Because neutrinos are effectively invisible particles, it is not possible to count them directly. In the future LBNF beam, complex nuclear physics processes will govern the particle interactions with the target and horns, and therefore, the number and spectrum of neutrinos emerging at the end of the beamline. Even the best available measurements of these types of interactions offer no better than 8% uncertainty on the number of neutrinos in any bin of energy.
A flux spectrometer would do the next best thing to counting neutrinos: measure the secondary particles that can decay to neutrinos — pions, kaons and muons — after they have been focused by the LBNF horns. The concept under development includes a magnetized tracker to measure the momentum of the secondary particles, followed by a suite of time-of-flight (TOF) counters, Ring Imaging Cherenkov detectors (RICH) and calorimeters, to collectively provide particle identification and background discrimination.
At the recent DUNE collaboration meeting at CERN, the collaboration agreed that the flux spectrometer has physics potential, and that the design and simulation work should continue. The collaboration also addressed the question of where to do this measurement. While the actual LBNF target chase would be ideal from a scientific perspective, doing a precise physics measurement in a small room that receives more than 10E13 high energy protons per second is challenging, to say the least! The collaboration is instead considering installing the spectrometer in a replica setup of the LBNF beam. This would require spare LBNF target and horns in a separate proton beamline at Fermilab, such as one of the beams in the Fermilab Test Beam Facility.
The figure shows a possible configuration for the flux spectrometer. The design would likely build on equipment already available at Fermilab, for example, large-aperture magnets and silicon tracking systems.
Fermilab scientists Paul Lebrun and Laura Fields are currently leading a small team of DUNE collaborators to develop the design of the spectrometer and demonstrate its physics potential. New collaborators are very welcome to head into the future, if not back to the future, by collaborating on the flux spectrometer with Paul, Laura and their team.