Experiment Proposal - opera - Infn

efficiently discriminate between signal and background by means of kinematical criteria. Due to brickto-brick
connection inefficiency, ∼ 96% of the electrons produced in τ decays can be fully measured.
In order to reduce the background from low energy electrons (γ → e + e − )theτ daughter is required
to have an energy higher than 1 GeV . This cut also allows to have a better electron identification (see
Section 7.1). In addition, an upper cut (15 GeV ) on the electron energy reduces the background from
prompt electrons produced in ν e CC interactions. The distribution of the τ daughter energy is shown in
Fig. 137. The overall efficiency of the above energy cuts is ε E ∼ 90% for both DIS and QE events.
The distribution of the transverse momentum at the decay vertex (p t ) is shown in Fig. 137. A lower
cut on p t (ε pt ) is applied in order to reduce the background from ν e CC interactions (see Section 7.4.3).
By requiring a p t larger than 100 MeV/c,morethan99% of the τ → e decays are kept while all
candidates from ν e CC events are rejected.
Finally, we determine the probability for an electron produced in a τ decay to be identified as a such
(ε e→e ). Results for both DIS and QE events show that ∼ 97% of the daughter electrons are identified
as such, 1% as hadrons and 2% cannot be classified.
Figure 137: Distribution of the reconstructed τ daughter energy (top) and of the reconstructed transverse
momentum at the decay vertex (bottom) for DIS τ → e decays. Vertical lines show the applied cuts.
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