Neutrinos are difficult to study because their interaction with matter is extremely rare. Nonetheless, neutrino experiments do what they can to improve the odds: They use a neutrino beam with as high an energy as possible and build detectors filled with as many protons and neutrons as possible. One thing these detectors have in common is that they see nothing coming in, only something going out when a neutrino does interact in the detector.
Neutrino scientists have to work their way back from those end products to study the neutrino and its interaction with matter. One of these interactions, known as a quasielastic interaction, takes place when the neutrino completely scatters off a neutron in an atom’s nucleus, producing a muon and a proton.
Since the 1970s, several experiments have measured the probability of this interaction using different types of detectors. Old experiments used deuterium (which contains exactly one proton and one neutron) as a target. Modern experiments use more complex nuclei such as carbon (six protons and six neutrons), which makes the study of quasielastic interactions very challenging. For example, the end products can interact with other protons and neutrons in the same nucleus after the main interaction, and thus what we measure in the detector is different from what was initially produced. This quasielastic interaction is very important for neutrino oscillation experiments, and current experiments use this interaction as the signal for the analysis.
Recent measurements of this interaction have shown discrepancies between data and theoretical models. Last year, the MINERvA experiment measured everything it could about the outgoing muon. The MINERvA detector is also capable of measuring the outgoing proton. MINERvA recently made a new measurement using information from the proton and including all the events with a muon plus a proton, no matter which way the muon travels through the detector. It is shown exiting through the side of the detector in the above figure.
The new measurement (see below) gives the probability of producing a proton and muon from a neutrino as a function of the momentum given the nucleus (Q2), where we calculate that using only information from the proton. This is the first time a measurement of the protons has ever been made. To see Tammy Walton present these results, please see the recording of the May 9 wine and cheese seminar.