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Register a new userLatest results of the OPERA experiment on nu-tau appearance in the CNGS neutrino beam
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OPERA is a long-baseline experiment designed to search for $ u_{mu}to u_{tau}$ oscillations in appearance mode. It was based at the INFN Gran Sasso laboratory (LNGS) and took data from 2008 to 2012 with the CNGS neutrino beam from CERN. After the discovery of $ u_tau$ appearance in 2015, with $5.1sigma$ significance, the criteria to select $ u_tau$ candidates have been extended and a multivariate approach has been used for events identification. In this way the statistical uncertainty in the measurement of the oscillation parameters and of $ u_tau$ properties has been improved. Results are reported.
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The OPERA neutrino experiment is designed to perform the first observation of neutrino oscillations in direct appearance mode in the $ u_mu to u_tau$ channel, via the detection of the $tau$-leptons created in charged current $ u_tau$ interactions. The detector, located in the underground Gran Sasso Laboratory, consists of an emulsion/lead target with an average mass of about 1.2 kt, complemented by electronic detectors. It is exposed to the CERN Neutrinos to Gran Sasso beam, with a baseline of 730 km and a mean energy of 17 GeV. The observation of the first $ u_tau$ candidate event and the analysis of the 2008-2009 neutrino sample have been reported in previous publications. This work describes substantial improvements in the analysis and in the evaluation of the detection efficiencies and backgrounds using new simulation tools. The analysis is extended to a sub-sample of 2010 and 2011 data, resulting from an electronic detector-based pre-selection, in which an additional $ u_tau$ candidate has been observed. The significance of the two events in terms of a $ u_mu to u_tau$ oscillation signal is of 2.40 $sigma$.
The OPERA experiment has conclusively observed the appearance of tau neutrinos in the muon neutrino CNGS beam. Exploiting the OPERA detector capabilities, it was possible to isolate high purity samples of $ u_{e}$, $ u_{mu}$ and $ u_{tau}$ charged current weak neutrino interactions, as well as neutral current weak interactions. In this Letter, the full dataset is used for the first time to test the three-flavor neutrino oscillation model and to derive constraints on the existence of a light sterile neutrino within the framework of the $3+1$ neutrino model. For the first time, tau and electron neutrino appearance channels are jointly used to test the sterile neutrino hypothesis. A significant fraction of the sterile neutrino parameter space allowed by LSND and MiniBooNE experiments is excluded at 90% C.L. In particular, the best-fit values obtained by MiniBooNE combining neutrino and antineutrino data are excluded at 3.3 $sigma$ significance.
The OPERA experiment was designed to search for $ u_{mu} rightarrow u_{tau}$ oscillations in appearance mode, i.e. by detecting the $tau$-leptons produced in charged current $ u_{tau}$ interactions. The experiment took data from 2008 to 2012 in the CERN Neutrinos to Gran Sasso beam. The observation of $ u_{mu} rightarrow u_{tau}$ appearance, achieved with four candidate events in a sub-sample of the data, was previously reported. In this paper, a fifth $ u_{tau}$ candidate event, found in an enlarged data sample, is described. Together with a further reduction of the expected background, the candidate events detected so far allow assessing the discovery of $ u_{mu}rightarrow u_{tau}$ oscillations in appearance mode with a significance larger than 5 $sigma$.
The OPERA experiment is designed to search for $ u_{mu} rightarrow u_{tau}$ oscillations in appearance mode i.e. through the direct observation of the $tau$ lepton in $ u_{tau}$ charged current interactions. The experiment has taken data for five years, since 2008, with the CERN Neutrino to Gran Sasso beam. Previously, two $ u_{tau}$ candidates with a $tau$ decaying into hadrons were observed in a sub-sample of data of the 2008-2011 runs. Here we report the observation of a third $ u_tau$ candidate in the $tau^-tomu^-$ decay channel coming from the analysis of a sub-sample of the 2012 run. Taking into account the estimated background, the absence of $ u_{mu} rightarrow u_{tau}$ oscillations is excluded at the 3.4 $sigma$ level.
The OPERA experiment was designed to study $ u_muto u_tau$ oscillations in appearance mode in the CNGS neutrino beam. In this letter we report the final analysis of the full data sample collected between 2008 and 2012, corresponding to $17.97cdot 10^{19}$ protons on target. Selection criteria looser than in previous analyses have produced ten $ u_tau$ candidate events, thus reducing the statistical uncertainty in the measurement of the oscillation parameters and of $ u_tau$ properties. A multivariate approach for event identification has been applied to the candidate events and the discovery of $ u_tau$ appearance is confirmed with an improved significance level of 6.1 $sigma$. $Delta m^2_{23}$ has been measured, in appearance mode, with an accuracy of 20%. The measurement of $ u_tau$ CC cross-section, for the first time with a negligible contamination from $bar{ u}_tau$, and the first direct observation of the $ u_tau$ lepton number are also reported.
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