[en] The OPERA experiment has observed muon neutrino to tau neutrino oscillations in the atmospheric sector in appearance mode. Five ν_τ candidate events have been detected, a number consistent with the expectation from the “standard” 3ν framework. Based on this result new limits on the mixing parameters of a massive sterile neutrino have been set. The analysis is performed in the 3+1 neutrino model

[en] In the last few years the experimental results on neutrino/anti-neutrino oscillations at Short-Baseline (SBL) showed a tension with several phenomenological models. The recent and carefully recomputed anti-neutrino fluxes from nuclear reactors have further increased this tension drawing a picture not fully compatible with the 3 neutrino oscillation scenario. A sterile neutrino is a neutral lepton which does not couple with W/Z bosons. it is not an exotic particle, its existence being a natural consequence of neutrinos having a non-zero mass. Sterile neutrinos can mix with the active ones through additional mass eigenstates, with no necessary mass scale. We will present an experimental search for sterile neutrinos with a new CERN-SPS neutrino beam using muon spectrometers and large LAr detectors. To definitely clarify the physics issue, the proposed experiment will study oscillations in a muon neutrino / antineutrino beam both in appearance and disappearance modes, exploring the Δm² ∼ 1 eV² range

[en] OPERA (Oscillation Project with Emulsion tRacking Apparatus) is a long baseline neutrino experiment aiming to detect for the first time the appearance of tau neutrinos in a pure muon neutrino beam. The OPERA detector is exposed to the CNGS beam (CERN Neutrinos to Gran Sasso), a high-intensity and high-energy beam of muon neutrinos produced at the CERN SPS and directed toward the Gran Sasso underground laboratory (LNGS), 730 km away. The LNGS laboratory is situated under an average depth of 3800 meters water equivalent, making OPERA the deepest experiment able to measure cosmic ray muons with a magnetized detector, e.g. the cosmic ray muon charge ratio. Cosmic ray events were observed, together with the first neutrino beam events, during the 2007 and 2008 CNGS runs. This paper reviews the experimental apparatus, presents the first results for neutrino data and describes the analysis of cosmic ray data.

[en] The OPERA detector at the Gran Sasso underground laboratory (LNGS) was used to measure the cosmic ray muon charge ratio R_μ=N_μ⁺/N_μ⁻ in the TeV energy region. R_μ is shown as a function of the “vertical surface energy” E_μcosθ. A fit to a simplified model of muon production in atmosphere allowed the determination of the pion and kaon charge ratios weighted by the cosmic ray energy spectrum.

[en] We present a review of past measurements of the vertical atmospheric muon charge ratio above 1 TeV. In particular we focus on the latest OPERA data which confirm with a higher statistical significance a drop of the charge ratio above ∼3 TeV. We attempt to interpret the result in terms of possible physical mechanisms that could manifest itself in this energy region.

[en] A tracking system composed of planes of triangular shape scintillator bars coupled to Silicon PhotoMultipliers in analog mode read-out has been developed for applications in neutrino experiments. A spatial resolution of O (1 mm) is required for the determination of momentum and charge of muons produced in ν_μ CC interactions at few GeV energy scale. The performance of the system has been studied by exposing it to charged particle beams at the CERN-PS. Preliminary results are discussed.

[en] The NESSiE Collaboration has been setup to undertake a conclusive experiment to clarify the muon-neutrino disappearance measurements at short baselines in order to put severe constraints to models with more than the three-standard neutrinos. To this aim the current FNAL-Booster neutrino beam for a Short-Baseline experiment was carefully evaluated by considering the use of magnetic spectrometers at two sites, near and far ones. The detector locations were studied, together with the achievable performances of two OPERA-like spectrometers. The study was constrained by the availability of existing hardware and a time-schedule compatible with the undergoing project of multi-site Liquid-Argon detectors at FNAL. The settled physics case and the kind of proposed experiment on the Booster neutrino beam would definitively clarify the existing tension between the ν_μ disappearance and the ν_e appearance/disappearance at the eV mass scale. In the context of neutrino oscillations the measurement of ν_μ disappearance is a robust and fast approach to either reject or discover new neutrino states at the eV mass scale. We discuss an experimental program able to extend by more than one order of magnitude (for neutrino disappearance) and by almost one order of magnitude (for antineutrino disappearance) the present range of sensitivity for the mixing angle between standard and sterile neutrinos. These extensions are larger than those achieved in any other proposal presented so far. (orig.)

[en] The OPERA detector, designed to search for ν_μ → ν_τ oscillations in the CNGS beam, is located in the underground Gran Sasso laboratory, a privileged location to study TeV-scale cosmic rays. For the analysis here presented, the detector was used to measure the atmospheric muon charge ratio in the TeV region. OPERA collected charge separated cosmic ray data between 2008 and 2012. More than 3 million atmospheric muon events were detected and reconstructed, among which about 110000 multiple muon bundles. The charge ratio R_μ ≡ N_μ+/N_μ- was measured separately for single and for multiple muon events. The analysis exploited the inversion of the magnet polarity which was performed on purpose during the 2012 Run. The combination of the two data sets with opposite magnet polarities allowed minimizing systematic uncertainties and reaching an accurate determination of the muon charge ratio. Data were fitted to obtain relevant parameters on the composition of primary cosmic rays and the associated kaon production in the forward fragmentation region. In the surface energy range 1-20 TeV investigated by OPERA, R_μ is well described by a parametric model including only pion and kaon contributions to the muon flux, showing no significant contribution of the prompt component. The energy independence supports the validity of Feynman scaling in the fragmentation region up to 200 TeV/nucleon primary energy. (orig.)

[en] TheOPERAexperiment, designed to perform the first observation of ν_μ → ν_τ oscillations in appearance mode through the detection of the τ leptons produced in ν_τ charged current interactions, has collected data from 2008 to 2012. In the present paper, the procedure developed to detect τ particle decays, occurring over distances of the order of 1mm from the neutrino interaction point, is described in detail and applied to the search for charmed hadrons, showing similar decay topologies as the τ lepton. In the analysed sample, 50 charm decay candidate events are observed while 54 ± 4 are expected, proving that the detector performance and the analysis chain applied to neutrino events are well reproduced by the OPERA simulation and thus validating the methods for ν_τ appearance detection. (orig.)

[en] The OPERA experiment was designed to search for ν_μ → ν_τ oscillations in appearance mode through the direct observation of tau neutrinos in the CNGS neutrino beam. In this paper, we report a study of the multiplicity of charged particles produced in charged-current neutrino interactions in lead. We present charged hadron average multiplicities, their dispersion and investigate the KNO scaling in different kinematical regions. The results are presented in detail in the form of tables that can be used in the validation of Monte Carlo generators of neutrino-lead interactions. (orig.)