New Results from OPERA
Matteo Tenti (right) gave a presentation of recent results from OPERA, a long baseline accelerator experiment at the Gran Sasso Laboratory. The aim of Opera is to verify the muon to tau neutrino oscillation. The muon neutrino beam comes from the CNGS at CERN, at 735 km distance, and has a mean energy of 17 GeV.
The tau neutrino detection technique is performed by loking at the tau decay, which at the energy of the beam gives a 0.6mm decay distance. The detector is made of bricks of lead and 56 sheets of nuclear emulsions. The extreme granularity allows for micrometer-size resolution. The detector is made up by two supermodules, a target section of brick walls, interleaved with planes of strip scintillator. This “target tracker” is used to choose the brick where the neutrino interaction has occurred, and to also provide some calorimetric measure of the event.
From a total integration of 1.8×10^20 protons on target at CNGS, 19505 neutrino interactions were reconstructed. 5 candidate events were finally extracted, with an expected S/N ratio of about 10. The background-only hypothesis was rejected at 5.1 sigma significance.
Afterwards looser kinematic cuts were made in order to increase the signal statistics. They deifined a minimal selection and applied a boosted decision tree classifier. This led to an expected background of 1.86+-0.5 events, with an expected signal of 5.88+-1.18, and 10 events were observed. This led to an improvement in the deltam^2_23, determined as a Feldman-Cousins interval of (1.98-3.95)x10^-3 eV^2.
Another search was made for electron-neutrino appearance in the predominantly muon neutrino beam. The result is not very sensitive to oscillation contribution (2.8 events expected out of a total of 35.7, dominated by beam contamination of electron neutrinos).