Pluta, Priyanka Roy-Chowdhury, Daniel Wielanek and Hanna Zbroszczyk.