Experiment Proposal - opera - Infn

which may enter in such a comparison are the fraction of back-scattering tracks, the mean energy per
scintillator strip in the target and the event shape (number of hits and energy per plane, number of
target/spectrometer planes hit per event, etc.).
Possible sources of systematic error have been considered and some studies have already started
• in the GEANT simulation particles are traced until their energy reaches a predefined threshold.
In particular, the results presented above have been obtained by using a threshold of 1 MeV for
the tracking of photons and electrons. When this threshold is lowered to 30 keV , the fraction of
back-scattering events increases by ∼ 15%, which leads to a decrease of 3% of the brick finding
efficiency. This result has been obtained using the same neural network for wall finding as the one
which has been trained on events with a 1 MeV threshold;
• the effect of misalignment of target scintillator strips has also been studied. Detector strips X
and Y positions have been randomly mismatched from generation values up to 6 mm. Fig. 83
shows the effect of alignment errors on the brick finding efficiency for ν µ CC events. It indicates
that mechanical or survey accuracies should be kept below 2 mm for internal tracker geometry.
The efficiency is much less sensitive to possible relative brick-tracker misalignments. This leads to
looser mechanical tolerances;
• the effect of electronic thresholds on the brick finding efficiency is shown in Fig. 84. For muonic
events one observes a reduction of efficiency if the muon track is lost. Neutral current events are
rather insensitive to the threshold. The same behaviour is seen for muonic events above 5 p.e.
once the muon is no more contributing to vertex finding;
• a study of the effect of mean number of p.e./mip is shown in Fig. 85. Again the muon track
degradation is responsible for the reduction in efficiency observed for muonic events. One can
deduce that the mean number of p.e./mip should be above 4.
In addition to the above studies the following issues will be addressed
• effect of changes of the fragmentation model parameters: this may affect hit multiplicity, hit energy
and event shapes;
• effects coming from the simulation of the support structure of brick walls and scintillator strips;
• uncertainties in the simulation of light production in the strips, and light collection and propagation
in the fibres.
More simulations are in progress to clarify those points. Test beam results have already been reported
[6] and further beam tests are planned.
5.6 Muon charge and momentum determination
The performance of the spectrometer for the determination of the muon charge and momentum has been
studied using Monte Carlo ν µ CC and τ → µ interactions randomly distributed in the target volume. An
iterative χ 2 minimisation procedure is used to fit the measured muon trajectory, following the method
described in [75]. This method is based on the linearisation of the constraint equations as a function of the
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