This hosted the MARATHON experiment in Hall A of the Thomas Jefferson National Accelerator Facility (JLab). Its precision in measuring the structure of these nucleons has achieved an unrivaled level of accuracy. This amazing experiment—officially known as the MeAsurement of the F₂ⁿ/F₂ᵖ Ratio and A=3 EMC Effect in Deep Inelastic Scattering off Tritium and Helium-3 Mirror Nuclei—has changed what we know about particle physics. It has produced unprecedented discoveries that elucidate the inelastic structure functions of single protons and neutrons.
The experiment ultimately occurred in 2018 after receiving approval in 2011. It used an 11 GeV beam from JLab’s accelerator to fire at gaseous tritium, helium-3, and deuterium targets. By utilizing high-resolution spectrometers and charge-coupled device (CCD) detectors, researchers were able to gather high quality data. This data is essential for interpreting the inner workings of three-nucleon systems.
Developing a tritium target turned out to be a formidable obstacle. The radioactive nature of the material and that it had not been used in over thirty years compounded the challenge. Our biggest lesson learned according to Dave Meekins, the provision requiring this target to be included was the most experimental part of the experiment.
Details of the Experiment
The MARATHON experiment’s primary goal was to measure the ratio of inelastic structure functions of protons and neutrons, which are fundamental components of atomic nuclei. Researchers targeted the interconversion of tritium and helium-3, also called mirror nuclei. Their negative counts for protons and neutrons provide a refreshing view of the two nucleons’ interactions.
The experiment required extensive preparation, including waiting for JLab’s 12 GeV energy upgrade and undergoing a lengthy safety review process. The complex arrangement included two high-resolution mass magnetic spectrometers, outfitted with enormous superconducting magnets and state-of-the-art radiation detection apparatus.
“Scattered electrons were detected in the two state-of-the-art mass magnetic spectrometers of the Hall A Facility of the Lab made up of powerful, high volume superconducting magnets and modern radiation detection apparatus,” – Makis Petratos
The careful engineering allowed for very accurate readings to be measured from them. Such measurements are essential for establishing the internal substructure of nucleons. As Makis Petratos, the project’s lead researcher, emphasized, “Knowledge of these distributions is critical for the understanding of the internal substructure of the two nucleons in terms of quarks and gluons.”
Significance of Findings
These results from the MARATHON experiment are key for moving our understanding of nucleon-nucleon interactions in three-nucleon systems to the next level. These measurements put all of those assumptions in jeopardy. They expose unpredicted mechanisms by which the inelastic nucleus structure function diverges from the incoherent sum of its nucleonic constituents.
Petratos noted that “this effect demonstrates that the inelastic structure function of a nucleus is not (as was naively originally expected) equal to the sum of the inelastic structure functions of its [nucleon] constituents.” This exciting discovery lays an important foundation for further studies of the intricate world of nuclear interactions.
“These measurements are considered essential for our understanding of the internal structure and dynamics of the three-nucleon systems of nature, and of the nature of nucleon-nucleon interactions inside them,” – Makis Petratos
The research team emphasized the need to understand the nuclear EMC effect. As Petratos explained, “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.”
Future Prospects
Focusing on the future, JLab is poised to deepen its luminosity-first exploration into subatomic physics with even more experimental pencil-like probes of nature. The MARATHON experiment is the second release from this exciting research collaborative. It all began through the leadership of committed psychologists such as Mina Katramatou and Javier Gomez.
Petratos expressed optimism regarding future developments, stating, “It is expected that Jefferson Lab will conduct additional experimental investigations in the future and promote the development of novel theoretical ideas that will advance fundamental subatomic physics.”