Nucleons—protons and neutrons—are the very building blocks of matter, the particles that make up atomic nuclei. These tiny, subatomic particles lie at the core of every atom, shaping the structure of all matter in the universe. Yet, despite decades of study, their inner workings remain shrouded in mystery. What are they truly made of? How do their quarks and gluons behave within the confines of the strong nuclear force that binds them together? Understanding nucleons goes beyond the basics of chemistry or physics; it is about delving into the fundamental forces and particles that govern the very fabric of existence.
Recent experiments, like the groundbreaking MARATHON project at the Jefferson Lab, are shedding new light on these enigmatic particles, pushing the boundaries of our knowledge and offering exciting new possibilities for the future of particle physics. In this article, we will explore the nature of nucleons, the scientific quest to understand them, and the implications of the latest discoveries in the field.
The Inner World of Nucleons: Quarks and Gluons
At the most fundamental level, nucleons are not indivisible particles, as once believed, but rather composite entities made up of even smaller constituents. These particles—protons and neutrons—are each composed of three quarks, held together by the strong nuclear force. Quarks, which are elementary particles, come in six “flavors” (up, down, charm, strange, top, and bottom), but only two flavors are involved in the composition of nucleons: the “up” quarks and “down” quarks.
A proton consists of two up quarks and one down quark, while a neutron is made up of one up quark and two down quarks. These quarks are held together by the exchange of gluons—force carriers of the strong nuclear force. Gluons are responsible for the interaction between quarks and play a pivotal role in maintaining the integrity of nucleons.
But nucleons are not just simple collections of quarks. Within their interior, there are also quark-antiquark pairs that constantly emerge and annihilate. This complex, dynamic environment makes the study of nucleons incredibly challenging, as understanding how quarks and gluons interact within such a system is no simple task. Even more elusive is the distribution of momentum and spin across these individual building blocks, which scientists have yet to fully uncover.
The MARATHON Experiment: A New Approach to Nucleon Structure
To better understand nucleons, researchers have designed new experiments that focus on measuring the behavior and interactions of quarks inside protons and neutrons. One such experiment is the MARATHON (MeAsurement of the F₂ⁿ/F₂ᵖ, d/u Ratio and A=3 EMC Effect in Deep Inelastic Scattering off Tritium and Helium-3 Mirror Nuclei) experiment, carried out by the Jefferson Lab Hall A Tritium Collaboration. This ambitious project aims to provide the most detailed measurements to date of the internal structure of nucleons, specifically focusing on the momentum distribution of quarks.
The MARATHON experiment has already produced significant results. It published the most precise measurement yet of the ratio of the neutron and proton structure functions (F₂ⁿ/F₂ᵖ), a vital quantity that reveals how quarks share momentum within nucleons. This ratio is an important piece of the puzzle, as it helps scientists better understand the quark dynamics in protons and neutrons. The results, published in Physical Review Letters, represent a significant milestone in our understanding of quantum chromodynamics (QCD), the theory that describes the strong force and the interactions between quarks and gluons.
According to Makis Petratos, the spokesperson for the MARATHON experiment, this groundbreaking paper builds on a project that has spanned nearly two decades. “The experiment had to wait for the 12 GeV energy upgrade of the Lab and a lengthy safety review process, as it required the use of a radioactive tritium gas target. It was fully approved in 2011 and took data in 2018—almost 20 years after its inception,” Petratos explained.
Understanding the EMC Effect
One of the key objectives of the MARATHON experiment is to study the EMC effect, a phenomenon first discovered in 1983 by the European Muon Collaboration at CERN. This effect reveals that the inelastic structure function of a nucleus is not simply the sum of the structure functions of its individual nucleons. In other words, when nucleons are bound together inside a nucleus, their properties are “modified” in a way that is not fully understood.
The MARATHON team focused on measuring the EMC effect in tritium and helium-3, which are mirror nuclei. In a mirror nucleus, the number of protons in one nucleus is equal to the number of neutrons in the other. By studying these mirror nuclei, researchers hoped to uncover clues about the nature of the EMC effect and why nucleons behave differently when bound in a nucleus.
“The reason for this apparent ‘modification’ of a free nucleon when it is embedded in a nucleus is not yet resolved. Theorists had argued that the measurement of this effect for tritium and helium-3 mirror nuclei would be essential for its explanation,” said Petratos. The results from the MARATHON experiment offer valuable new insights into this mysterious phenomenon, which continues to challenge our understanding of nuclear physics.
A Massive Technical Challenge: The Tritium Target
One of the most significant challenges of the MARATHON experiment was the development of a tritium target, as tritium is a radioactive gas. The team, led by Dave Meekins from the JLab Target Group, had to design a safe and effective system for handling the tritium target material. This was the first time in over 30 years that such a target had been used in an experiment of this type.
“Developing and implementing the tritium target was by far the biggest challenge for this experiment,” said Meekins. “Tritium being a radioactive gas, it was critical to ensure a safe and effective design.” The successful implementation of this target was crucial to the success of the MARATHON experiment and helped the team acquire high-quality data on the inelastic structure functions of tritium, helium-3, and deuterium—three key nuclei that are essential to understanding nucleon dynamics.
New Data, New Possibilities
The data collected during the MARATHON experiment has already provided a wealth of new information about the structure of nucleons. By bombarding the tritium, helium-3, and deuterium targets with an 11 GeV beam from the Jefferson Lab accelerator, researchers were able to measure inelastic electron scattering from these nuclei. The scattered electrons were detected using two advanced mass magnetic spectrometers, which are equipped with powerful superconducting magnets and modern radiation detection apparatus.
This high-quality data is essential for advancing our understanding of nucleons and their behavior. In particular, the measurements on tritium and helium-3—the so-called mirror nuclei—will provide valuable insights into the interactions between nucleons inside atomic nuclei. These interactions, and the dynamics of quarks and gluons within them, remain a fundamental challenge in nuclear and particle physics.
The first-ever measurement of the EMC effect in tritium, conducted as part of the MARATHON experiment, is a landmark achievement. This data will help to refine existing models of nucleon structure and may lead to new theoretical ideas that could reshape our understanding of nuclear physics.
The Future of Nucleon Research
The findings of the MARATHON experiment are a significant step forward, but they are far from the final word in the study of nucleons. As Petratos pointed out, “The understanding of the nuclear EMC effect and the structure of the nuclear ‘few-body’ (few-nucleon) systems remains one of the most important issues of modern, high-energy nuclear physics today.”
Jefferson Lab, where the MARATHON experiment was conducted, plans to continue investigating these mysteries. Future experiments will build on the insights gained from the MARATHON project, further testing and refining our models of quark-gluon interactions within nucleons and nuclei. The results will have far-reaching implications for the study of nuclear physics, including the search for new forms of matter, the understanding of neutron stars, and the development of novel technologies based on our understanding of subatomic particles.
The Bottom Line: Unlocking the Secrets of Matter
At the heart of the MARATHON experiment lies a deeper question: How do the most fundamental particles of nature behave when bound together? Nucleons are the essential building blocks of the universe, and understanding their internal structure is crucial for our understanding of matter itself.
Thanks to the efforts of researchers involved in the MARATHON experiment, we are one step closer to answering these questions. Their work not only expands our knowledge of particle physics but also opens up new possibilities for testing modern models of quantum chromodynamics (QCD) and refining our understanding of the forces that govern the universe.
The journey to unlock the secrets of nucleons is far from over, but with each new discovery, we are inching closer to understanding the intricate dance of quarks and gluons that form the very foundation of matter.
More information: D. Abrams et al, EMC Effect of Tritium and Helium-3 from the JLab MARATHON Experiment, Physical Review Letters (2025). DOI: 10.1103/31xz-s84d. On arXiv: DOI: 10.48550/arxiv.2410.12099
