Experiment Proposal - opera - Infn

π−
e+
γ
e-
e+ e-
γ
π−
Figure 141: Background to the electron channel from pion charge exchange processes, occurring when
the electron or the positron track is absorbed in the lead.
the π 0 conversion could mimic a τ to electron decay, as shown schematically in Fig. 141. Simulations
indicate that the expected background from this process amounts to 0.2 × 10 −6 × N CC events.
Possible sources of background for short decays are prompt electrons and electrons produced in the
conversion of a γ inside the vertex plate. By applying the kinematical cuts described in Section 7.3.8,
the expected background is ∼ 0.1 × 10 −6 × N CC .
7.4.4 Background from muon scattering
Muons produced in ν µ CC events and undergoing a scattering in the lead plate following the vertex plate
could mimic a muonic τ decay.
A first estimate of this background was made in [6] with a GEANT 3.21 simulation. About 70000 ν µ
CC events in the compact bricks were simulated according to the CNGS beam spectrum. In 0.5% of the
events, the prompt muon had an apparent kink angle larger than 25 mrad in the lead plate downstream
of the vertex plate. For these events (see Fig. 2.25 of [6]) the muon transverse momentum was shown as a
function of the momentum. In the same plot the τ candidates were also shown. The correlation between
the two quantities was used to apply a combined cut, as shown in the same figure. Two events survived
the cut, both with quite a large angle, greater than 80 mrad.
Taking into account the other factors contributing to the detection efficiency (ε geom , ε brick and ε vert ),
the two survivors correspond to a normalised background of 1.3 × 10 −5 × N CC events for a 1 mm lead
plate. Since decays are searched for in two consecutive lead plates behind the vertex one, the total
background becomes 2.6 × 10 −5 × N CC .
More recently it became clear that GEANT overestimates large angle muon scatters background.
This was clearly observed in the NOMAD experiment by comparing the muon reconstruction efficiencies
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