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LHCb Experiment Challenges Assumptions of Baryon Production

Recent research carried out by physicists associated with the LHCb Collaboration reveals that the density of the environment affects how quarks form protons and neutrons, suggesting new factors were at play in the early universe. These findings were published in the journal Physical Review Letters.

LHCb Experiment Challenges Assumptions of Baryon Production
Illustration of a proton-proton collision at the Large Hadron Collider. Quarks produced in these collisions can cluster in threes to produce baryons (green) or in twos to make mesons (red). Image Credit: May Napora.

The fundamental particles that make up the universe's visible matter are called quarks. Quarks' most fascinating and perplexing characteristic is that they are never found in isolation. Instead, they are only visible when contained inside composite particles like protons.

Nuclear physicists use massive particle accelerators to generate diverse quark varieties and examine their transformation into detectable particles. Pairs of quarks form mesons, while groups of three quarks form composite particles called baryons (such as protons and neutrons). Contrary to expectations, new measurements from the Large Hadron Collider beauty (LHCb) experiment reveal unexpected variations in the rate at which baryons are produced.

The Impact

The atomic nuclei that constitute all visible matter are made up of baryons (specifically, protons and neutrons), which scientists believe were formed in the early universe.

Baryons inside nuclei are stable particles that do not undergo radioactive decay, whereas all mesons are unstable and quickly decay into lighter particles that cannot form atoms. The existence of stable baryons versus unstable mesons is, therefore, what makes the existence of atoms and the universe as we know it possible. 

The LHCb experiment has demonstrated that the rate at which quarks form baryons versus mesons is heavily influenced by the density of their environment. This discovery helps explain the formation of the first stable particles in the early universe.


The fact that quarks must be confined is the defining feature of the strong interaction, as described by the theory of quantum chromodynamics (QCD). While the total number of heavy bottom quarks produced in particle collisions can be predicted using QCD calculations, the fraction of particles that emerge as baryons instead of mesons cannot be described. Assuming a universal baryon production rate, researchers typically fine-tune models to match data from earlier experiments involving electron-positron collisions.

This new research differs significantly from earlier experiments. It creates an environment with a much higher quark density due to protons and nuclei colliding at the Large Hadron Collider. Nuclear physicists from the LHCb experiment discovered in this study that the quantity of baryons containing b quarks varies with higher particle densities and is dependent on the environment that follows collisions.

This demonstrates that the assumption made by scientists regarding the universality of baryon production is false and that interactions between the produced quarks determine the number of baryons that emerge during their evolution into visible matter. These new findings demonstrate the need for additional theoretical mechanisms in dense collision systems to produce baryons, which may have been crucial during the formation of the first protons in the universe.


The Nuclear Physics program of the Department of Energy's Office of Science funded this study.

Journal Reference:

Aaij, R., et al. (2024) Enhanced Production of Λ0b Baryons in High-Multiplicity pp Collisions at √s=13  TeV. Physical Review Letters.

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