Study of nu-tau properties with the SHiP experiment

The SHiP experiment (Search for Hidden Particles) is a beam dump experiment proposed at the CERN SPS with the submission of a Technical Proposal in April 2015. SHiP aims at the observation of long lived particles very weakly coupled with ordinary matter. These particles are mostly produced in the decay of charmed hadrons whose production is therefore enhanced through the definition of the characteristics of both the beam and the proton target. This makes the SHiP experiment a Standard Model neutrino factory too, in particular of tau neutrinos produced by the Ds decay chain. My studies have mainly focused on the design of the neutrino detector and on the evaluation of its performances. The Neutrino Detector is placed in a magnetic field and it exploits the Emulsion Cloud Chamber Technology with the micrometric position resolution needed to disentangle the tau lepton decay vertex from the neutrino interaction vertex. This peculiarity, together with the high electron identification efficiency, makes this detector also suitable to search for sub-GeV Dark Matter (produced in the decay of the dark photon) through its scattering with the electrons in the emulsion target. The main unit of the Neutrino Detector is the brick (lead plates interleaved with emulsion films) followed downstream by a Compact Emulsion Spectrometer (CES, 3 emulsion films interleaved with light material) needed to measure the charge and momentum of hadrons produced in neutrino interactions and short lived particles decays. A Muon Magnetic Spectrometer is placed immediately downstream to measure the charge and the momentum of muons produced in charged current muon neutrino interactions or in tau to muon decays. In this thesis the signal and background yield for all the different neutrino flavours are presented: more than twenty thousand nutau and nutau-bar charged current interactions are expected in five years of data taking. This unprecedented statistics of tau neutrinos will allow to measure the structure functions F4 and F5 entering the neutrino-nucleon cross section. The SHiP performances in the measurement of the tau neutrino anomalous magnetic moment are also reported with the estimation of the background yield for this searches. A preliminary estimate of the background events expected for LDM searches is also shown. Detailed studies will be performed with more general assumptions on the dark photon and the dark matter masses. Thanks to the large flux of electron and muon neutrinos interacting in the neutrino target, the measurement of the strange quark content of the nucleon has also been studied. The second to last chapter of the thesis is devoted to the description of the optimisation studies which are on going in view of the production of a Comprehensive Design Report to hand in to the CERN SPS Committee by the end of 2018. In this optimised version of the SHiP detector, the Neutrino Detector is roughly 20 m closer to the proton target, with a resulting increment in the incoming neutrino flux. However, being closer to the proton target has also generated the need for a complete redesign of the detector layout to fit the muon free region. A study of muon background rates on the Neutrino Detector and on the downstream Muon Magnetic Spectrometer is also reported. The last chapter describes the Test Beam activities conducted at CERN to study the performances of both the Compact Emulsion Spectrometer and of the gaseous electronic detectors (GEM) which complement the Neutrino Detector. The data analysis was carried out in the Napoli Emulsion Laboratory. The test beam for the CES has led us to discard the option of using the Rohacell as a light material interleaved to the emulsion films. The test beam with the GEM-emulsion coupled detector has shown a rapid degradation of the GEM performances in terms of position resolution when dealing with inclined tracks also in absence of magnetic field. The degradation is enhanced when the polarisation of the magnetic field contributes to the avalanche displacement. In case of a compensating magnetic field, the position resolution shows the same behaviour as in absence of field, except for a phase-shift of 15 degrees corresponding to the Lorentz angle of the generated electron avalanche.