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OPERA

Prof. Dr. Ereditato Antonio, Prof. Dr. em. Vuilleumier Jean-Luc, Prof. Dr. Kreslo Igor, Dr. Ariga Akitaka, Dr. Pistillo Ciro, Dr. Ariga Tomoko

LHEP is involved in the OPERA neutrino oscillation experiment at the underground Gran Sasso laboratory.

Several key experiments conducted in the last decade with solar, atmospheric, reactor and accelerator neutrinos have contributed to build-up our present understanding of neutrino mixing. A wide program is currently being executed under the pressure of a very active international community. Several challenging projects are planned for the forthcoming years, aimed at pinning-down the oscillation parameters, and at sensitive measurements of the elements of the Pontecorvo, Maki, Nakagawa, Sakata (PMNS) neutrino mixing-matrix.

Among the above-mentioned precision measurements, the OPERA experiment will make use of the long baseline CNGS neutrino beam from CERN in Geneva to the underground Gran Sasso Laboratory (LNGS) in central Italy with the main goal of identifying which neutrino flavors take part in the oscillation relative to the atmospheric neutrino signal. This will be performed thanks to the unique possibility, among all existing and planned experiments, of detecting nm-nt oscillations in "appearance" mode. OPERA will also provide measurement of the Δm223 and sin223 parameters, and will attempt measuring for the first time the still unknown θ13 angle from the sub- leading νμ→νe oscillation channel, for which only an upper limit exists today. The importance of this measurement relies on the fundamental implications that a not vanishing θ13 value would have on particle physics and cosmology, allowing for the possibility of CP violation in the neutrino sector.

The detector has a neutrino target with a mass of 1.8 kton, consisting of "bricks" made of sandwiched lead plates and thin emulsion films. This target is complemented by scintillator tracker planes needed to localize the events within the individual target elements, and by muon spectrometers. The detector is in its construction and installation phase.

The main Bern activities include:

  • detector installation, operation and data taking with the CNGS beam;
  • scanning of the emulsion films with the automatic microscope facility in Bern;
  • studies on event reconstruction algorithms, background, detection efficiency and sensitivity;
  • physics analysis of the events, with emphasis on the νμ→ντ and νμ→νe oscillation channels, through the detection of τ-leptons and electrons in the final state, determination of the oscillation parameters, with the first, direct observation of ντ appearance.

Although OPERA should be eventually able to exclude (at the 2σ level) values of sin213 down to 0.06, a new generation of dedicated accelerator neutrino-beam experiments will be required in order to improve the sensitivity down to sin213 values as low as 0.005. This is one of the goals of the T2K experiment in Japan, exploiting the long-baseline, low-energy, high-intensity neutrino beam of the future JPARC accelerator complex at Tokai.

Since 2010, T2K is measuring sin213 through the precision study of νμ→νe oscillations and already established a non-zero value for θ13 at the 2.5 σ level. It also allows a high precision measurement of the parameters sin223 and Δm223. In a second phase, thanks to improved beam intensity, the experiment will attempt addressing the issue of a CP-violating phase in the mixing matrix and the question of the neutrino mass-hierarchy.

The experimental apparatus will be realized in the framework of an international Collaboration and is composed of three detector stations at different distances from the neutrino source (140m, 280m and 295km). The first two sections have the task to detect "not oscillated" neutrino interactions in order to compare the event physical quantities to those obtained with the third (far) detector where oscillations will occur, constituted by the 50 kton, water Cerenkov, Super-Kamiokande detector.

The Bern group considered T2K as the logical continuation of the studies initiated with OPERA. LHEP is now member of T2K with responsibilities in the 280 m detector complex. In particular, the group realized an automatic equipment for the precision measurement of the magnetic field of the large magnet (former UA1/NOMAD). Furthermore, the beam was monitored with the help of emulsions and the expertice gathered in this field in Bern. LHEP is also involved in the process of building a scintillator/iron sandwich calorimeter for the monitoring of the neutrino beam low energy componentent (LEM detector).

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