Neutrino beams, such as those currently operating at Fermilab in the USA and J-PARC in Japan, are typically produced by colliding high-energy proton beams with long, thin solid targets. These collisions result in a spray of particles including short-lived hadrons such as pions or kaons. The hadrons are focused using magnetic focusing horns, which direct the hadrons into long tunnels, where they decay to neutrinos. Thick volumes of rock and shielding stop all particles except neutrinos, creating a beam of neutrinos.
Neutrinos come in three flavors known as electron, muon and tau neutrinos. After a neutrino of one flavor is created, it can “oscillate” into a different flavor, with the probability of oscillation depending on the neutrino’s energy and distance traveled. These neutrino oscillations were the first discovery of physics beyond the Standard Model and were the subject of the Nobel Prize in 2015.
Modern neutrino experiments such as NOvA and T2K are studying neutrino oscillations in fine detail in order to understand whether there may be more unknown physics at play, and whether a phenomenon known as CP violation occurs in neutrino oscillations. CP violation would allow neutrinos and antineutrinos to oscillate differently, and could be a critical part of the answer to a big question not explained by the Standard Model: why our universe appears to be made out of mostly matter rather than equal parts matter and anti-matter.
Fig 1. NA61 Measurements of the pion and kaon inelastic scattering cross sections on Aluminum and Carbon, as a function of incident particle momentum, compared with previous measurements by Denisov et al.
Neutrino oscillations are studied by generating neutrino beams consisting mainly of one flavor of neutrino physics and then studying that beam after it has traveled a long distance. Because neutrino oscillations vary with neutrino energy, it is very important to have a precise prediction of the number of neutrinos in the beam before oscillation and their energy spectrum (often called the “neutrino flux”). Estimating the neutrino flux is difficult because neutrinos are neutral particles that interact very rarely and can’t be measured or controlled like most particle beams. To measure neutrino flux, experiments instead have to measure the number of hadrons that were produced and focused before decaying to neutrinos. These measurements can’t be made in neutrino beams themselves because of the extremely high intensities (more than 10^13 protons per second!) necessary to produce neutrino beams.
NA61/SHINE, thanks to its large-acceptance Time Projection Chambers, is able to make very precise measurements of the interactions that happen in neutrino beams. In the past, NA61/SHINE has executed a program of measurements aimed at improving neutrino flux predictions in Fermilab’s neutrino beams (including the currently operating NuMI and planned LBNF beams). Recent measurements of pion and kaon inelastic interaction cross sections in carbon and aluminum thin targets are shown in Figure 1.
Fig. 2: A NuMI beam target installed in the NA61 experiment. Photo: Eric Zimmerman, University of Colorado.
Measuring individual interactions using thin targets contributes significantly to our understanding of neutrino fluxes, but even better is directly measuring hadrons produced using replicas of the actual neutrino beam targets. NA61/SHINE is also able to do that. In 2018, the collaboration took data on a replica of the NuMI beam target (shown in Figure 2) that will be used by experiments in the NuMI beam, including NOvA and MINERvA. The resulting data set is currently being calibrated and analyzed.
Looking into the future, the NA61/SHINE neutrino program will focus on measurements needed by the next generation of neutrino oscillation experiments, including DUNE and T2HK. For example, the collaboration is considering upgraded tracking systems that will enhance the hadron production measurements using replica LBNF/DUNE targets (which will be much longer than currently-operating targets) to enable these experiments to produce high-precision neutrino physics.