Study of tau-neutrino production by measuring Ds -> tau events in 400 GeV proton interactions: Test of lepton universality in neutrino charged-current interactions

The muon-neutrino charged-current (CC) cross section has been measured by many experiments. However, there has been only one measurement of the tau-neutrino CC cross section by the DONuT experiment with a systematic uncertainty larger than 50%, mainly due to uncertainties in the Ds differential production cross section in high energy proton interaction. The evaluation of the tau-neutrino cross section would provide a test of lepton universality in neutrino CC interactions, which has never been well tested for tau-neutrinos. In this new program, we propose to study tau-neutrino production (Ds production fraction x decay branching ratio of Ds -> tau) and the energy distribution by analyzing Ds -> tau events in 400 GeV proton interactions. By employing the state-of-the-art emulsion particle detector technologies, we will analyze 10^8 proton interactions and detect the double kink topology of Ds -> tau -> X decays. Accomplishing this new measurement, we will re-evaluate the tau-neutrino cross section with the data from DONuT. The expected outcome will significantly reduce the total systematic uncertainty to the 10% level, which is well below the statistical uncertainty of DONuT. In 2016, we will perform a prototype test experiment with 20 m^2 emulsion surface, corresponding to a scale of about 1:20 of the final setup. This test will provide a proof of principle and the results will be used to optimize the module structure. We plan to carry out the first run of the real experiment in 2018.