The OPERA experiment (Oscillation Project with Emulsion-tRacking Apparatus) has been taking data from 2008 to 2012 at the Gran Sasso INFN Laboratory on the CNGS muon neutrino beam produced at CERN. The experiment was aiming at demonstrating the tau neutrino appearance in a muon neutrino beam due to neutrino oscillations. This goal was achieved in 2015 when the observation of five tau neutrino candidates was reported and the discovery of tau neutrino appearance was published with a significance larger than 5 sigma. The experiment is unique in its capability of detecting all three neutrino flavours. Tau neutrinos are identified through their charged-current interactions where the short-lived tau lepton is produced and its decay vertex is detected. This challenging goal is achieved thanks to the Emulsion Cloud Chamber technology based on nuclear emulsion films with the unsurpassed sub-micrometric accuracy, readout by fully-automated and high-speed optical microscopes. The OPERA Collaboration includes about 150 physicists from 26 Institutes in 11 Countries.
The OPERA detector
Tau neutrinos in OPERA are produced in target units called bricks made of nuclear emulsion films interleaved with lead plates. The OPERA target contains about 150 thousands of these bricks, for a total mass of 1.25 kton, arranged into walls interleaved with plastic scintillator strips. The detector is made of two identical units, called Super Modules. Each of them contains a target section followed by a magnetic spectrometer for momentum and charge measurement of muon particles. Real time information from the scintillators in the target section and from the spectrometers provides the identification of the bricks where the neutrino interactions occurred. The candidate bricks are extracted from the walls and, after X-ray marking and an exposure to cosmic rays for their alignment, the emulsion films are developed and sent to the emulsion scanning laboratories to perform the accurate analysis of the event. The data reported in this release are obtained from all sub-detectors, namely the emulsion target and the target tracker (TT) in the target section, and the muon spectrometer. For the user sake of clarity, we give a very brief description of the sub-detectors and the information we infer from, providing references to the corresponding papers.
The reconstruction of one event with the OPERA detector starts from the registration of the hits produced by the electronic detectors, i.e. the muon spectrometer and the target tracker. The muon spectrometer is made of magnetised iron blocks interleaved by Resistive Plate Chambers (RPC), with a tracking station made of Drift Tubes (DT). Both RPC and DT are gas detectors. RPC, with a position resolution of about 1 cm, are used for the coarse tracking of muon tracks within the iron blocks while DT, with a resolution better than 300 microns, are used for the measurement of the muon momentum by detecting the bending of the muon trajectory in the magnetic field. RPC are described in JINST 4 (2009) P04018 and DT in NIM A 555 (2005) pp. 435-450 (erratum here).
The task of the Target Tracker detector is to locate in which of the target elementary constituents, the lead/emulsion bricks, the neutrino interactions have occurred and also to give energy information about each event with a calorimetric approach. The technology consists of walls of two planes of plastic scintillator strips, one per transverse direction. Wavelength shifting fibres collect the light signal emitted by the scintillator strips and guide it to both ends where it is read by multi-anode photomultiplier tubes. The TT is described in NIM A 577 (2007), Issue 3, pp. 523-539. The procedure to locate the neutrino interaction vertex with the help of electronic detectors in the OPERA experiment is described in Physics of Particles and Nuclei Letters, Volume 12 (2015), Issue 1, pp. 89-99.
The emulsion target is made of emulsion films interleaved with lead plates: the lead is the neutrino target while emulsion films provide a very accurate (at the micron level) tracking of the particles produced in the neutrino collision and allow reconstructing the neutrino vertex. The vertex reconstruction procedure is described in JINST 4 (2009) P04018. Unlike muon neutrinos, tau neutrinos are characterised by the presence of two vertices, one produced by the neutrino interaction and the other one produced by the tau lepton decay: this topological feature is used to identify tau neutrinos. The two vertices are recognised thanks to the micrometric accuracy of nuclear emulsion films. The procedure to identify the two vertices is described in Eur. Phys. J. C (2014) 74:2986.
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