Experiment Proposal - opera - Infn

7.3.9 Detection efficiency for the µ channel
The detection strategy for τ → µ decays is similar to that of the electron channel.
In addition to the loss due to the muon identification, which amounts to ∼ 5%, one requires that the
energy of the muon lies in the range 1 − 15 GeV . The lower cut allows to reject events for which the µ
identification is poor and to reject possible background from low energy hadrons. The upper cut reduces
ν µ CC events by about 30% with a small loss of the signal. This selection reduces the background from
scattering of a prompt muon in the lead (see Section 7.4.4). The total efficiency of the energy cut is
ε E ∼ 90%.
Acutonthep t at the decay vertex is also applied. By requiring p t > 0.25 GeV/c an efficiency for
the signal ε pt ∼ 90% is obtained.
We take into account the probability that the daughter muon identified in the electronic detector is
not matched to the correct emulsion track. This results an additional loss of about 10% (ε µ−match ).
The brick to brick connection does not play a crucial role in the τ → µ channel (ε b2b > 99.5%).
The efficiency of the kinematical analysis (ε L kin = ε b2b × ε E × ε pt × ε µ→µ × ε µ−match )forlong
decays is ε L kin
= 78(76)% for DIS(QE) events.
Presently, muonic short decays are not considered because of charm background. However, we are
investigating the possibility of its rejection by kinematical criteria.
7.3.10 Detection efficiency for the h channel
The hadronic single-prong decays of the τ are selected among the events with no detected muon or
electron daughter.
In order to keep the background for this channel as low as possible severe kinematical cuts are applied
both at the decay and at the primary vertex.
The kinematical analysis at the decay vertex is qualitatively similar to that of the electronic and
muonic channels. However, the cut applied on p t is harder (p t > 600 MeV/c) and the daughter particle
is required to have a momentum larger than 2 GeV/c. The p t cut is justified by the relatively small
( 100 MeV/c) in elastic or inelastic pion interactions, while the average p t in hadronic τ
decays is ∼ 900 MeV/c. The cut on the daughter momentum eliminates low energy hadrons which are
copiously produced in ν µ CC interactions.
The kinematical analysis at the primary vertex uses the variables p miss
t , defined as the missing
transverse momentum at the primary vertex, and φ, which is the angle in the transverse plane between
the parent track and the shower direction. The p miss
t and φ distributions both for the signal and the
background are shown in Fig. 139.
Due to the unobserved outgoing neutrino, p miss
t is expected to be large in NC interactions. Conversely,
it is expected to be small in CC interactions. For τ candidates the measured p miss
t is required to be
lower than 1 GeV/c.
180