Faculté des sciences

## Study of v interactions and background estimation in the OPERA emulsion film detector

### Thèse de doctorat : Université de Neuchâtel, 2006 ; 1883.

The OPERA (Oscillation Project with Emulsion tRacking Apparatus ) experiment or CNGS1 was approved in 2001 by CERN and presently is under construction. Data-taking is expected to start in 2006. The experiment is designated to the νμ  ντ oscillation search. OPERA is a hybrid detector comprising a number of electronic detectors and a specially designed nuclear emulsion stack interlaced with... More

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The OPERA (Oscillation Project with Emulsion tRacking Apparatus ) experiment or CNGS1 was approved in 2001 by CERN and presently is under construction. Data-taking is expected to start in 2006. The experiment is designated to the νμ  ντ oscillation search. OPERA is a hybrid detector comprising a number of electronic detectors and a specially designed nuclear emulsion stack interlaced with lead plates. The total target mass of the detector will be about 1.8 kt. This impressive mass needed for neutrino detection is combined with an even more impressive spatial resolution of about a $\mu m$, characteristic of the nuclear emulsion technique. The detection of ντ is based on the observation of the decay of the τ lepton. The fine grained structure of nuclear emulsions offers the possibility to directly observe such a decay and by the means of kinematical analysis can be clearly separated from background events. Nuclear emulsions will be produced and processed in industrial quantities and the readout will be done with automatized optical microscopes. In this work are presented some of the analysis tools needed for the data analysis starting with the raw data produced by the scanning stations. An estimation of the background for the τ  μ+νμ + ντ and τ  h + ντ decay channels and an estimate of detection efficiency for the hadronic decay channels are given. In the last chapter experimental results from test-beam data are presented. In this study we estimated the background originating from hadron re-interactions in the ECC Brick. The total background rate for the five year run of the OPERA experiment, after the suggested kinematical cuts, is estimated at 0.1 or below for each of the τ  μ+νμ + ντ, τ  π+ ντ and τ  π+π0+ ντ channels. The background for the τ  π+nπ0+ ντ is estimated for the first time here. The results are consistent with another simulation done with FLUKA. This study is important for demonstrating that OPERA is a very low background experiment and the total number of background events will be below one for the five year run. The detection efficiency for all τ decay channels of the OPERA detector is currently estimated at 9%. Hence a small gain in detection efficiency would be considered as a major improvement. Based on a Geant4 simulation, here we estimated the detection efficiency of the decay channels with one charged pion. We exploited the possibility to identify the decay channels with neutral pions by attaching γ's to the secondary vertex. The found overall efficiencies are ετ = 2.82% for DIS and ετ = 2.99% for QE interactions. Showing our results with data coming from test-beam exposures, we proved that we will be able to analize the data when the experiment starts. The number of pion interactions found in the November 2004 test-beam data is compatible with the number of interactions in simulated events. In general, the real-data and the simulated one shows a good agreement. A preliminary kinematical analysis shows the feasibility of using cuts on the transverse momentum and the impact parameter. Studies of the momentum measurement showed that resolution below 20% is possible to achieve, up to 4 GeV momentum. With an alternative method we achieved resolution better than 15% at low momentum (2 GeV). Unfortunately we observed strong limitations in the maximal measurable momentum, not observed in Monte Carlo data. These studies has to be continued in order to fulfill the requirements of the kinematical analysis.