ISSN 1063-7796, Physics of Particles and Nuclei, 2013, Vol. 44, No. 4, pp. 703–727. © Pleiades Publishing, Ltd., 2013. Original Russian Text © N.Yu. Agafonova, A.B. Aleksandrov, A.M. Anokhina, A.V. Bagulya, M.S. Vladimirov, Yu.A. Gornushkin, T.A. Dzhatdoev, S.G. Dmitrievski, R.I. Enikeev, S.G. Zemskova, D.K. Levashev, A.S. Malgin, V.A. Matveev, D.V. Naumov, V.V. Nikitina, N.M. Okateva, A.G. Olshevsky, D.A. Podgrudkov, N.G. Polukhina, T.M. Roganova, O.G.Ryazhskaya, V.G. Ryasnyi, N.I. Starkov, V.L. Troshina, M.M. Chernyavskiy, A.V. Chukanov, I.R. Shakyrianova, A.S. Sheshukov, G.P. Shoziyoev, T.V. Shedrina, V.F. Yakushev, 2013, published in Fizika Elementarnykh Chastits i Atomnogo Yadra, 2013, Vol. 44, No. 4. 703 CONTENTS INTRODUCTION 703 PREDECESSORS OF THE OPERA EXPERIMENT 704 DONUT Experiment 705 CHORUS Experiment 705 NOMAD Experiment 705 OPERA EXPERIMENT 706 CNGS Beam 707 Electronic Detectors 708 Emulsion Detectors 709 Automatic Scanning Microscopes 711 RECONSTRUCTION OF EVENT CHARACTERISTICS BY THE OPERA HYBRID DETECTOR AND DATA ANALYSIS 712 Detector Response Simulation 713 Reconstruction of Event Characteristics by the Electronic Detectors 713 Reconstruction of Tracks in Nuclear Emulsion 715 Evaluation of Particle Parameters from Emulsion Data 717 Search for Short-Lived Particle Decay 718 Characteristics of Background Events and Standard Selection Criteria for ν τ Interactions 719 Possibility of Multidimensional Search for ν τ Interactions 720 CANDIDATES FOR THE ν τ INTERACTION IN THE OPERA EMULSION DETECTOR 721 CURRENT STATUS OF THE OPERA EXPERIMENT 722 CONCLUSIONS 724 INTRODUCTION Though neutrino oscillations were predicted over 50 years ago [1], it was only recently that the experi- mental results reliably interpreted as an evidence of oscillation have been obtained. The first results that could be interpreted as oscilla- tions were obtained in the solar neutrino experiments Search for the ν μ → ν τ Oscillation with the OPERA Hybrid Detector N. Yu. Agafonova a , A. B. Aleksandrov b , A. M. Anokhina c, d , A. V. Bagulya b , M. S. Vladimirov b , Yu. A. Gornushkin e , T. A. Dzhatdoev d , S. G. Dmitrievski e , R. I. Enikeev a , S. G. Zemskova e , D. K. Levashev c, d , A. S. Malgin a , V. A. Matveev a , D. V. Naumov e , V. V. Nikitina d , N. M. Okateva b , A. G. Olshevsky e , D. A. Podgrudkov c, d , N. G. Polukhina b , T. M. Roganova d , O. G. Ryazhskaya a , V. G. Ryasny a , N. I. Starkov b , V. L. Troshina c, d , M. M. Chernyavskiy b , A. V. Chukanov e , I. R. Shakyrianova a , A. S. Sheshukov e , G. P. Shoziyoev d , T. V. Shedrina b , and V. F. Yakushev a a Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, 117312 Russia b P.N. Lebedev Physical Institute of the Russian Academy of Sciences, Moscow, 119991 Russia c Faculty of Physics, M.V. Lomonosov Moscow State University, Moscow, 119991 Russia d Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University, Moscow, 119992 Russia e Joint Institute for Nuclear Research, Dubna, 141980 Russia e-mail: timur1606@gmail.com Abstract—This work reviews the status of the international OPERA experiment aimed at searching for the ν μ → ν τ oscillation. Development of the emulsion techniques in the preceding DONUT and CHORUS experi- ments is described. A brief overview of the NOMAD experiment is given, in which properties of ν τ interac- tions were thoroughly investigated. Characteristics of the CERN–Gran Sasso (CNGS) neutrino beam are given, the structure of the OPERA hybrid detector and the functions of its subsystems are considered, and the automatic emulsion scanning technique is briefly reviewed. Uncertainties in reconstruction of event param- eters using the electronic and emulsion detectors are discussed and the procedure adopted in the experiment for identification of ν τ interactions is described. The main sources of background in the search for ν μ → ν τ oscillation are considered. Topologies of the first two ν τ candidates observed in the OPERA emulsion detector are presented. Possibilities of increasing the ν τ interaction selection efficiency using multidimensional criteria are briefly discussed. DOI: 10.1134/S1063779613040023