OPERA is a long baseline neutrino oscillation experiment designed to detect the appearance of nt a in pure nm beam in the parameters region indicated by the anomaly in the fluxes of neutrinos produced by cosmic ray interactions in the atmosphere. The detector target of 1.8 kilotons is made of foils of lead interleaved with films of nuclear photographic emulsion. The high spatial resolution of the emulsion technology is exploited to detect unambiguously the decay vertices of the t- lepton produced in charged current nt interactions with the lead nuclei. The detector is installed at the Laboratori Nazionali del Gran Sasso (LNGS) in a cavern excavated under the Gran Sasso in the Italian Abruzzes. The cavern is in the line of sight of the CNGS beam of neutrinos originated from CERN, Geneva, at a distance of 730 km. The commissioning of the beam and the electronic components of the detector has taken place in August 2006. The filling of the target has started in October 2006 and is due to be completed by mid-2007. The data taking is planned to last for five years.
The OPERA physics goal
At the nominal value of âˆ†m223 = 2.5 10-3 eV2 and full nm-nt mixing (sin22q23=1), as measured by the Super-Kamiokande atmospheric neutrino experiment, OPERA will observe about than 20 nt CC interactions after 5 years of data taking, with an estimated background of only 1 event. The probability to establish the oscillation signal at a 4-sigma level is of 99.9%.
At the same value of âˆ†m2, OPERA will lower down the upper limit on the mixing angle sin22q13 of the sub-dominant oscillation channel nm-ne from 0.14, as mesured by the CHOOZ reactor neutrino experiment, to 0.06.
The OPERA detector
OPERA is a massive hybrid detector consisting of two identical super-modules. Each super-module has a 0.9-kton instrumented target followed by a 10x8 m2 magnetic muon spectrometer. One target is the repetition of 31 6.7x6.7 m2 modules each including a proper target wall followed by a dual X-Y plane of scintillator strips trackers for event location in the target. A wall is an assemblage of 8.3kg bricks made of 56 1-mm thick lead sheets, that provide the necessary mass, interleaved with 57 nuclear emulsion films that provide the necessary sub-millimetre high resolution.
Bricks in which a neutrino interaction has occured, typically 30 per day, are identified by the event reconstruction in the trackers. They will be extracted, disassembled and the emulsion films scanned and analysed by a battery of high speed high resolution automatic microscopes in order to locate the interaction vertex and search for candidates of the t- lepton decay topology.
Official OPERA website: http://operaweb.lngs.infn.it/