The OPERA experiment
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NEUTRINO PHYSICS AND ASTROPHYSICS (Elementary Particles and Fields. Experiment)
The OPERA Experiment* C. Jollet** IReS, IN2P3–CNRS and University Louis Pasteur, Strasbourg, France Received October 13, 2005
Abstract—The aim of the OPERA experiment is to provide unambiguous evidence for the νμ ↔ ντ oscillation by looking at the appearance of ντ in a pure νμ beam. This oscillation will be sought in the region of the oscillation parameters indicated by the atmospheric neutrino results. The experiment is part of the CNGS (CERN Neutrino beam to Gran Sasso) project. The νμ beam produced at CERN will be sent towards the Gran Sasso underground laboratory, where the OPERA detector is under construction. The detector, the physics potential, and performance for neutrino oscillation studies including the subleading νμ ↔ νe search are presented. PACS numbers : 95.55.Vj, 96.40.Tv DOI: 10.1134/S1063778806110093
1. INTRODUCTION The “appearance” long-baseline neutrino experiment OPERA [1] has been motivated by the atmospheric neutrino disappearance. Given the distance of 732 km between the neutrino source (at CERN) and the detectors (in the Gran Sasso underground laboratory), the CNGS beam (CERN Neutrino beam to Gran Sasso) [2] was designed in order to optimize the number of ντ charged-current (CC) interactions detectable at the Gran Sasso location. To be sensitive in the oscillation parameter region delimited mainly by the latest Super-Kamiokande results [3], the average energy of the CNGS beam is about 17 GeV. With the CERN SPS accelerator operating in a shared mode, 4.5 × 1019 protons on target will be delivered per year. The number of νμ CC interactions is 2900/(kt yr). If the νμ ↔ ντ oscillation hypothesis is confirmed, the number of τ produced via CC interaction at Gran Sasso will be of the order of 16/(kt yr) for Δm2 = 2.4 × 10−3 eV2 at full mixing.
1 μm. This technique will allow one to localize the τ decay vertex (“kink”). Moreover, in order to increase the number of ντ CC interactions, lead plates will be used for the target. The detection principle depicted in Fig. 1 takes into account these two conditions. The neutrinos interact with the 1-mm-thick lead target plates and the charged tracks are detected by the emulsion films. The kink between the τ track and the tracks produced by the particles following the τ decay makes an angle required to be greater than 25 mrad to avoid background coming from multiple scattered tracks. Each track segment is reconstructed using 15 to 20 visible grains produced by the charged particles. The spatial resolution is of the order of 0.21 μm, while the angular resolution is about 2.1 mrad. On the reconstructed events, the kink search is performed, as well as the energy reconstruction of electromagnetic showers and the determination of momenta of charged particles by multiple scattering. All this inPlastic base (200 μm) Pb Pb
ντ
e, μ, h
2. DETECTION TECHNIQUE The principle is to detect the τ leptons produced by ντ interactions. According to the mean lifetime of the τ lepton, an unambiguous
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