STUDY OF PROTON-NUCLEUS INTERACTIONS IN THE DsTau(NA65) EXPERIMENT AT THE CERN-SPS  ABSTRACT: The DsTau (NA65) experiment at CERN was proposed to measure an inclusive differential cross-section of Ds production, and its decay branching ratios in p-A interactions. The DsTau detector is based on the nuclear emulsion technique providing an excellent spatial resolution for detecting short-lived particles like charmed hadrons. The first results of the analysis of the pilot-run data are presented. The accuracy of the proton interaction vertex reconstruction is reported. A high precision in vertex reconstruction allows one to measure the proton interaction length and charged particle multiplicities accurately in a high-track density environment. The measured data have been compared with several Monte Carlo event generators in terms of multiplicity and angular distribution of charged particles. The results presented in this study can be used to validate event generators of p-A interactions. Elena FIRU* on behalf of the DsTau (NA65) Collaboration *Institute of Space Science – subsidiary of INFLPR PHYSICS GOALS STUDY OF TAU NEUTRINOS Tau neutrino is one of the least studied particles • only a few measurements: i. Direct 𝜈𝜏beam: DONUT (DIS) - first direct evidence of tau- neutrino interaction ii. Oscillated 𝜈𝜏: OPERA (DIS), Super-K (QE), IceCube (DIS) • cross section error >50% (DIS) due to systematic uncertainty in 𝜈𝜏production A new precise measurement of the ντ cross section • test lepton universality • new physics effects in ντ CC interactions Future ντ measurements • SHiP: high statistics measurement at the SPS • reduce statistical uncertainty from 33% in DONuT • indirectly FASER DATA PROCESSING full surface scanning is done to accumulate all charged tracks segments by Hyper Track Selector (HTS) • HTS scans emulsion tracks with the speed of 5000 cm2⁄h Preselect events in the precession measurement to search for small angle decay of Ds→𝜏 ALIGMENT AND TRACK RECONSTRUCTION reconstructed visual of a Ds decay candidate alignment algorithm - very precise - sub micron track reconstruction - positions and angular correspondences average efficiency is higher than 95%. proton beam tracks were checked in detail processing in sub-volumes 1.5 cm x 1.5 cm x 30 plates not interacted protons Computing infrastructure protons interacted in the last tungsten yellow line segments show the trajectories of charged particles in the emulsion films June 16-22, 2024 Milan, Italy EXPOSURE AT SPS, CERN DETECTOR ASSEMBLY AT CERN NUCLEAR EMULSIONS FILMS PRODUCTION detector assembly take place at CERN Dark room DEVELOPMENT AT CERN take place at dark room of the CERN Emulsion Facility PRIMARY VERTEX RECONSTRUCTED for estimation of vertex reconstruction efficiency, the true vertex position is compared with the reconstructed Monte Carlo DATA ANALYSIS a sample from 2018 pilot run data is used for proton interaction characterization measurement of interaction length in tungsten in presented Summary: ●DsTau(NA65) experiment aims to study tau neutrino production by Ds decays ●2018 pilot run data sample is analyzed to study proton interactions in tungsten ●proton interaction length in tungsten is measured for the first time, results will be submitted for publication ●data analysis of physics runs data is going on References: 1. Aoki S, Ariga A., Ariga T., all, Development of proton beam irradiation system for the NA65/DsTau experiment, Journal of Instrumentation, Volume 18, October 2023 2. Ariga, A. Status and plans of the DsTau Experiment, SPSC Open session June 2021 3. A. M. Guler, NA65(DsTau): study of tau neutrino production in p-A interactions, PoS TAUP2023 (2024) 157 4. Ariga, A. DsTau status report 2023 (https://cds.cern.ch/record/2868526/files/SPSC-SR-334.pdf) 5. E. Yüksel, NA65/DsTau: Study of Tau Neutrino Production in p-A interactions, 31st International Workshop on Deep Inelastic Scattering, April 2024, Grenoble, France Cross section measureme