OPERA

Home page of the Experiment: http://opera.web.cern.ch/opera/

The measurements of neutrino fluxes performed by the Super-Kamiokande experiment indicate a deficit of muon neutrinos and an anomaly in their zenith angle distribution, consistent with oscillations nm --> nt with Dm2 = 1.8 4 x 10-3 eV2 (90% C.L.) and full mixing. The Soudan2 and the MACRO experiments also made observations compatible with this result. Recently the K2K experiment observed, by using a nm beam produced at KEK and direct toward the 250 km far Super-Kamiokande detector, indications of neutrino oscillations: a reduction of flux together with a distortion of the energy spectrum. Therefore, the primary goal of OPERA is to obtain direct evidence for nt appearance, which could confirm the oscillation hypothesis.

Although the detector has been not optimized to search for nm --> ne, thanks to the very good electron identification of the ECC technique, OPERA also has a sensitivity in searching for q13 which, in case of null observation, would give a limit sin2 2q13 < 0.06 at 90% C.L.

A long baseline of 732 km is used between the neutrino source (the CERN beam line) and the detector (located in the underground Gran Sasso Laboratory) in order to be sensitive to the oscillation parameters indicated by the Super-Kamiokande data. The CNGS neutrino beam has been optimized for the detection of nm charged-current (CC) interactions and provides an average nm energy of about 20 GeV. For the evaluation of the experiment an integrated fluency of 2.25 x 10-3 protons on target is assumed, corresponding to a 5 years SPS operation in a shared mode.

The main principle of the nt search is the “direct" detection of the decay of the t lepton produced by CC interactions. This is achieved by a massive (about 1.8 Kton) neutrino target based on the ECC design which combines, in a sandwich-like cell, the high precision tracking capabilities of nuclear emulsion (two 50 mm layers on both sides of a 200 mm plastic base) and the large target mass provided by lead plates (1 mm thick). This technique has been recently demonstrated to be effective for t detection by the DONUT Collaboration.

The basic element of the target structure is the “brick", composed by a consecutive series of individual cells with transverse dimensions of 10.2 x 12.7 cm2. Bricks are arranged into planar structures (“walls"), which are interleaved with electronic tracker planes. These planes are built from vertical and horizontal strips of plastic scintillator, 2.6 cm wide. The main purpose of the electronic target tracker is to localize the particular brick in which the neutrino interaction occurred, once an interaction trigger is recorded. In the past year a new emulsion detector, the so-called Changeable Sheet (CS), has been proposed and included in the final design of the experiment. It has a twofold purpose: to help in the identification of the brick where the neutrino interaction occurred and to reduce the emulsion scanning load. After the CS has been removed and scanned, a brick is then extracted for the emulsion development and scanning in a quasi-online sequence. Large emulsion areas can be scanned with automatic microscopes equipped with fast track recognition processors. This allows the measurement of both track momenta from their multiple scattering in the brick and electron and gamma energies from shower development. The target and the tracker sections are further arranged into two independent super-modules. Each super-module is followed by a downstream muon spectrometer. A spectrometer consists of a dipolar magnet made of two iron walls, interleaved by pairs of vertical drift tube planes. Planes of RPC's are inserted between the magnet iron plates to allow a coarse tracking inside the magnet and a measurement of the stopping particles from the upstream super-module.

The OPERA design is optimized to achieve low background levels for the nt appearance search. The experiment aims at the analysis of all single-prong decay modes (e,m,h). Signal events are classified as “long" or “short" decays depending whether the t track traverses an emulsion sheet or not. The main background sources are charm production in CC interactions, hadronic interactions in lead and large angle muon scatterings. These events are rejected by the identification of the primary lepton in CC interactions and either by requiring the presence of a t-like kink topology (long decays) or by an impact parameter method (short decays). In addition, a kinematic analysis is used to enhance the signal to background ratio. Overall, a total background of about 0.5 events is expected. If nm --> nt oscillations occur, the average number of detected events ranges from 6 at Dm2 = 1.8 x 10-3 eV2 to 30 at Dm2 = 4 x 10-3 eV2. The achieved sensitivity at 90% C.L. covers the region of parameter space allowed by the Super-Kamiokande data. Within this region, the probability to obtain a statistical significance on the detected signal of at least 4s (gaussian equivalent) is larger 90% after 5 year run.

Since the submission of the Status Report in 2001 the sensitivity has been improved and further studies are still in progress and we expect to finalize these calculations by the end of 2003.