Experiment DsTau (NA65) - study of tau neutrino production at CERN SPS

Abstract

The goal of the DsTau (NA65) experiment at CERN SPS is the measurement of tau neutrino production. The dominant source of tau neutrinos is decay of $D_s$ mesons, produced in proton-nucleon interactions, to tau neutrino and tau lepton with further decay of tau lepton. Thus, the experiment aims to measure differential cross section of this reaction. This measurement will allow to re-evaluate DONuT results reducing the systematic error in tau neutrino interaction cross section caused by the tau neutrino flux uncertainty in beam dump experiments from 50\% to 10\%. Such accuracy of cross section measurement will allow testing the Lepton Flavour Universality for neutrinos. The experiment exploits nuclear emulsion detectors with $\mu$m space resolution allowing to recognize the peculiar $D_s$ to $\tau$ decay topology in a few mm range. In addition, about $10^5$ charmed particle decays are expected in the experiment, which makes possible to study charm physics, in particular to search for intrinsic charm component in proton. About 10\% of whole data were collected during 2018 pilot run and analysis of this sample is ongoing. Given the relevance of the study and encouraging results of the data analysis, CERN had approved the DsTau project as a new experiment NA65 in 2019. The main data sample will be collected in 2021-22 physics runs. In this paper, the status and prospects of NA65 as well as the results of the pilot run are presented.