Despite being one of the most abundant particles in our universe, neutrinos are notoriously difficult to detect and study. That is because they have no electrical charge and mass. That’s why they are called “ghost particles” because they rarely interact with atoms. But they can be used to answer fundamental questions about the universe.

For the first time, scientists from the University of Rochester – scientists from the international collaboration MINERvA – have investigated the structure of protons using these ghost particles. Specifically, they used a beam of neutrinos in the Fermilab to investigate the structure of protons.

Tejin Cai, the first author of the article, said: “At first we weren’t sure if it would work, but eventually we discovered that we could use neutrinos to measure the shape and size of the protons that make up the nuclei of atoms. It’s like using a ghost ruler to take a measurement.”

The first measurement of the size of a proton was made in the 1950s. At that time, scientists used an electron-beam accelerator at Stanford University’s linear accelerator facility. However, scientists’ new method uses neutrino rays instead of accelerated electron beams. This new method gives scientists more information about the interaction of neutrinos and protons.

Kevin S. McFarland, Dr. Steven Chu Professor of Physics at Rochester, said: “Using our new measurement to improve our understanding of these nuclear effects will allow us to better perform future measurements of neutrino properties.”

Usually scientists only use hydrogen atoms in experiments to measure protons. But a pure hydrogen target wouldn’t be dense enough for enough neutrinos to interact with the atoms. So, scientists in this study performed a chemical trick by binding the hydrogen into hydrocarbon molecules that enable it to detect subatomic particles.

The MINERvA group performed their experiments using a powerful, high-energy particle accelerator at Fermilab. The accelerator produces the strongest source of high-energy neutrinos in the world.

Scientists hit their detector made of hydrogen and carbon atoms with the beam of neutrinos and recorded data for nearly nine years of operation. The background noise from the carbon atoms must be removed to focus solely on the information from the hydrogen atoms.

Kai said, “The hydrogen and carbon are chemically bonded together, so the detector sees interactions on both at the same time. I realized that a technique I used to study interactions with carbon could also be used to see hydrogen all by itself if you look at the carbon interactions. Much of our work has been subtracting the very large background of neutrinos scattering on the protons in the carbon nucleus.”

Deborah Harris, a professor at the University of York and a fellow MINERvA spokesperson said: “When we proposed MINERvA, we never imagined that we could get measurements from the hydrogen in the detector. Making this work required great detector performance, creative analysis from scientists, and years of using “the accelerator at Fermilab.”

Kai said, “The analysis result and the new techniques that have been developed underline the importance of creativity and collaboration in understanding data. While many of the analysis components already existed, putting them together in the right way made a difference, and this cannot be done without experts from different technical backgrounds sharing their knowledge to make the experiment a success.”

Magazine reference:

  1. Cai, T., Moore, ML, Olivier, A. et al. Measurement of the axial vector shape factor of antineutrino-proton scattering. Nature 614, 48-53 (2023). DOI: 10.1038/s41586-022-05478-3