The OPERA neutrino experiment at the underground Gran Sasso Laboratory has measured the velocity of neutrinos from the CERN CNGS beam over a baseline of about 730 km. The measurement is based on data taken by OPERA in the years 2009, 2010 and 2011. Dedicated upgrades of the CNGS timing system and of the OPERA detector, as well as a high precision geodesy campaign for the measurement of the neutrino baseline, allowed reaching comparable systematic and statistical accuracies.An arrival time of CNGS muon neutrinos with respect to the one computed assuming the speed of light in vacuum of (6.5 ± 7.4 (stat.) +8.3 −8.0 (sys.)) ns was measured corresponding to a relative difference of the muon neutrino velocity with respect to the speed of light (v − c)/c = (2.7 ± 3.1 (stat.) +3.4 −3.3 (sys.)) × 10 −6 . The above result, obtained by comparing the time distributions of neutrino interactions and of protons hitting the CNGS target in 10.5 µs long extractions, was confirmed by a test performed at the end of 2011 using a short bunch beam allowing to measure the neutrino time of flight at the single interaction level.

A first result of the search for ν µ → ν e oscillations in the OPERA experiment, located at the Gran Sasso Underground Laboratory, is presented. The experiment looked for the appearance of ν e in the CNGS neutrino beam using the data collected in 2008 and 2009. Data are compatible with the non-oscillation hypothesis in the three-flavour mixing model. A further analysis of the same data constrains the non-standard oscillation parameters θ new and ∆m 2 new suggested by the LSND and MiniBooNE experiments. For large ∆m 2 new values (>0.1 eV 2 ), the OPERA 90% C.L. upper limit on sin 2 (2θ new ) based on a Bayesian statistical method reaches the value 7.2 × 10 −3 .

Limits on muon-neutrino to tau-neutrino oscillations induced by a sterile neutrino state obtained by OPERA at the CNGS beamThe OPERA collaboration E-mail: alessandro.paoloni@lnf.infn.it, alessandra.pastore@ba.infn.it Abstract: The OPERA experiment, exposed to the CERN to Gran Sasso ν µ beam, collected data from 2008 to 2012. Four oscillated ν τ Charged Current interaction candidates have been detected in appearance mode, which are consistent with ν µ → ν τ oscillations at the atmospheric ∆m 2 within the "standard" three-neutrino framework. In this paper, the OPERA ν τ appearance results are used to derive limits on the mixing parameters of a massive sterile neutrino. The OPERA collaboration 11 IntroductionThe OPERA experiment [1] operated in the CERN Neutrinos to Gran Sasso (CNGS) beam produced at CERN and directed towards the Gran Sasso Underground Laboratory of INFN (LNGS), 730 km away, where the detector is located. The experiment is unique in its capability to observe ν τ appearance on an event-by-event basis. Nuclear emulsion films instrumenting the target allow the detection of the short-lived τ lepton decay, and hence the identification of ν τ Charged Current (CC) interactions. The standard three-neutrino oscillation framework predicts ν µ → ν τ oscillations with close-to-maximal mixing at the so-called atmospheric scale, ∆m 2 32 ∼ 2.4 × 10 −3 eV 2 [2], i.e. in the oscillation parameters region discovered by detecting atmospheric neutrinos [3]. OPERA has observed four ν τ CC interaction candidate events [4][5][6][7], consistent with the expectation of the standard oscillation framework at this scale. This result represents the first direct evidence of ν µ → ν τ oscillation in appearance mode.In the present paper, limits are derived on the existence of a massive sterile neutrino. The excess of ν e (ν e ) observed by the LSND [8] and MiniBooNE [9] collaborations and the so-called reactor [10] and Gallium [11,12] neutrino anomalies are also interpreted as due to the existence of a fourth sterile neutrino with mass at the eV scale. In relation to this issue, it is worth mentioning that the effective number of neutrino-like species decoupled from the primeval plasma measured by the Planck collaboration is 3.15 ± 0.23 at 95% Confidence Level (CL) [13].Neutrino oscillations at large ∆m 2 have been searched for by several short baseline experiments. The most stringent limits on ν µ → ν τ oscillations were set by the NOMAD [14] and CHORUS [15] experiments, with high sensitivity for ∆m 2 values larger than 10 eV 2 .In the following, a short description of the OPERA experimental setup and of the procedure used to detect ν τ interactions is given, the data analysis is described and exclusion regions in the parameter space are derived.

The OPERA long-baseline neutrino-oscillation experiment has observed the direct appearance of ν τ in the CNGS ν µ beam. Two large muon magnetic spectrometers are used to identify muons produced in the τ leptonic decay and in ν µ CC interactions by measuring their charge and momentum. Besides the kinematic analysis of the τ decays, background resulting from the decay of charmed particles produced in ν µ CC interactions is reduced by efficiently identifying the muon track. A new method for the charge sign determination has been applied, via a weighted angular matching of the straight track-segments reconstructed in the different parts of the dipole magnets. Results obtained for Monte Carlo and real data are presented. Comparison with a method where no matching is used shows a significant reduction of up to 40% of the fraction of wrongly determined charges.

Characteristics of γ -ray families with halos (XREC, Pamir) and data of experiments with EAS are analyzed to estimate the proton and helium (p+He) fractions in the primary cosmic radiation at E 0 = 1-100 PeV. It is shown that at energies E 0 ∼ 1-100 PeV the fraction of p+He remains significant, namely, the fraction of p+He is near 40% at E 0 = 10 PeV.