Tau neutrinos were predicted after the discovery of the tau particle in 1975 and were first observed in 2000. However, they have not been studied to the same degree as their electron and muon counterparts, the electron neutrinos and the muon neutrinos.
A precise measurement of the tau-neutrino production rate would allow researchers to test a fundamental principle of the Standard Model of particle physics, called lepton flavour universality, in neutrino interactions. According to this principle, different kinds of leptons, of which electrons and neutrinos are examples, should have the same properties once their difference in mass is accounted for. A precise measurement could also help to interpret data from current and future experiments measuring the change, or “oscillation”, of neutrinos of one type into another type as they travel.
To measure the tau-neutrino production rate, NA65 studies the main source of tau neutrinos – the decay of particles called Ds mesons into tau leptons and tau neutrinos. Using a high-resolution emulsion detector, it should detect some 1000 such decays in about 200 million proton interactions in a tungsten target. These decays will reduce the relative uncertainty of tau-neutrino production from more than 50% to about 10%.
NA65 was approved by the CERN Research Board in 2019 and started taking data on 22 September 2021.