Abstract
The OPERA experiment has discovered the tau neutrino appearance in the CNGS muon neutrino beam, in agreement with the 3 neutrino flavour oscillation hypothesis. The OPERA neutrino interaction target, made of Emulsion Cloud Chambers, was particularly efficient in the reconstruction of electromagnetic showers. Moreover, thanks to the very high granularity of the emulsion films, showers induced by electrons can be distinguished from those induced by π0s, thus allowing the detection of charged current interactions of electron neutrinos. In this paper the results of the search for electron neutrino events using the full dataset are reported. An improved method for the electron neutrino energy estimation is exploited. Data are compatible with the 3 neutrino flavour mixing model expectations and are used to set limits on the oscillation parameters of the 3+1 neutrino mixing model, in which an additional mass eigenstate m4 is introduced. At high Δm412 (≳0.1 eV2), an upper limit on sin2 2θμe is set to 0.021 at 90% C.L. and Δm412 ≳ 4 × 10− 3 eV2 is excluded for maximal mixing in appearance mode.
Highlights
The CNGS beam was an almost pure muon neutrino beam having a mean energy of 17 GeV
A search is performed in the CS doublet for track segments less than 2 mm apart from the extrapolation point of each track originated from the interaction vertex
Since the number of hit Target Tracker (TT)/Resistive Plate Chambers (RPC) planes and presence of long tracks in neutrino events are correlated with the neutrino energy, as a result of 0μ tagging criterion, a drop in the efficiency is expected at high energies
Summary
The brick acted as a high sampling calorimeter with more than five active layers every X0 over a total thickness of 10 X0. The search for electrons is performed in an extended scanned volume applying a dedicated procedure to the 0μ tagged events, i.e. events having no reconstructed three-dimensional muon track and less than 20 fired TT/RPC planes. 5868 events have a reconstructed neutrino interaction vertex in the first or second most probable brick and 1281 are tagged as 0μ events. The first type of background arises when the τ -lepton and its daughter electron track cannot be distinguished It is estimated by MC simulation assuming the 3-flavour νμ → ντ oscillation scheme and oscillation parameters from [24]. The background due to π0 is derived from the data by counting the number of events fulfilling the criteria for a νe candidate with a γ converting in the second or third lead plate downstream of the interaction vertex. Summarizing, the expected number of background events, Nbkg, amounts to 1.2 ± 0.5 (stat.) ± 0.2 (syst.), while the sum of expected events from νe and νe beam components and backgrounds is 31.9 ± 1.0 (stat.) ± 3.1 (syst.)
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