Experiment Proposal - opera - Infn

1 Introduction
In this document we propose an experiment (OPERA) for the direct observation of ν τ appearance from
ν µ ↔ ν τ oscillations in the CNGS long baseline beam [1] from the CERN SPS to the Gran Sasso Laboratory.
OPERA aims at high sensitivity in the parameter region indicated by the deficit of atmospheric
muon neutrinos and by its zenith angle dependence as observed by the Super-Kamiokande experiment.
The experiment exploits nuclear emulsions as very high resolution tracking devices for the direct detection
of the decay of the τ produced in the charged current (CC) interaction of the ν τ with the target. Preliminary
ideas and designs of the experiment can be found in [2–5] and in the Progress Report presented
to the Scientific Committees in 1999 [6].
The technique of nuclear emulsion has found a large scale application in the target of the CHORUS
experiment [7] in which the automatic scanning of a large sample of events has first been applied. This
technique can be further improved and lead to the much larger scale of the OPERA target. Its mass
is made of lead plates, with emulsion films used as high precision trackers, unlike in CHORUS in which
emulsions constitute the bulk of the target mass.
The experiment design is based on the Emulsion Cloud Chamber (ECC) detector, a modular structure
made of a sandwich of passive material plates interspaced with emulsion layers (Fig. 1). By assembling
a large quantity of such modules, it is possible to conceive and realise a ∼ 2000 ton fine-grained vertex
detector optimised for the study of ν τ appearance.
Beam
Emulsion Sheet (ES)
Passive material plate
Figure 1: Schematic structure of an Emulsion Cloud Chamber (ECC).
This target is complemented by arrays of electronic trackers for the real time determination of the
event position and by magnetised iron spectrometers for muon identification and for the reconstruction
of their charge and momentum.
5