Experiment Proposal - opera - Infn

momentum is deduced from the measurement of the multiple scattering in the lead plates. As discussed
above, the muon momentum is also measured by the electronic detectors in a large fraction of the cases.
2.1.2 Event reconstruction with electronic detectors
Electronic detectors placed downstream of each emulsion brick wall are used to select the brick (to be
removed for analysis) where the neutrino interaction took place and to guide the scanning, by defining
the region of the films to be scanned. Fast and efficient scanning of the emulsion films, as by the Net
Scan method described in Section 6, removes the need for a precise reconstruction of individual tracks by
the electronic trackers. Therefore, a moderate spatial resolution can be tolerated, which allows to reduce
the cost of the electronic trackers covering a large surface.
The target electronic detectors are also used to sample the energy of hadronic showers and to contribute
to the identification and reconstruction of penetrating tracks.
The selection of the brick containing the neutrino interaction vertex is performed by combining
different algorithms based on the observed transverse and longitudinal event profiles as well as on the
presence of individual reconstructed tracks. As an illustration, Fig. 7 shows a simulated ν τ event with a
muonic decay for one of the two projections transverse to the beam direction. The transverse granularity
of the trackers is assumed to be 2.5 cm.
Figure 7: Display of a simulated τ → µ event in the OPERA target. The beam comes from the left of
the figure. The primary vertex occurs in the third brick wall. Each wall of bricks is followed by the target
tracker planes. These planes are oriented along the X and Y directions, perpendicular to the beam. The
muon track corresponds to the longest track escaping on the right of the figure.
Fig. 8 shows the multiplicity distributions of particles traversing the tracker planes downstream of
the vertex brick and downstream of the following brick wall. The simulated sample corresponds to
ν µ CC interactions. In the simulations, electron and photons have an energy above 1 MeV ,charged
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