Nuclear physics meets neutrinos: NA61/SHINE measurements for Fermilab neutrino beams

The NA61/SHINE experiment at CERN was originally designed to study a phenomenon known as the “onset of deconfinement,” or the transition between ordinary matter and quark-gluon plasma in collisions of heavy ions. But it turns out that the experiment is also able to make essential measurements for a very different field: accelerator-based neutrino physics.

Neutrino beams, such as those currently operating at Fermilab and at 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 pipes, 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 antimatter.

This plot shows the 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 S.P. Denisov and collaborators at the Institute for High-Energy Physics in Russia.

Neutrino oscillations are studied by generating neutrino beams consisting mainly of one flavor of neutrino 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 as a function of energy, which is referred to as 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.

Fermilab’s neutrino beamlines aren’t the right places to make these hadron measurements. That’s because of the extremely high intensities (more than 1013 protons per second!) necessary to produce neutrino beams. We need a separate particle beamline to produce the hadrons you’d see in a Fermilab neutrino experiment, but less intense.

That’s where the CERN-based NA61/SHINE experiment comes in. It uses particles from CERN’s Super Proton Synchrotron to make very precise measurements of the interactions that happen in neutrino beams. Over the past several years, 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). The above plot shows recent measurements of pion and kaon inelastic interaction cross sections in carbon and aluminum thin targets.

A NuMI beam target is installed in the NA61 experiment. Photo: Eric Zimmerman, University of Colorado

NA61/SHINE also uses replicas of the actual Fermilab neutrino beam targets to directly measure hadrons, contributing to our understanding of neutrino fluxes in a way no other experiment can. In 2018, the collaboration took data on a replica of the NuMI beam target that will be used by experiments in the NuMI beam, including NOvA and MINERvA. The above photo shows the NuMI target installed in NA61/SHINE. The resulting data 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 larger than currently operating targets) to enable the high-precision neutrino physics to be produced by these experiments.

Laura Fields is a Fermilab scientist and co-spokesperson for the MINERvA experiment.