Neutrinos are notoriously difficult particles to study: For every 50 billion neutrinos that pass through the MINERvA detector at Fermilab, only about one will interact leaving a trace in our detector, producing particles that we can observe directly.
In spite of this, we are starting to use neutrinos to learn more about protons and neutrons and how they behave when they’re together inside an atomic nucleus. We already understand a lot about the nucleus: We know that it’s made of protons and neutrons, and we know the number of protons and the number of neutrons in the nucleus for every chemical element. But there is much we still don’t fully understand, especially about what those protons and neutrons are doing inside the nucleus.
We can study the protons’ and neutrons’ behavior in the nucleus the way we might study how people act at a party. Do the party-goers mingle according to the general spirit of the party, or do they break off into pairs? We could determine the party’s nature by sending in very shy folks and observing how quickly they leave and whether they leave through the same door they entered.
In a nucleus, does each proton and neutron react to just the average effect of the others, or do they occasionally pair up? One way to answer this question is to fire neutrinos at nuclei and measure the particles produced when neutrinos do interact with the nuclei of atoms in our detector. By studying those particles, we can try to infer the behavior of the protons and neutrons.
The MINERvA collaboration has done this by studying one of the simplest types of neutrino interactions: quasi-elastic scattering, in which the neutrino changes into a muon (a heavier cousin of the electron), in the process knocking some protons and neutrons out of the nucleus. We look at quasi-elastic scattering of both neutrinos and antineutrinos and study them in two ways: First, we look at the directions and energies of the muons produced in these interactions and find that the data agree better with predictions in which the protons and neutrons spend some of their time in the nucleus joined together in pairs. In this case, we’d expect that both of the particles are knocked out of the nucleus. Second, we look for these knocked-out particles directly and find an amount of energy consistent with two particles being emitted from the nucleus.
These results support the more complex model of the nucleus, with pairs of protons and neutrons continually joining up and splitting apart, and open the door for future studies of nuclear behavior using neutrinos.