Following a bold suggestion from a postdoc researcher, an international team has discovered a robust technique for probing the internal structure of the proton by using neutrino scattering. Tejin Cai at the University of Rochester and colleagues working on Fermilab’s MINERvA experiment have showed how information about the proton can be extracted from neutrinos that have been scattered by the detector’s plastic target.
As early as the 1950s, physicists were using high-energy electron beams to determine the size of the proton. By measuring how these electrons scatter from targets, researchers have since managed to probe the interior structure of the proton and measure the charge distributions of their constituent quarks in detail.
In principle, similar measurements should also be possible using a beam of neutrinos, such as the beam generated at Fermilab. Despite being chargeless and almost massless, a tiny fraction of neutrinos in a beam will interact with protons, and scatter at characteristic angles. If this scattering can be measured, it would not only complement electron scattering experiments in probing proton structures; it may also provide important new insights into how neutrinos and protons interact.
Far too diffuse
So far, researchers have only considered the possibility of firing neutrino beams into gaseous hydrogen targets. However, the protons in these targets are far too diffuse to scatter neutrinos in high enough numbers to gain any conclusive results using existing experimental techniques.
In the new study, Cai’s team found a solution to this problem almost by accident. The physicists are currently using the MINERvA experiment at Fermilab to study neutrinos by firing a high-energy beam of the particles into plastic scintillator targets. These are dense, solid polymers that contain lots of hydrogen and carbon.
Subtracting carbon
Cai realized that the hydrogen atoms in this solid target are far more densely packed than they are in hydrogen gas. If the neutrinos scattered by carbon atoms in MINERvA’s detector could be subtracted from measurements, he suggested that the team would be left with the signal scattered by hydrogen nuclei.
Proton’s puzzling electromagnetic structure is observed in new experiment
Since far more neutrinos are scattered by carbon than hydrogen, many of Cai’s colleagues were not convinced by the proposal. To test his idea, the researchers subtracted simulated neutrino–carbon interactions from nine years of measurements of neutrino scattering at MINERvA. Just as Cai predicted, they were left with scattering data that closely resembled the results of electron-scattering experiments – clearly indicating their technique had worked as intended.
Based on this initial success, the team now hopes the approach could lead to deeper insights into the proton’s interior structure. It could bring researchers a step closer to answering many remaining questions surrounding the nature of neutrinos. This includes neutrino’s elusive interaction with other types of matter and their spontaneous transformation through neutrino oscillation.
The research is described in Nature.
