Study of tau-neutrino production at CERN SPS

Lepton universality in tau-neutrino (ν_τ) scattering has been poorly tested. More precise measurement of the ν_τ cross section would enable a search for new physics effects in ν_τ-nucleon CC interactions. Such measurement also has practical importance for next-generation neutrino oscillation experiments and astrophysical ν_τ observations. The DsTau project has been proposed at CERN SPS to study tau-neutrino production with the aim of providing important data for future ν_τ measurements. For ν_τ cross section measurements, the practical method of producing an artificial ν_τ beam employs the sequential decay of D_s mesons produced in high-energy proton interactions. However, there has been no experimental measurement of the D_s differential production cross section in fixed-target experiments using proton beams, which leads to a large systematic uncertainty on the ν_τ flux estimation. The DsTau project aims to reduce this uncertainty in the current cross section measurement from about 50% to 10% by measuring the D_s differential production cross section. For this purpose, emulsion detectors with a 50-nm spatial resolution will allow the detection of D_s → τ → X double kinks in a few mm range. Results from the beam tests conducted in 2016-2017 are presented together with a prospect for the pilot run in 2018 and the physics run in 2021.