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Register a new userSearch for u_mu -> u_tau oscillations in appearance mode in the OPERA experiment
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Authors
Umut Kose
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The OPERA experiment in the underground Gran Sasso Laboratory (LNGS) has been designed to perform the first detection of neutrino oscillations in direct appearance mode in the muon neutrino to tau neutrino channel. The detector is hybrid, being made of an emulsion/lead target and of electronic detectors. It is placed in the CNGS neutrino beam 730 km away from the neutrino source. Runs with CNGS neutrinos were successfully carried out in 2008, 2009, and 2010. After a brief description of the beam and the experimental setup, we report on event analysis of a sample of events corresponding to 1.89times 10^{19} p.o.t. in the CERN CNGS u_{mu} beam that yielded the observation of a first candidate u_{tau} CC interaction. The topology and kinematics of this candidate event are described in detail. The background sources are explained and the significance of the candidate is assessed.
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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 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 $sigma$.
A first result of the search for umu $rightarrow$ ue oscillations in the OPERA experiment, located at the Gran Sasso Underground Laboratory, is presented. The experiment looked for the appearance of ue 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 $theta_{new}$ and $Delta m^2_{new}$ suggested by the LSND and MiniBooNE experiments. For large $Delta m^{2}_{new}$ values ($>$0.1 eV$^{2}$), the OPERA 90% C.L. upper limit on sin$^{2}(2theta_{new})$ based on a Bayesian statistical method reaches the value $7.2 times 10^{-3}$.
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.
The MiniBooNE experiment at Fermilab reports results from an analysis of $bar u_e$ appearance data from $11.27 times 10^{20}$ protons on target in antineutrino mode, an increase of approximately a factor of two over the previously reported results. An event excess of $78.4 pm 28.5$ events ($2.8 sigma$) is observed in the energy range $200<E_ u^{QE}<1250$ MeV. If interpreted in a two-neutrino oscillation model, $bar{ u}_{mu}rightarrowbar{ u}_e$, the best oscillation fit to the excess has a probability of 66% while the background-only fit has a $chi^2$-probability of 0.5% relative to the best fit. The data are consistent with antineutrino oscillations in the $0.01 < Delta m^2 < 1.0$ eV$^2$ range and have some overlap with the evidence for antineutrino oscillations from the Liquid Scintillator Neutrino Detector (LSND). All of the major backgrounds are constrained by in-situ event measurements so non-oscillation explanations would need to invoke new anomalous background processes. The neutrino mode running also shows an excess at low energy of $162.0 pm 47.8$ events ($3.4 sigma$) but the energy distribution of the excess is marginally compatible with a simple two neutrino oscillation formalism. Expanded models with several sterile neutrinos can reduce the incompatibility by allowing for CP violating effects between neutrino and antineutrino oscillations.
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