Development of proton beam irradiation system for the NA65/DsTau experiment

Tau neutrino is the least studied lepton of the StandardModel (SM). The NA65/DsTau experiment targets to investigate Ds, the parent particle of the ντ, using the nuclearemulsion-based detector and to decrease the systematic uncertaintyof ντ flux prediction from over 50 % to 10 % for futurebeam dump experiments. In the experiment, the emulsion detectorsare exposed to the CERN SPS 400 GeV proton beam. To provideoptimal conditions for the reconstruction of interactions, theprotons are required to be uniformly distributed over the detector'ssurface with an average density of 10$^{5}$ cm$^{-2}$ and thefluctuation of less than 10%. To address this issue, we developeda new proton irradiation system called the target mover. The newtarget mover provided irradiation with a proton density of1.01 × 10$^{5}$ cm$^{-2}$ and the density fluctuation of1.9 ± 0.3% in the DsTau 2021 run.