A New Search for $ \nu _{\mu} - \nu _{\tau} $ Oscillations

% WA95\\ \\ The question whether neutrino flavours mix at some level - and the related question whether neutrinos have non-zero mass - is one of the remaining great challenges of experimental physics. Neutrinos from supernovae, from the sun, from the earth's atmosphere, from nuclear reactors and from radioactive decays are currently under study; in this frame, experiments using accelerators play a privileged role because the well known neutrino source properties allow high precision measurements and background control.\\ \\The main goal of the CHORUS experiment is to search for neutrino oscillations in the $\nu_{\mu} \rightarrow \nu_{\tau}$ sector, by detecting the occurrence of the reaction $\nu_{\tau} N \rightarrow \tau X$ within a large sample of $\nu_{\mu}$ interactions. The $\tau^$ is mainly identified by its characteristic decay topology, and separated from topological backgrounds by charge and kinematical analysis. A mixing probability P ($\nu_{\mu} \rightarrow \nu_{\tau}$) < 10^{-4}$ at 90~\% C.L. can be explored in the region where $\Delta$m$^2_{\tau}_{\mu}$ > 1 eV$^2$, using the neutrino sample already collected. The main feature of CHORUS consists in reaching the mentioned sensitivity by keeping any background source below one event (zero-background experiment).\\ \\The experimental setup consists of a target, an air-core magnet, a high resolution calorimeter and a muon spectrometer. Nuclear emulsion stacks form the 800 kg active target; decay of short lived particles - ${\tau}^-$ for example - is visualised with a spatial resolution better than a micron, and by about 300 grains/mm along the tracks. The interactions are located inside the emulsion target with large scale scintillating fiber trackers, read out with opto-electronic image intensifiers coupled to CCD cameras.\\ \\The air-core magnet (instrumented by scintillating fiber arrays, honeycomb chambers and emulsion trackers) allows to evaluate the charge and momentum of hadrons and low momentum particles. The high resolution hadronic calorimeter is used in the estimate of energy and transverse momentum of the interactions (or of individual particles). The spectrometer identifies muons and measures their charge and momentum.\\ \\A significative part of the experiment takes place after the neutrino beam exposure, when the emulsion data are read. This phase is accomplished by means of computer-assisted automatic microscopes, that allow to reconstruct the interactions at unprecedented scanning speed. The automatic scanning technology - involving developments in optics, electronics and computer science - is being pioneered by the CHORUS experiment. \\ \\CHORUS took data in the Wide Band neutrino Beam of the SPS from May 1994 to November 1997. After two years the emulsion target had been completely replaced. A sample corresponding to about 800,000 $\nu_{\mu}$ charged current interactions has been recorded permanently in the emulsion target and also by the electronic detectors. Results - based on a subsample of the available data - have already been published: new limits on $\nu_{\mu} \rightarrow \nu_{\tau}$ oscillation are set and rare charm events have been observed. The location, measurement and analysis of the complete set of events in the target will take a few years of "emulsion data taking". In addition to the mentioned ${\tau}$ hunting, a significative part of the program consists in studying the very large sample of neutrino interactions in the emulsion: charm production, diffractive processes, nuclear effects, rare events search. Analysis of neutrino interactions originated in the calorimeter and in the spectrometer allows additional measurements such as structure functions evaluation, neutral heavy lepton search, cross sections and neutrino beam studies.